Non-aqueous electrolyte and lithium ion battery thereof

Abstract

This invention discloses a non-aqueous electrolyte and its lithium-ion battery. The non-aqueous electrolyte of this invention comprises a lithium salt, a non-aqueous organic solvent, and additives. The additives include at least one first additive having the structure shown in Formulas 1, 2, and 3, and a second additive having the structure shown in Formula 4. R1 and R2 are each independently selected from hydrogen, α, β, or α, * denotes a connecting end, or R1 and R2 are combined to form a cyclic structure; R3, R4, and R5 are each independently selected from C1-C6 alkyl, C2-C6 alkenyl, or alkoxyalkyl groups. The non-aqueous electrolyte of this invention includes a first additive and a second additive with special structures. Through their synergistic effect, the overall electrochemical performance of lithium manganese iron phosphate batteries can be effectively improved, especially their long cycle life and storage stability under high temperature and high voltage conditions. Formulas 1, 2, 3, and 4 are also mentioned.

Description

Technical Field

[0001] This invention relates to the field of new energy technology, and in particular to a non-aqueous electrolyte and its lithium-ion battery. Background Technology

[0002] As a crucial electrochemical energy storage device, the performance of the cathode material in lithium-ion batteries is paramount. Lithium manganese iron phosphate (LMP), a cathode material with an olivine structure, combines the excellent thermal stability and safety of lithium iron phosphate with the high operating voltage of lithium manganese phosphate (approximately 4.1V vs. Li / Li+). Its theoretical energy density is about 20% higher than that of lithium iron phosphate, making it considered one of the most promising next-generation high-safety cathode materials. However, the practical application of LMP still faces significant challenges: First, manganese is prone to disproportionation during electrochemical cycling, especially under high-temperature conditions, leading to the degradation of Mn... 2+ Dissolving in the electrolyte, the dissolved manganese ions migrate to the negative electrode, damaging the solid electrolyte interphase (SEI) film of the graphite negative electrode, catalyzing the continuous decomposition of the electrolyte, and consuming active lithium, ultimately leading to rapid capacity decay and impedance increase. Secondly, the material itself has low electronic conductivity and lithium-ion diffusion coefficient, limiting its rate performance. Thirdly, at high voltages (>4.2V), the material-electrolyte interface stability is poor; conventional carbonate electrolytes are prone to oxidative decomposition, leading to increased electrode interface impedance, gas generation, and deteriorated cycle performance.

[0003] To address the core issues of interfacial instability and manganese leaching, conventional lithium salt (such as LiPF6) and carbonate solvent systems are insufficient. Therefore, there is an urgent need for a non-aqueous electrolyte and its lithium-ion battery to overcome the shortcomings of existing technologies. Summary of the Invention

[0004] In view of the above problems, the purpose of this invention is to provide a non-aqueous electrolyte and its lithium-ion battery. The non-aqueous electrolyte includes a first additive and a second additive with a special structure. Through the synergistic effect of the two, the overall electrochemical performance of the lithium manganese iron phosphate battery can be effectively improved, especially its long cycle life and storage stability under high temperature and high voltage conditions.

[0005] To achieve the above objectives, the present invention provides a non-aqueous electrolyte comprising a lithium salt, a non-aqueous organic solvent, and an additive, wherein the additive includes at least one first additive having the structure shown in Formulas 1, 2, and 3.

[0006] Formula 1 Formula 2 Formula 3 And, a second additive having the structure shown in Formula 4,

[0007] Formula 4 Among them, R1 and R2 are each independently selected from hydrogen, , or * indicates a connecting end, or R1 and R2 are combined to form a ring structure; R3, R4 and R5 are each independently selected from C1~C6 alkyl, C2~C6 alkenyl or alkoxyalkyl groups.

[0008] Compared with existing technologies, the non-aqueous electrolyte of the present invention includes a first additive with the structure shown in Formulas 1, 2, and 3, and a second additive with the structure shown in Formula 4. The combination of the first and second additives can improve ion transport rate, form a stable SEI film, reduce battery internal resistance, and improve the high-temperature storage and cycling performance of lithium manganese iron phosphate batteries under high voltage. Specifically, the cyclic sulfonic anhydride structure contained in the first additive can construct a dense and stable interfacial protective film on the positive and negative electrode surfaces through preferential redox reactions. This film not only effectively inhibits the decomposition and loss of the electrolyte under high voltage, but also physically blocks the dissolution of manganese ions from the positive electrode and prevents their migration to the negative electrode to destroy the SEI film, thereby significantly improving the battery's cycle life and coulombic efficiency. However, its cyclic structure is prone to ring-opening polymerization or decomposition at high temperatures, leading to gas production, battery swelling, and safety risks. On the other hand, its byproducts can hinder Li + Transport degradation and deterioration of high-temperature cycling performance. However, the second additive can synergistically regulate the composition of the solid electrolyte membrane through the interaction of amide groups in the molecule with Li. + The strong coordination effect of the additive preferentially reduces / oxidizes at the electrode interface, forming an interface film rich in organic amide lithium salt with relatively low impedance. Simultaneously, the amide group (-CONH-) is a strong Lewis base, preferentially reacting with Lewis acids (PF5) generated from the decomposition of lithium salts (such as LiPF6) in the electrolyte, thereby significantly suppressing gas generation and improving the battery's thermal safety and cycle stability. Therefore, through the synergistic effect of the first and second additives, the overall electrochemical performance of lithium manganese iron phosphate batteries can be effectively improved, especially their long cycle life and storage stability under high temperature and high voltage.

[0009] As a preferred technical solution, R1 and R2 are each independently selected from hydrogen, , or * indicates a connecting end, or R1 and R2 combine to form an aromatic structure; R3 is selected from C1~C6 alkyl, C2~C6 alkenyl or alkoxyalkyl, and R4 and R5 are each independently selected from C1~C6 alkyl.

[0010] As a preferred technical solution, the first additive with the structure shown in Formula 1 of the present invention includes at least one of compound 1 to compound 4.

[0011] Compound 1 Compound 2

[0012] Compound 3 and Compound 4.

[0013] Specifically, the CAS number of compound 1 is 4074-55-9, the CAS number of compound 2 is 1431298-10-0, the CAS number of compound 3 is 2520352-90-1, the CAS number of compound 4 is 2520352-94-5, the CAS number of formula 2 is 4720-58-5, and the CAS number of formula 3 is 201419-80-9.

[0014] As a preferred embodiment, the second additive of the present invention comprises at least one of compounds 5 to 7.

[0015] Compound 5, Compound 6, Compound 7.

[0016] Specifically, the CAS number of compound 5 is 78191-00-1, the CAS number of compound 6 is 132289-57-7, and the CAS number of compound 7 is 193634-77-4.

[0017] As a preferred technical solution, based on the total mass of lithium salt, non-aqueous organic solvent, and additives being 100%, the mass percentage of the first additive is 0.05% to 3.00%. More preferably, the mass percentage of the first additive is 0.10% to 2.00%, and more preferably, the mass percentage of the first additive is 0.10% to 1.00%. Specifically, the mass percentage of the first additive may be, but is not limited to, 0.05%, 0.10%, 0.50%, 1.00%, 1.50%, 2.00%, 2.50%, and 3.00%.

[0018] As a preferred technical solution, based on the total mass of lithium salt, non-aqueous organic solvent, and additives being 100%, the mass percentage of the second additive is 0.05% to 3.00%. More preferably, the mass percentage of the second additive is 0.10% to 2.00%, and more preferably, the mass percentage of the second additive is 0.10% to 1.00%. Specifically, the mass percentage of the second additive may be, but is not limited to, 0.05%, 0.10%, 0.50%, 1.00%, 1.50%, 2.00%, 2.50%, and 3.00%.

[0019] As a preferred technical solution, the lithium salt of the present invention is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalate-borate)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorooxalate-borate (LiODFB), lithium difluorodioxalate-phosphate (LiDFOP), and lithium bis(fluorosulfonyl)imide (LiFSI).

[0020] Specifically, with the total mass of lithium salt, non-aqueous organic solvent, and additives being 100%, the mass percentage of lithium salt is 5% to 25%. Further, the mass percentage of lithium salt is 9% to 22%, and even further, it is 10% to 18%. As examples, the mass percentage of lithium salt may be, but is not limited to, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, and 25%.

[0021] As a preferred technical solution, the non-aqueous organic solvent of the present invention is selected from γ-butyrolactone, γ-valerolactone, δ-valerolactone, methyl acetate (MA), ethyl acetate (EA), ethyl propionate (EP), butyl acetate (n-Ba), propyl propionate (n-PP), butyl propionate, ethylene carbonate (EC), propylene carbonate, butyl carbonate (BC), amyl carbonate, vinylene carbonate (VC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EM). C) At least one of the following: methyl propyl carbonate, ethyl propyl carbonate, propylene carbonate (PC), 1,3-dioxolane (DOL), 1,4-dioxolane (DX), crown ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran (2-CH3-THF), 2-trifluoromethyltetrahydrofuran (2-CF3-THF), dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

[0022] Specifically, taking the total mass of lithium salt, non-aqueous organic solvent, and additives as 100%, the mass percentage of non-aqueous organic solvent is 65-90%, further, 75-89%, and even further, 78-88%. As examples, the mass percentage of non-aqueous organic solvent may be, but is not limited to, 65%, 67%, 69%, 70%, 72%, 74%, 75%, 77%, 79%, 80%, 81%, 84%, 85%, 88%, 89%, and 90%.

[0023] Another aspect of the present invention provides a lithium-ion battery, including a positive electrode active material, a negative electrode active material, and the aforementioned non-aqueous electrolyte.

[0024] As a preferred technical solution, the positive electrode active material of the present invention is selected from lithium manganese iron phosphate. Specifically, the chemical formula of lithium manganese iron phosphate is LiFe. 1-x Mn x PO4, 0.5 <x<1。

[0025] As a preferred technical solution, the negative electrode active material of the present invention is selected from at least one of artificial graphite, natural graphite, soft carbon, hard carbon, lithium titanate, silicon-carbon composite material, and silicon-oxygen composite material. As an example, the negative electrode active material may be artificial graphite, but is not limited thereto. Detailed Implementation

[0026] To better illustrate the purpose, technical solution, and beneficial effects of this invention, the following will further describe the invention in conjunction with specific embodiments. It should be noted that the methods described below are further explanations of the invention and should not be construed as limiting the invention. Where specific conditions are not specified in the embodiments, they can be performed according to conventional conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0027] Example 1 In an argon atmosphere and a vacuum glove box with a moisture content of <1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a weight ratio of EC:EMC=3:7 to obtain 86g of non-aqueous organic solvent. Then, 1.0g of Formula II and 0.5g of Compound 7 were added, dissolved, and stirred thoroughly. After that, 12.5g of lithium hexafluorophosphate (LiPF6) was added and mixed evenly to obtain a non-aqueous electrolyte.

[0028] (2) Preparation of positive electrode Lithium manganese iron phosphate (LiFe) 0.7 Mn 0.3PO4, PVDF binder, and SuperP conductive agent are mixed evenly at a mass ratio of 96:2.5:1.5 to prepare a lithium secondary battery positive electrode slurry of a certain viscosity. The mixed slurry is coated on both sides of aluminum foil, dried, and rolled to obtain the positive electrode sheet.

[0029] (3) Preparation of negative electrode Artificial graphite, binder PVDF and conductive agent SuperP are mixed evenly in a mass ratio of 90:2:8 to prepare a lithium secondary battery negative electrode slurry of a certain viscosity. The mixed slurry is coated on both sides of copper foil, dried and rolled to obtain the negative electrode sheet.

[0030] (4) Preparation of lithium-ion batteries The positive electrode, separator, and negative electrode are stacked in sequence, and then layered as needed. After the tabs are welded, they are placed in the aluminum-plastic film of the battery outer packaging. The prepared non-aqueous electrolyte is injected into the dried bare cell. Vacuum sealing, standing, formation (0.05C constant current charging to 3.9V, then 0.1C constant current charging to 4.3V), shaping, and capacity testing are performed sequentially to obtain a 1Ah soft-pack lithium secondary battery.

[0031] The non-aqueous electrolyte formulations of Examples 1-14 and Comparative Examples 1-5 are shown in Table 1. The steps for preparing the electrolyte and manufacturing the battery in Examples 2-14 and Comparative Examples 1-5 are the same as those in Example 1.

[0032] Table 1. Non-aqueous electrolyte formulations for Examples 1-14 and Comparative Examples 1-5

[0033] The lithium-ion batteries prepared in Examples 1-14 and Comparative Examples 1-5 were subjected to high-temperature storage tests and high-temperature cycling tests, respectively. The specific test conditions are as follows, and the performance test results are shown in Table 2.

[0034] (1) High-temperature storage performance test Under normal temperature (25℃) conditions, a lithium-ion battery was subjected to one 0.3C / 0.3C charge and discharge cycle (battery discharge capacity recorded as C0), with an upper limit voltage of 4.3V, and the initial battery thickness H1 was measured. The battery was then placed in a 60℃ oven for 30 days, removed, and placed in a 25℃ environment for a 0.3C discharge, with the discharge capacity recorded as C1. The lithium-ion battery was then subjected to another 0.3C / 0.3C charge and discharge cycle (battery discharge capacity recorded as C2), and the battery thickness H2 after storage was measured. The capacity retention rate, capacity recovery rate, and thickness expansion rate of the lithium-ion battery were calculated using the following formulas.

[0035] Capacity retention rate = C1 / C0 × 100% Capacity recovery rate = C2 / C0 × 100% Thickness expansion rate = (H2 - H1) / H1 × 100% (2) High-temperature cycling performance test The lithium-ion battery was placed in a 45°C constant temperature chamber and allowed to stand for 30 minutes to reach a constant temperature. It was then charged at a constant current of 1C until the voltage reached 4.3V, followed by constant voltage charging at 4.3V until the current reached 0.05C. Next, it was discharged at a constant current of 1C until the voltage reached 2.5V. The first discharge capacity was recorded as C0. This constitutes one charge-discharge cycle. Then, 1000 cycles of 1C / 1C charge and discharge were performed at 45°C, and the discharge capacity was recorded as C1. The capacity retention rate of the lithium-ion battery was calculated using the following formula.

[0036] Capacity retention rate = C1 / C0 × 100% (3) The rate of increase of internal resistance after 500 cycles at high temperature At 45℃, the capacity-balanced battery was charged at room temperature using a constant current and constant voltage of 1C until the cutoff voltage of 4.3V. It was then discharged using a constant current and constant voltage until the capacity decreased to 50% SOC. The battery's internal resistance was measured. Then, it was charged again using a constant current of 1C until 4.3V. This cycle was repeated 500 times. The battery's internal resistance after the first full charge and the last full charge were recorded. The internal resistance growth rate of the lithium-ion battery after 500 high-temperature cycles is calculated using the following formula.

[0037] The increase in internal resistance after 500 high-temperature cycles is calculated as follows: (Internal resistance of the battery after the last full charge - Internal resistance of the battery after the first full charge) / Internal resistance of the battery after the first full charge × 100%.

[0038] Table 2 Performance test results of lithium-ion batteries in Examples 1-14 and Comparative Examples 1-5

[0039] Based on the results of Examples 1-14 and Comparative Examples 2-5 in Table 2, it can be seen that the lithium-ion batteries of Examples 1-9 have better high-temperature cycling and high-temperature storage performance at a high voltage of 4.3V. This is because the synergistic effect of the first and second additives can improve the ion transport rate of the lithium manganese iron phosphate battery, form a stable SEI film, reduce the internal resistance of the battery, and improve the high-temperature storage and high-temperature cycling performance of the lithium-ion battery.

[0040] A comparison of Examples 1-6 shows that when the first additive is Compound 1, the overall performance is relatively poor, possibly because the conjugated structure of the benzene ring has a certain influence on the polymerization function of the sulfate ester.

[0041] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention. Although the present invention has been described in detail with reference to preferred embodiments, it is not limited to those listed in the embodiments. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the essence and scope of the technical solutions of the present invention.

Claims

1. A nonaqueous electrolyte comprising a lithium salt, a nonaqueous organic solvent, and an additive, characterized in that, The additive includes at least one first additive having the structure shown in Formula 1, Formula 2 and Formula 3. Formula 1 Formula 2 Formula 3 And, a second additive having the structure shown in Formula 4, Formula 4 wherein R1and R2are each independently selected from hydrogen, , or , * indicates a connecting end, or R1and R2combine with each other to form a ring structure; R3, R4and R5are each independently selected from C1-C6 alkyl, C2-C6 alkenyl or alkoxyalkyl.

2. The non-aqueous electrolyte according to claim 1, characterized in that, R1and R2are each independently selected from hydrogen, , or , * indicates the connecting end, or R1and R2combine with each other to form an aromatic structure; R3is selected from C1-C6 alkyl, C2-C6 alkenyl or alkoxyalkyl, R4and R5are each independently selected from C1-C6 alkyl.

3. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The first additive with the structure shown in Formula 1 includes at least one of compound 1 to compound 4. Compound 1 Compound 2 Compound 3 and Compound 4.

4. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The second additive includes at least one of compounds 5 to 7. Compound 5, Compound 6, Compound 7.

5. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, With the total mass of the lithium salt, the non-aqueous organic solvent, and the additive being 100%, the mass percentage of the first additive is 0.05~3.00%, and the mass percentage of the second additive is 0.05~3.00%.

6. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(oxalate-borate), lithium difluorophosphate, lithium difluorooxalate-borate, lithium difluorodioxalate-phosphate, and lithium bis(oxalate-imide).

7. The non-aqueous electrolyte according to claim 1 or 2, characterized in that, The non-aqueous organic solvent is selected from at least one of γ-butyrolactone, γ-valerolactone, δ-valerolactone, methyl acetate, ethyl acetate, ethyl propionate, butyl acetate, propyl propionate, butyl propionate, ethylene carbonate, propylene carbonate, butyl carbonate, pentylenetene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl n-propyl carbonate, ethyl n-propyl carbonate, propylene carbonate, 1,3-dioxolane, 1,4-dioxane, crown ether, tetrahydrofuran, 2-methyltetrahydrofuran, 2-trifluoromethyltetrahydrofuran, dimethoxymethane, diethoxymethane, ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

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

9. The lithium-ion battery according to claim 8, characterized in that, The positive active material is selected from lithium manganese iron phosphate, with the chemical formula LiFe 1-x Mn x PO4, 0.5 < x < 1.

10. The lithium-ion battery according to claim 8, characterized in that, The negative electrode active material is selected from at least one of artificial graphite, natural graphite, soft carbon, hard carbon, lithium titanate, silicon-carbon composite material, and silicon-oxygen composite material.

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