Electrolyte and lithium ion battery

Abstract

The application discloses an electrolyte and a lithium ion battery. The electrolyte comprises a mixed solvent of fluorinated carbonate and linear carbonate, a lithium salt, an additive and a stabilizer. The mass ratio of the fluorinated carbonate to the linear carbonate in the mixed solvent is 1:(1-5). The electrolyte has good thermal stability, and the high-temperature decomposition defect of the cyclic carbonate is improved. The lithium ion battery assembled by using the electrolyte has good high-temperature storage performance, and the gas production of the assembled lithium ion battery at high temperature is significantly reduced. The safety performance of the assembled lithium ion battery is greatly improved, and the heat resistance temperature is as high as 160 DEG C. In addition, the battery also has good cycle performance, and when the cycle is 600 times at a current density of 1C, the capacity retention rate can still reach more than 94%.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to an electrolyte and a lithium-ion battery. Background Technology

[0002] In response to increasingly severe environmental pollution and the energy crisis, the demand for green energy is growing. Lithium-ion batteries, with their long lifespan, high operating voltage and energy density, and relatively low environmental pollution, are widely used in various portable electronic devices. Using high-nickel ternary cathode materials paired with high-silicon graphite composite anode materials is one of the important methods to improve battery energy density. However, as the nickel content increases, the battery's safety and stability decrease significantly, and as the silicon content of the anode increases, the battery's cycle performance and high-temperature performance gradually decline.

[0003] One of the main obstacles limiting the development of high-nickel ternary cathode materials is the rapid oxidative decomposition of the electrolyte under high voltage and high temperature. To address this, researchers have conducted extensive studies and developed many novel electrolyte systems, such as high-concentration, sulfone-based, and nitrile-based electrolytes. Although these new electrolyte systems significantly improve the compatibility of the electrolyte with the cathode at high voltage, most systems cannot simultaneously meet the requirements of high-temperature storage and safety performance in lithium-ion batteries.

[0004] Therefore, there is an urgent need to develop an electrolyte that can improve the battery's cycle performance, high-temperature storage performance, and safety performance when used in high-voltage cathode materials. Summary of the Invention

[0005] This invention aims to solve at least one of the technical problems existing in the prior art. To this end, this invention proposes an electrolyte and a lithium-ion battery. The electrolyte of this invention comprises a mixed solvent of fluorinated carbonate and linear carbonate, a lithium salt, additives, and a stabilizer, which improves the disadvantage of carbonate solvents being easily decomposed at high temperatures and enhances the thermal stability of the electrolyte. Lithium-ion batteries assembled using this electrolyte exhibit better high-temperature storage performance and significantly reduced gas production. Furthermore, the electrolyte of this invention improves the safety and cycle performance of the assembled lithium-ion batteries.

[0006] In a first aspect, the present invention provides an electrolyte comprising the following components: a solvent, a lithium salt, an additive, and a stabilizer.

[0007] According to a first aspect of the invention, in some embodiments of the invention, the solvent includes fluorocarbonate and linear carbonate.

[0008] In some preferred embodiments of the present invention, the mass ratio of fluorocarbonate to linear carbonate in the solvent is 1:(1-5).

[0009] In some preferred embodiments of the present invention, the electrolyte contains 75-85% solvent by mass.

[0010] In some preferred embodiments of the present invention, the electrolyte contains 2 to 5% by mass of additives.

[0011] In some preferred embodiments of the present invention, the electrolyte contains 10-20% lithium salt by mass.

[0012] In some preferred embodiments of the present invention, the electrolyte contains 0.05 to 5% by mass of a stabilizer.

[0013] In some more preferred embodiments of the present invention, the electrolyte contains 78-82% solvent by mass.

[0014] In some more preferred embodiments of the present invention, the electrolyte contains 3 to 4% by mass of an additive.

[0015] In some more preferred embodiments of the present invention, the electrolyte contains 13-17% lithium salt by mass.

[0016] In some more preferred embodiments of the present invention, the electrolyte contains 0.1% to 3% by mass of a stabilizer.

[0017] In some preferred embodiments of the present invention, the fluorocarbonate includes one or more of fluoroethylene carbonate, fluoropropylene carbonate, difluoroethylene carbonate, 2,2-trifluoroethyl carbonate, and 4-trifluoromethylethylene carbonate.

[0018] In some preferred embodiments of the present invention, the linear carbonate includes one or more of methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

[0019] In some preferred embodiments of the present invention, the additive includes one or more of ethoxypentafluorocyclotriphosphazene, trifluoroethoxypentafluorocyclotriphosphazene, and phenoxypentafluorocyclotriphosphazene.

[0020] In some preferred embodiments of the present invention, the lithium salt includes one or more of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorooxalate borate, and lithium dioxalate borate.

[0021] In some preferred embodiments of the present invention, the stabilizer includes one or more of N,N'-diisopropylcarbodiimide, N,N-diisopropylethylamine, 4-imidazolium carboxaldehyde, and 1-methylimidazolium.

[0022] In this invention, the reaction between the perfluorinated carbonate solvent and the high-nickel interface in the cathode material is relatively low. The synergistic effect of the perfluorinated carbonate solvent and the linear carbonate solvent improves the oxidation resistance of the electrolyte, effectively reducing the interfacial impedance of batteries assembled using the electrolyte of this invention, thereby improving the rate performance and cycle performance of the battery. Fluorophosphazene additives have a certain flame-retardant effect and can form a protective film on the cathode surface during battery cycling, providing some protection to the cathode surface. Furthermore, the electrolyte of this invention includes stabilizers such as amines and imidazoles. The addition of stabilizers effectively prevents the decomposition of fluorinated carbonates during high-temperature storage, thereby improving the high-temperature resistance and limiting temperature of the battery cell.

[0023] In a second aspect, the present invention provides a method for preparing the electrolyte described in the first aspect of the present invention. The specific preparation method is as follows: the components are mixed according to the proportions described in the first aspect of the present invention to obtain the electrolyte.

[0024] In a third aspect, the present invention provides a lithium-ion battery, wherein the electrolyte used in the lithium-ion battery is the electrolyte described in the first aspect of the present invention.

[0025] According to a third aspect of the present invention, in some embodiments of the present invention, the lithium-ion battery may be square or cylindrical.

[0026] In some preferred embodiments of the present invention, the lithium-ion battery includes lithium cobalt oxide lithium-ion battery, lithium iron phosphate lithium-ion battery, lithium manganese oxide lithium-ion battery and nickel cobalt manganese oxide lithium-ion battery.

[0027] In a preferred embodiment of the present invention, the lithium-ion battery is a nickel-cobalt-manganese lithium oxide battery with a nickel molar fraction greater than 0.83.

[0028] In a preferred embodiment of the present invention, the negative electrode of the lithium-ion battery comprises silicon-carbon, natural graphite, artificial graphite, and lithium titanate.

[0029] The beneficial effects of this invention are: the electrolyte in this invention has good thermal stability, overcoming the disadvantage of cyclic carbonates being easily decomposed at high temperatures. Lithium-ion batteries assembled using the electrolyte of this invention exhibit good high-temperature storage performance, and the gas production of the assembled lithium-ion batteries at high temperatures is significantly reduced. The safety performance of the assembled lithium-ion batteries is greatly improved, with a heat resistance temperature up to 160℃. Furthermore, the battery also has good cycle performance; at a 1C current density, after 600 cycles, the capacity retention rate can still reach over 94%. Attached Figure Description

[0030] Figure 1Statistics on the high-temperature gas production of lithium-ion batteries in Embodiment 12 and Comparative Example 4 of the present invention;

[0031] Figure 2 Statistics on the capacity retention rate of lithium-ion batteries in Example 12 and Comparative Example 4 of this invention;

[0032] Figure 3 This is a statistical analysis of the high-temperature resistance performance of the lithium-ion batteries in Example 12 and Comparative Example 4 of the present invention. Detailed Implementation

[0033] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as a result. However, these embodiments are merely exemplary and do not constitute any limitation on the scope of the present invention. Those skilled in the art should understand that modifications or substitutions can be made to the details and form of the technical solutions of the present invention without departing from the spirit and scope of the present invention, but all such modifications and substitutions fall within the protection scope of the present invention.

[0034] Preparation of electrolyte

[0035] Example 1

[0036] 13.5g of lithium hexafluorophosphate and 5g of ethoxypentafluorocyclotriphosphazene were weighed and dissolved in a mixed solvent consisting of 26g of fluoroethylene carbonate and 60.5g of methyl ethyl carbonate to obtain the electrolyte in this example.

[0037] Example 2

[0038] 13.5g of lithium hexafluorophosphate and 5g of ethoxypentafluorocyclotriphosphazene were weighed and dissolved in a mixed solvent consisting of 26g of difluoroethylene carbonate and 60.5g of methyl ethyl carbonate to obtain the electrolyte in this example.

[0039] Example 3

[0040] 13.5g of lithium hexafluorophosphate and 5g of ethoxypentafluorocyclotriphosphazene were weighed and dissolved in a mixed solvent consisting of 26g of 4-trifluoromethyl ethylene carbonate and 60.5g of methyl ethyl carbonate to obtain the electrolyte in this example.

[0041] Example 4

[0042] 13.5g of lithium hexafluorophosphate, 5g of ethoxypentafluorocyclotriphosphazene and 0.1g of N,N'-diisopropylcarbodiimide were weighed and dissolved in a mixed solvent consisting of 26g of fluoroethylene carbonate and 60.4g of methyl ethyl carbonate to obtain the electrolyte in this example.

[0043] Example 5

[0044] The electrolyte in this example was prepared by weighing 13.5g of lithium hexafluorophosphate, 5g of ethoxypentafluorocyclotriphosphazene and 0.1g of N,N-diisopropylethylamine and dissolving them in a mixed solvent consisting of 26g of fluoroethylene carbonate and 60.4g of methyl ethyl carbonate.

[0045] Example 6

[0046] 10.5g of lithium hexafluorophosphate, 3g of lithium difluorosulfonylimide, 5g of ethoxypentafluorocyclotriphosphazene, 0.1g of N,N'-diisopropylcarbodiimide, and 0.05g of pentafluorocyclotriphosphazene were weighed and dissolved in a mixed solvent consisting of 26g of fluoroethylene carbonate, 50.35g of methyl ethyl carbonate, and 10g of diethyl carbonate to obtain the electrolyte in this example.

[0047] Comparative Example 1

[0048] Weigh 13.5g of lithium hexafluorophosphate and dissolve it in a mixed solvent consisting of 26g of ethylene carbonate and 60.5g of methyl ethyl carbonate to obtain the electrolyte in this comparative example.

[0049] Comparative Example 2

[0050] 13.5g of lithium hexafluorophosphate was weighed and dissolved in a mixed solvent consisting of 17.3g of ethylene carbonate, 60.5g of methyl ethyl carbonate and 8.7g of fluoroethylene carbonate to obtain the electrolyte in this comparative example.

[0051] Assembly of lithium-ion batteries

[0052] Example 7

[0053] (1) Preparation of the positive electrode sheet: The active material in the positive electrode sheet is LiNiCoMnO2 (NCM), the conductive agent is Super-P, and the binder is polyvinylidene fluoride (PVDF). The ratio of LiNiCoMnO2 (NCM):Super-P:PVDF is 98:1:1. According to the weight ratio, PVDF is dissolved in NMP (N,N-dimethylpyrrolidone) and stirred evenly to obtain a gel solution. Then, the active material NCM and the conductive agent are added to the gel solution according to the above ratio and stirred evenly to obtain a slurry for later use. The theoretical solid content of the obtained slurry is 75%, and the viscosity is about 3000 mPa·s. Then, the obtained slurry is uniformly coated on both sides of a 12 μm thick positive current collector (aluminum foil) with a certain width and thickness. After cold pressing, slitting, and cutting, the positive electrode sheet is obtained.

[0054] (2) Preparation of the negative electrode sheet: In the negative electrode sheet, the ratio of silicon-carbon composite material: Super-P: single-walled carbon nanotubes (SW-CNT): CMC (carboxymethyl cellulose sodium): styrene-butadiene rubber solution (SBR): polyacrylic acid (PAA) is 96.2:0.95:0.05:1:0.8:1. Among them, Super-P and SW-CNT act as conductive agents, and SBR and PAA are composite binders. According to the weight ratio, CMC is dissolved in deionized water and mixed and stirred to obtain a uniform adhesive solution. The silicon-carbon composite material, conductive agent (SP+SW-CNT) and binder (SBR+PAA) are added to the obtained adhesive solution and stirred to obtain a uniform slurry. The theoretical solid content of the slurry is 50%, and the viscosity is about 2000 mPa·s. Then, the slurry is coated on both sides of a 6 μm thick negative electrode current collector (copper foil) according to a certain width and thickness. After cold pressing, slitting, and cutting, the negative electrode sheet is obtained.

[0055] Among them, in the silicon-carbon composite material, SiC>10%Wt, and specific capacity>450mAh / g.

[0056] (3) Preparation of cylindrical lithium-ion battery: The positive and negative electrodes of the cylindrical lithium-ion battery are the electrodes prepared in steps (1) and (2), respectively, and the separator is a double-sided coated ceramic separator (the ceramic coating layer of the separator is >2um). The winding is carried out according to the conventional technical means in the art by setting the process parameters, wherein the width of the separator exceeds the width of the negative electrode by more than 2mm, and the width of the negative electrode exceeds the width of the positive electrode by more than 1mm; then, according to the conventional technical means in the art, the wound cell is hot-pressed and the tabs are welded to the blank areas of the positive and negative electrodes, and then packaged to obtain the cell. After the packaged cell is baked at 90°C to remove moisture, it is injected with electrolyte. The injected electrolyte is the electrolyte prepared in Example 1, and the injected amount of electrolyte is 3-5g / Ah, to obtain a cylindrical lithium-ion battery.

[0057] Example 8

[0058] The only difference between this embodiment and embodiment 7 is that the electrolyte in embodiment 2 is used to assemble the lithium-ion battery; the other steps are the same as in embodiment 7.

[0059] Example 9

[0060] The only difference between this embodiment and embodiment 7 is that the electrolyte in embodiment 3 is used to assemble the lithium-ion battery; the other steps are the same as in embodiment 7.

[0061] Example 10

[0062] The only difference between this embodiment and embodiment 7 is that the electrolyte in embodiment 4 is used to assemble the lithium-ion battery; the other steps are the same as in embodiment 7.

[0063] Example 11

[0064] The only difference between this embodiment and embodiment 7 is that the electrolyte in embodiment 5 is used to assemble the lithium-ion battery; the other steps are the same as in embodiment 7.

[0065] Example 12

[0066] The only difference between this embodiment and embodiment 7 is that the electrolyte in embodiment 6 is used to assemble the lithium-ion battery; the other steps are the same as in embodiment 7.

[0067] Comparative Example 3

[0068] The only difference between this comparative example and Example 7 is that the electrolyte in Comparative Example 1 is used to assemble the lithium-ion battery; the other steps are the same as in Example 7.

[0069] Comparative Example 4

[0070] The only difference between this comparative example and Example 7 is that the electrolyte in Comparative Example 2 is used to assemble the lithium-ion battery; the other steps are the same as in Example 7.

[0071] The high-temperature storage performance, cycle performance, and extreme temperature of the lithium-ion batteries assembled in Examples 7-12 and Comparative Examples 3-4 were tested respectively.

[0072] High-temperature storage performance test: The battery was charged to 4.2V at a constant current density of 1C and stored at 70°C for 14 days at 100% SOC.

[0073] Cyclic performance testing: Tested at room temperature, voltage window 2.8V~4.2V, constant current charging to 4.2V at 1C current density, then constant voltage charging to 0.05C, then 1C discharge to 2.8V, the cycle count is 600 cycles.

[0074] High temperature resistance test: The cell was heated to 130°C at a rate of 5°C / min and held for 30 min. Then the temperature was increased at a gradient of 10°C and held for 30 min until the cell thermally ran away.

[0075] Table 1 shows the high-temperature storage performance, cycle performance, and extreme temperature of the lithium-ion batteries in Examples 7-12 and Comparative Examples 3-4:

[0076] Table 1. High-temperature storage performance, cycle performance, and extreme temperature of the lithium-ion batteries in Examples 7-12 and Comparative Examples 3-4.

[0077]

[0078] As shown in Table 1, after 14 days of storage at 70°C, the residual fluorocarbonate content in the electrolyte of Examples 7-12 and Comparative Examples 3-4 remained above 45% in the batteries assembled in this invention. Conversely, the residual fluorocarbonate content in the electrolyte of Comparative Example 4 was below 40% after 14 days of storage. This is because the addition of the stabilizer protected the fluorocarbonate, thus preventing its decomposition under high-temperature storage. Furthermore, the capacity retention rate of the lithium-ion batteries assembled in Examples 7-12 after 600 cycles was higher than that of the lithium-ion batteries in Comparative Examples 3-4. Regarding extreme high temperatures, the heat resistance temperature of the lithium-ion batteries in Examples 7-12 was 160°C, while that of the lithium-ion batteries in the comparative examples was 140°C. In other words, using the electrolyte of Examples 7-12 can improve the cell's storage performance at high temperatures, increase the cell's extreme temperature, and also improve the battery's cycle performance and capacity retention rate.

[0079] Figure 1 This is a statistical analysis of the gas production during high-temperature storage of the lithium-ion batteries in Example 12 and Comparative Example 4. The battery storage conditions were: the batteries were stored at 70°C for 14 days at 100% SOC. Figure 1 As can be seen, after high-temperature storage, the gas production of the lithium-ion battery in Example 12 is much lower than that of the lithium-ion battery in Comparative Example 4. The gas production of the lithium-ion battery in Example 12 is about 20% of that of the lithium-ion battery in Comparative Example 4.

[0080] Figure 2 The capacity retention rate of the lithium-ion batteries in Example 12 and Comparative Example 4 is calculated from... Figure 2 As can be seen, as the number of cycles increases, the capacity retention rate of the lithium-ion battery in Comparative Example 4 decreases rapidly, while the capacity retention rate of the lithium-ion battery in Example 12 decreases more slowly. When the cycle count reaches 600, the capacity retention rate of Example 12 is 94.2%, while the capacity retention rate of Comparative Example 4 is 91.1%.

[0081] Figure 3 The results of the stepped heating test of the lithium-ion batteries in Example 12 and Comparative Example 4 are as follows: Figure 3 As can be seen, the lithium-ion battery in Comparative Example 4 exhibited a sharp increase in temperature at 140°C, while the lithium-ion battery in Example 12 remained stable up to 160°C, indicating that the battery prepared in Example 12 has better high-temperature resistance and a higher limiting temperature.

[0082] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An electrolyte, characterized in that, The electrolyte is composed of the following components by mass percentage: 1) Solvents with a content of 78-82%; 2) 3-4% additives; 3) Lithium salts of 13-17%; 4) 0.1%–3% stabilizer; The solvent is composed of fluorocarbonate and linear carbonate, wherein the mass ratio of fluorocarbonate to linear carbonate in the solvent is 1:((60.4 / 26)~5); The stabilizer includes one or more of N,N'-diisopropylcarbodiimide, N,N-diisopropylethylamine, 4-imidazolium carbaldehyde and 1-methylimidazolium; The additives include one or more of ethoxypentafluorocyclotriphosphazene, trifluoroethoxypentafluorocyclotriphosphazene, and phenoxypentafluorocyclotriphosphazene.

2. The electrolyte according to claim 1, characterized in that, The fluorocarbonate includes one or more of fluoroethylene carbonate, fluoropropylene carbonate, difluoroethylene carbonate, 2,2-trifluoroethyl carbonate, and 4-trifluoromethylethylene carbonate; the linear carbonate includes one or more of methyl ethyl carbonate, dimethyl carbonate, diethyl carbonate, and methyl propyl carbonate.

3. The electrolyte according to claim 1, characterized in that, The lithium salt includes one or more of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorooxalate borate, and lithium dioxalate borate.

4. A method for preparing the electrolyte according to any one of claims 1 to 3, characterized in that, Includes the following steps: The components are mixed to obtain the electrolyte.

5. A lithium-ion battery, characterized in that, The electrolyte used in the lithium-ion battery is the electrolyte according to any one of claims 1 to 3.

6. The lithium-ion battery according to claim 5, characterized in that, The lithium-ion batteries include lithium cobalt oxide lithium-ion batteries, lithium iron phosphate lithium-ion batteries, lithium manganese oxide lithium-ion batteries, and lithium nickel cobalt manganese oxide lithium-ion batteries.

Images