Electrolyte and lithium ion battery

Abstract

The invention provides an electrolyte and a lithium ion battery, and relates to the technical field of lithium ion batteries. The electrolyte comprises a lithium salt, an organic solvent and an additive, and the structural formula of the additive is shown as a formula I. And the additive is selected from one or more of 2-(trifluoromethoxy) phenyl isocyanate, 3-(trifluoromethoxy) phenyl isocyanate and 4-(trifluoromethoxy) phenyl isocyanate. The invention further discloses a preparation method of the polyurethane adhesive. The LUMO orbital energy of the electrolyte additive is low, a film can be preferentially formed at a negative electrode, the stability of an interfacial film is improved, meanwhile, the electrolyte additive is combined with constant acid and water in the electrolyte, the acid and the water are removed, and then the cycle stability of the battery is improved.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more particularly to an electrolyte and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries play a vital role due to their advantages such as high specific energy, long cycle life, and high specific power. Among them, lithium iron phosphate batteries have been widely used due to their advantages such as high safety and low cost. In order to further improve battery performance and ensure the stability of lithium iron phosphate lithium-ion batteries, the selection of a suitable electrolyte is crucial. Summary of the Invention

[0003] Based on the technical problems existing in the background art, the present invention proposes an electrolyte and a lithium-ion battery.

[0004] The present invention proposes an electrolyte comprising a lithium salt, an organic solvent, and an additive, wherein the additive has the structural formula shown in Formula I:

[0005]

[0006] Formula I.

[0007] The electrolyte additive proposed in this invention has a low LUMO orbital energy, which allows it to preferentially form a film on the negative electrode, improving the stability of the interfacial film. At the same time, it combines with a constant amount of acid and water in the electrolyte to remove acid and water, thereby improving the cycle stability of the battery.

[0008] Preferably, the additive is selected from one or more of 2-(trifluoromethoxy)phenyl isocyanate, 3-(trifluoromethoxy)phenyl isocyanate, and 4-(trifluoromethoxy)phenyl isocyanate.

[0009] The selection of additives can help them work synergistically with solvents to improve the overall performance of the electrolyte.

[0010] More preferably, the LUMO orbital energy of the additive is -2.1 to -1.8 eV.

[0011] The additive has a low LUMO orbital energy, which allows it to preferentially form a film on the negative electrode, thus improving the stability of the interfacial film.

[0012] Preferably, the amount of the additive is 0.1% to 1.5% of the total mass of the electrolyte.

[0013] Controlling the amount of additives helps to form a film on the negative electrode, improves the stability of the interfacial film, and combines with the constant amount of acid and water in the electrolyte to remove acid and water, thereby improving the cycle stability of the battery.

[0014] More preferably, the additives include 0.5% 2-(trifluoromethoxy)phenyl isocyanate and 0.5% 3-(trifluoromethoxy)phenyl isocyanate by weight of the total electrolyte.

[0015] Preferably, the lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium methanesulfonate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium difluorooxalate phosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

[0016] Lithium salts can form stable solvation structures with solvents, promoting ion conduction, reducing electrolyte viscosity, and increasing ion transport number; sulfonic acid lithium salts have high dissociation constants, high thermal stability, and are not sensitive to water.

[0017] Preferably, the amount of lithium salt added accounts for 10% to 15% of the total mass of the electrolyte.

[0018] Controlling the amount of lithium salt added helps to balance the number of ions and migration efficiency, as well as to form a stable interfacial film.

[0019] Preferably, the organic solvent is selected from at least one of methyl ethyl carbonate, propylene carbonate, ethylene carbonate, and diethyl carbonate.

[0020] Organic solvents interact with lithium salts through their high dielectric constant, breaking the lithium salt lattice and causing it to dissociate into lithium ions and anions in solution, forming a conductive ionic solution that provides an ion source for battery charging and discharging. Low-viscosity organic solvents reduce the migration resistance of lithium ions in the electrolyte, increasing ionic conductivity and ensuring efficient lithium ion transport between the positive and negative electrodes.

[0021] A lithium-ion battery includes a positive electrode, a negative electrode, a separator, and the electrolyte described above.

[0022] The electrolyte provided by this invention, when applied to lithium-ion batteries, helps to improve the cycle stability of the batteries.

[0023] Preferably, the positive electrode active material in the positive electrode sheet is lithium iron phosphate.

[0024] Preferably, the negative electrode active material in the negative electrode material is graphite.

[0025] The beneficial effects of this invention are as follows:

[0026] The electrolyte additive of this invention has a low LUMO orbital energy, which allows it to preferentially form a film on the negative electrode, improving the stability of the interfacial film. At the same time, it combines with a constant amount of acid and water in the electrolyte to remove acid and water, thereby improving the cycle stability of the battery. Attached Figure Description

[0027] Figure 1The distribution of LUMO orbitals between lithium-ion battery solvents and the electrolyte additives proposed in this invention. Detailed Implementation

[0028] The technical solution of the present invention will be described in detail through specific embodiments.

[0029] Unless otherwise specified, all materials and reagents used in the following examples and comparative examples are commercially available.

[0030] Example 1

[0031] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are uniformly mixed in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate and 0.5% 2-(trifluoromethoxy)phenyl isocyanate, accounting for 12% of the total mass of the electrolyte, are added to obtain a colorless and transparent liquid as the electrolyte.

[0032] Example 2

[0033] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are mixed uniformly in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate and 1% 2-(trifluoromethoxy)phenyl isocyanate, accounting for 1% of the total mass of the electrolyte, are added to finally obtain a colorless and transparent liquid as the electrolyte.

[0034] Example 3

[0035] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are mixed uniformly in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate and 2% 2-(trifluoromethoxy)phenyl isocyanate, accounting for 12% of the total mass of the electrolyte, are added to obtain a colorless and transparent liquid as the electrolyte.

[0036] Example 4

[0037] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are uniformly mixed in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate and 1% 3-(trifluoromethoxy)phenyl isocyanate, accounting for 1% of the total mass of the electrolyte, are added to finally obtain a colorless and transparent liquid as the electrolyte.

[0038] Example 5

[0039] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are mixed uniformly in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate and 1% 4-(trifluoromethoxy)phenyl isocyanate, accounting for 1% of the total mass of the electrolyte, are added to finally obtain a colorless and transparent liquid as the electrolyte.

[0040] Example 6

[0041] The preparation of an electrolyte includes the following steps: In a glove box under an argon atmosphere, methyl ethyl carbonate, ethylene carbonate, and propylene carbonate are uniformly mixed in a mass ratio of 60:30:10. While stirring, 12% lithium hexafluorophosphate, 0.5% 2-(trifluoromethoxy)phenyl isocyanate, and 0.5% 3-(trifluoromethoxy)phenyl isocyanate, accounting for 0.5% of the total mass of the electrolyte, are added to finally obtain a colorless and transparent liquid as the electrolyte.

[0042] Comparative Example 1

[0043] The only difference between Comparative Example 1 and Example 1 is that 2-(trifluoromethoxy)phenyl isocyanate is not added; the other steps are the same as in Example 1.

[0044] Comparative Example 2

[0045] The only difference between Comparative Example 2 and Example 2 is that “2-(trifluoromethoxy)phenyl isocyanate” is replaced with “phenyl isocyanate”, and the rest of the steps are the same as in Example 2.

[0046] The above electrolyte is assembled into a soft-pack battery, specifically as follows: 1) Preparation of positive electrode sheet: LiFePO4, conductive carbon black, and polyvinylidene fluoride are mixed in a weight ratio of 95:2.5:2.5 and dispersed evenly in 1-methyl-2-pyrrolidone to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of an aluminum foil, and after drying, rolling, and die-cutting, a positive electrode sheet is obtained; 2) Preparation of negative electrode sheet: Graphite, conductive carbon black, binder SBR, and carboxymethyl cellulose are mixed in a mass ratio of 86:6:4:4 and dispersed evenly in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on both sides of a copper foil, and after drying, rolling, and die-cutting, a negative electrode sheet is obtained; 3) The positive electrode sheet, separator, and negative electrode sheet are sequentially stacked, hot-pressed, formed, welded with tabs, encapsulated with aluminum-plastic film, and baked into a cell. The above electrolyte is then injected, and the battery is formed and tested for capacity to obtain a lithium-ion soft-pack battery.

[0047] The lithium-ion pouch batteries described above were subjected to cycle stability testing. Specifically, at a temperature of 25°C, the lithium-ion pouch batteries prepared in Examples 1-6 and Comparative Examples 1-2 were subjected to charge-discharge cycle testing at a 1C charge-discharge rate within a voltage range of 2-3.65V. The test results were recorded until the battery's discharge capacity decreased to 80% of its initial value. The cycle stability of the battery was evaluated based on the number of cycles at this point, denoted as N. 80 The test results are shown in Table 1.

[0048] Table 1

[0049] Group Electrolyte additive mass fraction <![CDATA[N 80 ]]> Example 1 0.5% 656 Example 2 1% 694 Example 3 2% 673 Example 4 1% 731 Example 5 1% 710 Example 6 1% 758 Comparative Example 1 0 482 Comparative Example 2 1% 546

[0050] Density functional theory simulations were performed on methyl ethyl carbonate, propylene carbonate, ethylene carbonate, 2-(trifluoromethoxy)phenyl isocyanate, 3-(trifluoromethoxy)phenyl isocyanate, and 4-(trifluoromethoxy)phenyl isocyanate. The LUMO orbital energies of the electrolyte additives were calculated using Materials Studio software. Relevant data are shown in Table 2. Figure 1 .

[0051] Table 2

[0052] Group methyl ethyl carbonate propylene carbonate Ethylene carbonate 2-(trifluoromethoxy)phenyl isocyanate 3-(trifluoromethoxy)phenyl isocyanate 4-(trifluoromethoxy)phenyl isocyanate LUMO orbital energy (eV) -0.798 -0.747 -0.808 -1.86 -2.06 -1.92

[0053] As can be seen from the data in Table 1, the lithium-ion battery assembled with the electrolyte of the present invention exhibits excellent cycle stability. Comparing the experimental results of Examples 1-3 with Comparative Example 1, it can be concluded that the electrolyte additives significantly improve the cycle stability of the lithium-ion pouch battery, with a mass fraction of 1% being optimal. 2-(trifluoromethoxy)phenyl isocyanate has a low LUMO orbital energy, allowing it to preferentially form a film on the negative electrode, improving the stability of the interfacial film. Simultaneously, it combines with the constant amounts of acid and water in the electrolyte, removing acid and water, thereby improving the battery's cycle stability. Comparing Example 4 with Examples 2 and 5, it can be seen that 3-(trifluoromethoxy)phenyl isocyanate has superior performance compared to 2-(trifluoromethoxy)phenyl isocyanate and 4-(trifluoromethoxy)phenyl isocyanate. 3-(trifluoromethoxy)phenyl isocyanate has the lowest LUMO orbital energy. Comparing Comparative Example 2 with Examples 2, 4, and 5, it can be seen that the compound in this invention contains trifluoromethoxy groups compared to the compound in Comparative Example 2. Trifluoromethoxy groups can participate in the formation of the SEI film at the negative electrode of the battery, improving the stability of the interfacial film and exhibiting superior cycle stability. Comparing Example 6 with Example 4, the mixed addition of 2-(trifluoromethoxy)phenyl isocyanate and 3-(trifluoromethoxy)phenyl isocyanate is superior to using 3-(trifluoromethoxy)phenyl isocyanate alone. The main reason is that the mixed use synergistically improves film formation performance, interfacial stability, or charge transport efficiency, resulting in a more uniform and stable film coverage, thereby improving the cycle life of the battery.

[0054] This invention demonstrates through experiments and theoretical calculations that electrolyte additives significantly improve the cycle stability of lithium-ion pouch batteries. The electrolyte additives provided by this invention have low LUMO orbital energy, allowing them to preferentially form a film at the negative electrode. The trifluoromethoxy group can participate in the formation of the SEI film at the battery negative electrode, improving the stability of the interfacial film. The isocyanate can react with a constant amount of acid and water in the electrolyte to remove acid and water, further enhancing the battery's cycle stability.

[0055] In summary, the electrolyte provided by this invention helps to improve the cycle stability of the battery.

[0056] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An electrolyte, characterized in that, It includes lithium salts, organic solvents, and additives, wherein the structural formula of the additives is shown in Formula I: Formula I.

2. The electrolyte according to claim 1, characterized in that, The additive is selected from one or more of 2-(trifluoromethoxy)phenyl isocyanate, 3-(trifluoromethoxy)phenyl isocyanate, and 4-(trifluoromethoxy)phenyl isocyanate.

3. The electrolyte according to claim 1 or 2, characterized in that, The additive has a LUMO orbital energy of -2.1 to -1.8 eV.

4. The electrolyte according to any one of claims 1 to 3, characterized in that, The amount of the additive added is 0.1% to 1.5% of the total mass of the electrolyte.

5. The electrolyte according to claim 1, characterized in that, The lithium salt is selected from one or more of lithium hexafluorophosphate, lithium perchlorate, lithium methanesulfonate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium difluorooxalate phosphate, lithium bis(oxalate)borate, and lithium difluorooxalateborate.

6. The electrolyte according to claim 1 or 5, characterized in that, The amount of lithium salt added accounts for 10% to 15% of the total mass of the electrolyte.

7. The electrolyte according to claim 1, characterized in that, The organic solvent is selected from at least one of methyl ethyl carbonate, propylene carbonate, ethylene carbonate, and diethyl carbonate.

8. A lithium-ion battery, characterized in that, It includes a positive electrode, a negative electrode, a separator, and the electrolyte as described in any one of claims 1 to 7.

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