A high-performance electrolyte and a lithium-ion battery containing the electrolyte

Abstract

The present invention discloses a high-performance electrolyte, which comprises a lithium salt, a solvent and an electrolyte additive. The solvent is composed of a fluorinated carbonate compound and a linear carbonate compound in a mass ratio of (3-7):(3-5), and the electrolyte additive includes a fluorinated phosphazene compound. The present invention also discloses a lithium-ion battery containing the electrolyte. In the negative electrode material of the battery, N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine is used as a modifier. On the one hand, the modifier can improve the binding ability of the negative electrode material and enhance the cycling performance; on the other hand, its residual amount has the ability to remove acid, which can solve the problems of acid production and gas production during high-temperature storage of the fluorinated carbonate solvent in the electrolyte. By designing the negative electrode slurry formula and the electrolyte formula, and simultaneously using a double-sided ceramic separator, a new chemical system of a high-nickel and high-capacity silicon negative electrode is designed, and a battery with high energy density, excellent cycling performance and safety performance is prepared.

Description

Technical Field

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

[0002] Adopting a high-nickel ternary material to match a graphite composite negative electrode material with a high silicon content is one of the important means to improve the energy density of lithium batteries. However, with the increase in nickel content, the safety thermal stability of the battery core deteriorates rapidly; with the increase in the use content of the silicon negative electrode, the cycle life of the battery deteriorates rapidly, and the entire battery system cannot well meet the product performance requirements. Summary of the Invention

[0003] In order to solve the problems of battery safety, cycle stability, etc. caused by high-nickel positive electrodes and high-capacity silicon negative electrodes, the present invention optimizes and modifies the components of the electrolyte and the negative electrode material to improve the heat resistance temperature, cycle performance, and safety performance of the battery.

[0004] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0005] The first object of the present invention is to provide a high-performance electrolyte, comprising a solvent, an electrolyte additive, and a lithium salt, wherein:

[0006] The solvent is composed of a fluorinated carbonate compound and a linear carbonate compound in a mass ratio of (3-7):(3-5); the fluorinated carbonate compound includes at least one of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and 4-trifluoromethyl ethylene carbonate (TEPC). The fluorinated carbonate compound can participate in the film formation of the silicon negative electrode at a high potential. The SEI film is rich in LIF, enhancing the SEI strength and improving the cycle of the silicon negative electrode; the linear carbonate compound includes at least one of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), and dimethyl carbonate (DMC).

[0007] The electrolyte additive includes fluorophosphazene compounds, and the fluorophosphazene compounds include at least one of ethoxy pentafluorocyclotriphosphazene (PEPN) and phenoxy pentafluorocyclotriphosphazene (FPPN). The content of the fluorophosphazene compounds in the electrolyte is 5-10 wt%. Such additives can form an oxide film on the positive electrode in an environment of about 4.2 V to prevent further oxidation of the positive electrode and the electrolyte, and can also be used as flame retardant additives. When heated and decomposed, they can capture flammable free radicals and reduce the degree of combustion. Further preferably, the electrolyte additive further includes N,N'-diisopropylcarbodiimide (DIC) or N,N-diisopropylethylamine (DIPEA). DIC or DIPEA can inhibit the high-temperature decomposition of fluorocarbonates, improve the cycle performance of the battery, and can alleviate problems such as gas generation caused by the acid production due to the high-temperature decomposition of fluorocarbonate compounds in the solvent.

[0008] The lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI); preferably, the lithium salt is composed of LiPF6 and LiFSI in a mass ratio of 3-8:2-7; the content of the lithium salt in the electrolyte is 5-10 wt%. Such a lithium salt ratio can reduce the generation of PF5 due to the decomposition of lithium hexafluorophosphate at high temperatures, and the thermal stability of lithium bis(fluorosulfonyl)imide is better than that of lithium hexafluorophosphate, improving the thermal stability of the system.

[0009] The second object of the present invention is to provide a lithium-ion battery, including a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte as described above; the negative electrode sheet includes a negative electrode current collector and a negative electrode material coated on the surface of the negative electrode current collector. 0.05-2 wt% of a modifier is added to the negative electrode material, and the modifier is N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine. This modifier has a certain moisturizing effect, can prevent the binder from floating up with the rapid evaporation of moisture, can reduce the surface tension, thereby ensuring the uniformity of the baking of the electrode plate, preventing cracking, and avoiding poor adhesion of the silicon negative electrode, which may lead to problems such as increased volume expansion during the charge and discharge process of the electrode plate. And after baking, the modifier has a residue of >300 ppm, and the residue can react with HF generated by the high-temperature storage of substances such as fluorocarbonates in the electrolyte, thereby inhibiting the reaction of HF with the residual alkali Li2CO3 of the positive electrode and reducing the generation of gases such as CO2 and CO.

[0010] Further solution: the negative electrode material includes a carbon-silicon composite material, a negative electrode conductive agent, an aqueous binder, and a composite binder; the negative electrode conductive agent includes conductive carbon black (Super-P) and carbon nanotubes (SW-CNT); the composite binder includes styrene-butadiene rubber (SBR) and polyacrylic acid (PAA). SBR has strong extensibility, and PAA has strong tensile strength. The combination of the two is beneficial to inhibiting the expansion of the silicon negative electrode; the aqueous binder is carboxymethyl cellulose sodium (CMC). Further, the mass ratio of the carbon-silicon composite material: Super-P: SW-CNT: CMC: composite binder is 94-97: 0.5-2: 0.05-0.1: 1.0-2.0: 1.0-2.0.

[0011] Further solution: the positive electrode sheet includes a positive electrode current collector and a positive electrode material coated on the surface of the positive electrode current collector; the positive electrode current collector is carbon-coated aluminum foil, which can prevent lithium bis(fluorosulfonyl)imide from corroding the light foil; the positive electrode material includes lithium nickel cobalt manganese oxide, a positive electrode conductive agent, and a positive electrode binder, and the Ni content in the lithium nickel cobalt manganese oxide is greater than 88 wt%.

[0012] Further solution: the lithium-ion battery separator is a double-sided ceramic separator. This double-sided ceramic separator is formed by coating a ceramic layer on a base film using a high-temperature resistant PI adhesive (polyimide adhesive). Since the PI adhesive does not melt and shrink at high temperatures, it ensures that the separator has the characteristic of low thermal shrinkage rate. The shrinkage rate of this separator is less than 0.5%, which can prevent short circuits during high-temperature resistance tests and increase the safety of the battery.

[0013] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0014] In the electrolyte provided by the present invention, by using a fluorinated carbonate compound as the main component of the solvent, the cycling performance of the negative electrode is improved; by using a fluorophosphazene compound as an electrolyte additive, the thermal stability and safety performance of the electrolyte are improved; and further, by using substances such as N,N'-diisopropylcarbodiimide and N,N-diisopropylethylamine in combination, the high-temperature decomposition of the fluorinated carbonate compound is inhibited, and problems such as gas generation caused by the production of acid due to the high-temperature decomposition of the fluorinated carbonate are alleviated, thereby improving the safety performance of the battery.

[0015] In the lithium-ion battery provided by the present invention, N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine is used as a modifier in the anode material. On the one hand, this modifier can improve the binding ability of the anode material and enhance the cycling performance; on the other hand, its residual amount has the ability to remove acids, which can remove acidic substances or acidic gases generated by the high-temperature storage of fluorinated carbonate compounds in the electrolyte solvent. By designing the anode slurry formula and the electrolyte formula, and simultaneously using a double-sided ceramic separator, a new chemical system of a high-nickel and high-capacity silicon anode is designed to manufacture a high-energy-density battery. This battery has excellent cycling performance, the gas generation amount of the battery is reduced by 80%, the battery safety is improved, the heat-resistant temperature is increased to 160°C, and the comprehensive performance is excellent, having excellent market application prospects. Description of the Drawings

[0016] Figure 1 It is the test result of the cycling performance of the batteries prepared in Comparative Example 2 and Example 8. Detailed Embodiments

[0017] The following further illustrates the present invention in conjunction with embodiments, so that those skilled in the art can better understand the present invention and be able to implement it, but the examples cited do not limit the present invention.

[0018] In addition, in the preparation processes in the following embodiments, if not otherwise specified, they are all conventional means in the prior art in this field, so they will not be described in detail; the parts in the following embodiments all refer to parts by weight. The raw materials used in the present invention are all commercially available products and can be obtained through purchase.

[0019] A high-performance electrolyte, comprising a solvent, an electrolyte additive, and a lithium salt, wherein:

[0020] The solvent is composed of fluorinated carbonate compounds and linear carbonate compounds in a mass ratio of (3 - 7):(3 - 5); the fluorinated carbonate compounds include at least one of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), and 4-trifluoromethyl ethylene carbonate (TEPC); the linear carbonate compounds include at least one of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), and dimethyl carbonate (DMC).

[0021] The electrolyte additive includes fluorophosphazene compounds, and the fluorophosphazene compounds include at least one of ethoxy pentafluorocyclotriphosphazene (PEPN) and phenoxy pentafluorocyclotriphosphazene (FPPN), and the content of the fluorophosphazene compounds in the electrolyte is 5 - 10 wt%. Further preferably, the electrolyte additive further includes N,N'-diisopropylcarbodiimide (DIC) or N,N-diisopropylethylamine (DIPEA).

[0022] The lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium bis(fluorosulfonyl)imide (LiFSI); preferably, the lithium salt is composed of LiPF6 and LiFSI in a mass ratio of 3-8:2-7; the content of the lithium salt in the electrolyte is 5-14 wt%, specifically, it can be 8 wt%, 10 wt%, 14 wt%, etc.

[0023] A lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte as described above; the negative electrode sheet includes a negative electrode current collector and a negative electrode material coated on the surface of the negative electrode current collector, and 0.05-2 wt% of a modifier is added to the negative electrode material, and the modifier is N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine.

[0024] Preferably, the negative electrode material includes a carbon-silicon composite material, a negative electrode conductive agent, an aqueous binder, and a composite binder; the negative electrode conductive agent includes conductive carbon black (Super-P) and carbon nanotubes (SW-CNT); the composite binder includes styrene-butadiene rubber (SBR) and polyacrylic acid (PAA); the aqueous binder is carboxymethyl cellulose sodium (CMC). Further, the mass ratio of the carbon-silicon composite material:Super-P:SW-CNT:CMC:composite binder is 94-97:0.5-2:0.05-0.1:1.0-2.0:1.0-2.0.

[0025] Preferably, the positive electrode sheet includes a positive electrode current collector and a positive electrode material coated on the surface of the positive electrode current collector; the positive electrode current collector is carbon-coated aluminum foil; the positive electrode material includes lithium nickel cobalt manganese oxide (LiNCM), a positive electrode conductive agent, and a positive electrode binder, and the Ni content in the lithium nickel cobalt manganese oxide is greater than 88% in molar ratio;

[0026] Preferably, the separator is a double-sided ceramic separator.

[0027] Examples

[0028] Referring to the raw material dosages in Table 1 for experiments, the preparation method of the lithium-ion battery includes the following steps:

[0029] Preparation of the positive electrode sheet: The positive electrode material includes 97.0% LiNCM, 1.8% Super-P, and 1.2% PVDF; the preparation method is: Mix NMP (N,N-dimethylpyrrolidone) and PVDF and stir evenly to obtain a glue solution, then set the solid content to 73% according to the formula ratio, add LiNCM and Super-P to the glue solution and stir evenly to obtain a uniformly mixed slurry with a viscosity of 4000 mPa·s. Then coat the slurry evenly on both sides of a 12-μm-thick positive electrode current collector (carbon-coated aluminum foil), and after cold pressing, slitting, and cutting into pieces, a positive electrode sheet is prepared. Among them: The molecular formula of LiNCM is LiNi88 Co4Mn4)O2;

[0030] Preparation of the negative electrode sheet: The negative electrode material consists of a carbon-silicon composite (Si:GR = 15:85) material: Super-P: SW-CNT: CMC83: composite binder (SBR:PAA = 1.5:0.6) in a weight ratio of 96:0.85:0.05:1.0:2.1. The preparation method is as follows: Mix deionized water with CMC83 and stir evenly to obtain a glue solution. Then, according to the formula ratio, set the solid content to 51%. Add the silicon-carbon composite material, Super-P, and SWCNT to the glue solution and stir evenly. Then add the composite binder and stir for about 30 min. On this basis, add 0.3 wt% of a modifier (N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine) based on the total mass of the negative electrode material and stir for about 30 min to obtain a uniformly mixed slurry with a viscosity of 3000 mPa·s. Then coat the slurry on both sides of a 4.5-μm-thick negative electrode current collector (copper foil), and obtain the negative electrode sheet through cold pressing and slicing; and test that the peel strength increases by 20% compared to the electrode sheet coated with the negative electrode slurry without the additive. The carbon-silicon composite material is a silicon-carbon composite product (Si:GR = 15:85), and the negative electrode specific capacity is 500 mAh / g;

[0031] Wind the positive electrode plate, the negative electrode sheet, and a double-sided ceramic separator (the thickness of the ceramic layer in the double-sided ceramic separator is 3 μm), where the separator wraps the negative electrode by 2 mm and the negative electrode wraps the positive electrode by 2 mm; then hot press the wound battery cell, weld the electrode tabs in the blank areas of the positive and negative electrodes, and then encapsulate the welded battery cell with an aluminum-plastic film;

[0032] Bake the encapsulated battery cell to remove moisture, inject the electrolyte into the battery cell at 3 - 5 g / Ah according to the electrolyte components in Table 1, and finally perform formation, high-temperature storage, secondary encapsulation, and grading through the formation process parameters to prepare a lithium-ion battery.

[0033] Perform performance tests on the batteries prepared in each example and comparative example. The test results show that the positive electrode specific capacity of the battery prepared in the example is greater than 210 mAh / g, and the battery energy density is greater than 320 Wh / Kg. Test the gas generation performance, cycle performance, and heat resistance performance of the battery. The test results are shown in Table 1 and Figure 1 . Among them, the test method for the heat resistance performance is as follows: Use the step heating method to heat the battery from room temperature to 120 °C at a heating rate of 2 °C / min, keep it warm for 30 min, continue to heat to 130 °C, keep it warm for 30 min, and so on until the battery experiences thermal runaway. The experimental results show that the heat resistance temperature of the battery prepared in the example is increased by 20 °C, and the heat resistance temperature reaches 160 °C; 1C cycle > 1200 cyc, and the volume increase of the gas generated at 70 °C is reduced by nearly 80%.

[0034] Table 1 Relationship of raw material dosages in each comparative example and example

[0035]

[0036]

[0037] Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

Claims

1. A lithium-ion battery, characterized in that: It includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte; the negative electrode sheet includes a negative electrode current collector and a negative electrode material coated on the surface of the negative electrode current collector, and 0.05-2 wt% of a modifier is added to the negative electrode material, and the modifier is N,N-bis(tert-butyl)ethylenediamine or N,N-diethylethylenediamine; The negative electrode material includes a carbon-silicon composite material, a negative electrode conductive agent, an aqueous binder and a composite binder; the negative electrode conductive agent includes conductive carbon black and carbon nanotubes; the composite binder includes styrene-butadiene rubber and polyacrylic acid; the aqueous binder is sodium carboxymethyl cellulose; The electrolyte contains a lithium salt, a solvent and an electrolyte additive, and the solvent consists of a fluorinated carbonate compound and a linear carbonate compound in a mass ratio of (3-7); (3-5), and the electrolyte additive includes a fluorophosphazene compound.

2. The lithium ion battery according to claim 1, characterized in that: The electrolyte additive also includes N,N'-diisopropylcarbodiimide or N,N-diisopropylethylamine.

3. The lithium-ion battery according to claim 1, characterized in that: The lithium salt includes at least one of lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide; the content of the lithium salt in the electrolyte is 5-10 wt%.

4. The lithium ion battery according to claim 3, characterized in that: The mass ratio of the lithium hexafluorophosphate to the lithium bis(fluorosulfonyl)imide is (3-8):(2-7).

5. The lithium-ion battery according to any one of claims 1 to 4, characterized in that: The fluorinated carbonate compound includes at least one of fluorinated ethylene carbonate, difluoroethylene carbonate, 4-trifluoromethyl ethylene carbonate; the linear carbonate compound includes at least one of ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, dimethyl carbonate.

6. The lithium-ion battery according to any one of claims 1 to 4, characterized in that: The fluorophosphazene compound includes at least one of ethoxy pentafluorocyclotriphosphazene and phenoxy pentafluorocyclotriphosphazene; the content of the fluorophosphazene compound in the electrolyte is 5-10 wt%.

7. The lithium ion battery according to claim 1, characterized in that: The positive electrode sheet includes a positive electrode current collector and a positive electrode material coated on the surface of the positive electrode current collector; the positive electrode current collector is carbon-coated aluminum foil; the positive electrode material includes lithium nickel cobalt manganese oxide, a positive electrode conductive agent and a positive electrode binder.

8. The lithium-ion battery according to claim 1, characterized in that: The lithium ion battery separator is a double-sided ceramic separator.

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