Electrolyte and lithium-ion battery using same

Abstract

An electrolyte and a lithium-ion battery using same. The electrolyte comprises an aprotic solvent and an ester solvent. In the electrolyte, the mass content of the aprotic solvent is 10% or less, and the mass content of the ester solvent is 70% or more; and the aprotic solvent includes at least one of a nonpolar aprotic solvent and a polar aprotic solvent.

Description

An electrolyte and a lithium-ion battery using the same.

[0001] This application claims priority to Chinese Patent Application No. 202510368651.4, filed on March 26, 2025, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to the field of lithium-ion battery technology, specifically to electrolytes and lithium-ion batteries using the same. Background Technology

[0003] Due to their numerous advantages, ester solvents are commonly used organic solvents in lithium-ion batteries. For example, ester solvents have a high dielectric constant, effectively dissociating lithium salts and resulting in high ionic conductivity in the electrolyte. Ester solvents also exhibit good chemical stability under typical electrochemical conditions, rarely reacting with lithium salts or the positive and negative electrode materials of the battery, ensuring electrolyte stability and battery safety. Furthermore, ester solvents show good compatibility with the positive and negative electrode materials of lithium-ion batteries under typical electrochemical conditions, forming a stable interfacial film on the electrode surface, reducing side reactions between the electrode materials and the electrolyte, and improving battery cycle performance and lifespan. As can be seen from the above, ester solvents in the electrolyte play roles such as dissolving lithium salts, providing ion conduction channels, and regulating the electrolyte viscosity and electrochemical window. Therefore, in existing technologies, ester solvents constitute a significant proportion of the electrolyte by mass.

[0004] However, electrolytes with ester solvents as the main component are prone to parasitic reactions with the electrodes or corrosion of the current collector. Specifically, parasitic reactions between the electrolyte and electrodes refer to unintended and harmful chemical reactions that occur between the electrolyte and electrodes in electrochemical systems such as batteries, affecting battery performance and lifespan. Common parasitic reactions include decomposition reactions and dendrite growth. Decomposition reactions occur during the charging and discharging of lithium-ion batteries, where ester solvents may decompose on the electrode surface, altering the composition and properties of the electrolyte and thus affecting battery performance and lifespan. Dendrite growth occurs under low-temperature conditions or at high-rate charging and discharging, where the deposition rate of lithium ions on the electrode surface may accelerate, leading to the growth of lithium dendrites. These dendrites can puncture the separator, causing a short circuit and consuming lithium salts in the electrolyte, further impacting battery performance and lifespan. Invention Overview

[0005] Battery operating temperature, charge / discharge rate, and voltage range all affect parasitic reactions between the electrolyte and electrodes. For example, the degree of parasitic reactions may be exacerbated at high temperatures or high charge / discharge rates. Currently, common methods to improve parasitic reactions between the electrolyte and electrodes include introducing special additives or improving electrode materials. Currently, significant attention is being paid to introducing special additives to optimize the electrolyte composition; for example, adding an appropriate amount of fluoroethylene carbonate (FEC) can form a stable SEI film on the negative electrode surface, inhibiting solvent decomposition and lithium dendrite growth. However, at high temperatures, FEC is prone to gas generation and the production of HF.

[0006] This application provides an electrolyte comprising an aprotic solvent and an ester solvent, wherein the mass content of the aprotic solvent in the electrolyte is less than 10%, and the mass content of the ester solvent is more than 70%; wherein the aprotic solvent comprises at least one of an aprotic nonpolar solvent and an aprotic polar solvent.

[0007] This application also provides a lithium-ion battery comprising the electrolyte as described above. Beneficial effects

[0008] The electrolyte provided in this application embodiment is prepared by introducing a small amount of aprotic solvent, which can stabilize the solvation structure in the electrolyte. This not only significantly improves the stability of the electrolyte, but also inhibits and reduces parasitic reactions between the electrolyte and the electrode, reduces current collector corrosion, and improves the inherent cycle performance of the battery.

[0009] The battery provided in this application embodiment can improve the inherent cycle performance of the battery by using the electrolyte described above. Embodiments of the present invention

[0010] In a first aspect, embodiments of this application provide an electrolyte comprising an aprotic solvent and an ester solvent, wherein the mass content of the aprotic solvent in the electrolyte is less than 10%, and the mass content of the ester solvent is more than 70%; wherein the aprotic solvent comprises at least one of an aprotic nonpolar solvent and an aprotic polar solvent.

[0011] Aprotic solvents are solvents that cannot donate protons in the reaction system. These solvents exhibit extremely weak or no tendency for proton self-renewal. On one hand, aprotic solvents generally have poor electrical conductivity because they cannot conduct electricity through proton transfer like protic solvents. On the other hand, aprotic solvents have low dielectric constants. The dielectric constant is a physical quantity describing the polarization ability of a dielectric material in an electric field; a higher dielectric constant indicates a stronger polarization ability of the dielectric under the same electric field, and the capacitance will increase accordingly. Therefore, for these reasons, aprotic solvents are rarely introduced into electrolytes in this field.

[0012] However, the inventors unexpectedly discovered that by introducing a small amount of aprotic solvent to prepare the electrolyte, this application can stabilize the solvation structure in the electrolyte, which not only significantly improves the stability of the electrolyte, but also inhibits and reduces parasitic reactions between the electrolyte and the electrode, reduces current collector corrosion, and improves the inherent cycle performance of the battery.

[0013] In some embodiments, the mass content of the ester solvent is 70% to 80%.

[0014] In some embodiments, the ester solvent includes at least one of ethyl acetate (EA), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), and propylene carbonate (PC).

[0015] In some embodiments, the aprotic nonpolar solvent includes substituted or unsubstituted saturated alkanes.

[0016] In some embodiments, the substituted saturated alkane includes at least one of 1,2-dichloroethane, chloroform, carbon tetrachloride, and phosphorus pentachloride.

[0017] In some embodiments, the unsubstituted saturated alkane includes at least one of n-hexane, n-heptane, and n-octane.

[0018] In some embodiments, the aprotic polar solvent includes at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

[0019] In some embodiments, the ratio of aprotic nonpolar solvent to aprotic polar solvent is calculated by mass ratio as 0.5~1:1~1.5.

[0020] In some embodiments, the mass content of the aprotic solvent in the electrolyte is 5-10%.

[0021] In some embodiments, the ester solvents include ethyl acetate (EA), ethylene carbonate (EC), and ethyl methyl carbonate (EMC); calculated by mass ratio, ethyl acetate: ethylene carbonate: ethyl methyl carbonate = 4~5: 2~3: 0.5~0.8.

[0022] In some embodiments, the electrolyte further includes additives, including at least one of vinylene carbonate, trimethyl phosphate, fluoroethylene carbonate, and vinyl sulfite.

[0023] In some embodiments, the additives are prepared in a mass ratio of vinylene carbonate: trimethyl phosphate: fluoroethylene carbonate: vinyl sulfite = 2.5~4: 3.5~5: 1.5~3: 0.5~2. Combining these additives helps to form a stable solid electrolyte interphase (SEI) film, significantly reducing the first irreversible capacity loss of lithium-ion batteries and improving their rate performance and cycle life.

[0024] In some embodiments, the mass content of the additive in the electrolyte is 2-5%.

[0025] In some embodiments, the electrolyte further includes a lithium salt, wherein the mass content of the lithium salt in the electrolyte is 8-12%.

[0026] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and lithium tetrafluoroborate.

[0027] In some embodiments, the steps for preparing the electrolyte include: S1. Pretreating the ester solvent to reduce the water content in the ester solvent to below 5 ppm; S2. Sequentially adding an aprotic solvent, an additive, and a lithium salt to the ester solvent to obtain the electrolyte.

[0028] Secondly, this application also provides a lithium-ion battery comprising the electrolyte as described above.

[0029] The present application is described below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally performed according to the conditions recommended by the manufacturer.

[0030] Example 1

[0031] 1. Composition of the electrolyte

[0032] Prepare the raw materials required for the electrolyte provided in this embodiment according to the formula provided in Table 1. Among them, calculated by mass ratio, in the ester solvent, ethyl acetate: ethylene carbonate: methyl ethyl carbonate = 4.5: 2.5: 0.5; in the additive, vinylene carbonate: trimethyl phosphate: fluoroethylene carbonate: vinyl sulfite = 3: 4: 2: 1.

[0033] Table 1. Formulation of the electrolyte prepared in this embodiment

[0034]

[0035] 2. Preparation of electrolyte

[0036] S1. Pretreatment of ester solvents to reduce the water content in the ester solvents to below 5 ppm;

[0037] S2. In an environment with an oxygen content of less than 2 ppm and a moisture content of less than 5 ppm, an aprotic solvent and an additive are added sequentially to an ester solvent to obtain a mixed solution. The temperature of the mixed solution is controlled at 25°C. Lithium salt is added while stirring for 2 hours. Then, stirring is continued at 25°C for 4 hours to obtain an electrolyte.

[0038] 3. Preparation of lithium-ion batteries

[0039] Using lithium iron phosphate as the positive electrode and graphite as the negative electrode, positive and negative electrode sheets are obtained through coating and cold pressing processes. Then, a double-layer coated separator is used to obtain a core through winding or stacking technology. The core is placed in an outer packaging shell (such as an aluminum shell or soft pack), vacuum baked, and then injected with the electrolyte prepared above. After vacuum sealing, standing, formation, and capacity testing, a lithium-ion battery is obtained.

[0040] Example 2

[0041] This embodiment refers to the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using it. The difference from Example 1 is that the mass content of the aprotic solvent used in preparing the electrolyte in this embodiment is 2% (the content of the aprotic solvent is changed by increasing or decreasing the content of the ester solvent, and the mass ratio of each specific material in the ester solvent is consistent with that in Example 1). Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0042] Example 3

[0043] This embodiment refers to the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using it. The difference from Example 1 is that the mass content of the aprotic solvent used in preparing the electrolyte in this embodiment is 5% (the content of the aprotic solvent is changed by increasing or decreasing the content of the ester solvent, and the mass ratio of each specific material in the ester solvent is consistent with that in Example 1). Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0044] Example 4

[0045] This embodiment refers to the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using it. The difference from Example 1 is that the mass content of the aprotic solvent used in preparing the electrolyte in this embodiment is 10% (the content of the aprotic solvent is changed by increasing or decreasing the content of the ester solvent, and the mass ratio of each specific material in the ester solvent is consistent with that in Example 1). Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0046] Example 5

[0047] This embodiment refers to the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using the same electrolyte. The difference from Example 1 is that in this embodiment, dimethyl sulfoxide is used instead of n-hexane in equal parts by mass when preparing the electrolyte. Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using the same electrolyte in this embodiment are strictly consistent with those in Example 1.

[0048] Example 6

[0049] This embodiment refers to the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using the same formulation. The difference from Example 1 is that in this embodiment, chloroform is used instead of n-hexane in equal parts by mass when preparing the electrolyte. Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using the same formulation in this embodiment are strictly consistent with those in Example 1.

[0050] Example 7

[0051] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, the aprotic solvent used in preparing the electrolyte is n-hexane and dimethyl sulfoxide, wherein the mass percentage of n-hexane in the electrolyte is 4% and the mass percentage of dimethyl sulfoxide is 4%. Apart from the above differences, the operational steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0052] Example 8

[0053] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, the aprotic solvent used in preparing the electrolyte is chloroform and dimethyl sulfoxide, wherein the mass percentage of chloroform in the electrolyte is 4% and the mass percentage of dimethyl sulfoxide is 4%. Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0054] Example 9

[0055] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, when preparing the electrolyte, the mass ratio (upper limit) of ethyl acetate: ethylene carbonate: methyl ethyl carbonate in the ester solvent is 5:3:0.8. Apart from the above differences, the operational steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0056] Example 10

[0057] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, when preparing the electrolyte, the mass ratio (lower limit) in the ester solvent is ethyl acetate: ethylene carbonate: methyl ethyl carbonate = 4:2:0.5. Apart from the above differences, the operational steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0058] Example 11

[0059] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in preparing the electrolyte in this embodiment, the specific materials used in the ester solvent are ethyl acetate (EA), dimethyl carbonate (DMC), and propylene carbonate (PC) (the combination of ester solvents is changed to others to demonstrate the broad applicability of introducing aprotic solvents, not for a specific combination of ester solvents). The mass ratio of ethyl acetate:dimethyl carbonate:propylene carbonate is 4.5:2.5:0.5. Apart from the above differences, the operational steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0060] Example 12

[0061] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, the additives used in preparing the electrolyte are only vinylene carbonate and fluoroethylene carbonate, with a mass ratio of vinylene carbonate:fluoroethylene carbonate:=3:2. Apart from the above differences, the operational steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0062] Example 13

[0063] This embodiment refers to the formulation and method provided in Example 1 for preparing the electrolyte and the lithium-ion battery using it. The difference from Example 1 is that in this embodiment, in the preparation of the electrolyte, ethylene ethylene carbonate (VEC) is used instead of vinylene carbonate in equal parts by mass as an additive. Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using it in this embodiment are strictly consistent with those in Example 1.

[0064] Comparative Example 1

[0065] This comparative example prepares an electrolyte and a lithium-ion battery using the same formulation and method as provided in Example 1. The difference between this comparative example and Example 1 is that, in preparing the electrolyte, an ester solvent of equal mass parts is used instead of an aprotic solvent (i.e., no aprotic solvent is used in this comparative example). Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using the same in this comparative example are strictly consistent with those in Example 1.

[0066] Comparative Example 2

[0067] This comparative example uses the formulation and method provided in Example 1 to prepare an electrolyte and a lithium-ion battery using the same. The difference between this comparative example and Example 1 is that the mass content of the aprotic solvent used in preparing the electrolyte is 20% (the content of the aprotic solvent is changed by increasing or decreasing the content of the ester solvent, and the mass ratio of each specific material in the ester solvent is consistent with that in Example 1). Apart from the above differences, the operation steps for preparing the electrolyte and the lithium-ion battery using the same in this comparative example are strictly consistent with those in Example 1.

[0068] Test case

[0069] 1. Test Object

[0070] The electrolytes prepared in Examples 1-13 and Comparative Examples 1-2, and the lithium-ion batteries using them.

[0071] 2. Testing Methods

[0072] (1) Electrolyte moisture test: Take 1±0.5 g of electrolyte and place it in a glove box. Use a moisture tester to measure the electrolyte moisture.

[0073] (2) Electrolyte color test: Take 20±1g of electrolyte and place it in a cuvette. Use a colorimeter to measure the color of the electrolyte.

[0074] (3) Electrolyte acidity test: Prepare a 10% sodium hydroxide-ethanol solution and test the electrolyte acidity using a potentiometric titrator.

[0075] (4) Electrolyte conductivity test: Take 20±1g of electrolyte and place it in a test tube. Use a conductivity meter to test the electrolyte conductivity.

[0076] (5) Electrochemical performance of the battery: The test results of cycle performance are characterized by the state of health (SOH) after 300 cycles. Specifically, the finished battery cell is placed in an environment of (25±2)℃ and left to stand for 30 min. The battery is charged at 1C constant current and constant voltage to 3.65V, the cutoff current is 0.05C, and left to rest for 10 min. Then it is discharged at 1C constant current to the cutoff voltage of 2.5V. The highest discharge capacity of the first three cycles is recorded as the initial capacity Q1. When the cycle reaches 300, the discharge capacity of the battery in the last cycle is recorded as Q2, and the SOH of the battery is calculated. The formula used is as follows: SOH (%) = Q2 / Q1 * 100%.

[0077] 3. Test Results and Analysis

[0078] Excessive moisture not only leads to the decomposition of lithium salts and other components in the electrolyte, but also causes corrosion and damage to the positive and negative electrode materials and current collectors. According to industry standards, the moisture content in the electrolyte is generally below 20 ppm, with below 5 ppm indicating very low moisture content and superior performance in this regard. Furthermore, color indicates the clarity of the electrolyte, reflecting its purity and impurity content; a higher color value indicates a greater susceptibility to side reactions with the electrodes. Conversely, a higher acid value in the electrolyte makes it more prone to internal battery side reactions, affecting battery stability and cycle performance. An acid value above 20 ppm indicates a higher incidence of internal side reactions, potentially impacting battery safety.

[0079] The State of Health (SOH) of a battery describes its degree of aging or degradation and is an important parameter used in a Battery Management System (BMS) to evaluate battery performance. SOH is the percentage of a battery's current maximum capacity relative to its original capacity. As batteries are used and over time, a series of physical and chemical changes occur inside the battery, such as a reduction in active material and an increase in internal resistance. These changes lead to a gradual decrease in battery capacity and a gradual degradation in performance. Therefore, by measuring the battery's current maximum capacity and comparing it with its original capacity, the SOH value can be obtained, thereby assessing the battery's state of health. In this field, under the same test conditions, a 1% difference in SOH values ​​between different battery samples indicates a significant difference in their performance.

[0080] As can be seen from Examples 1-13 and Comparative Examples 1 and 2, although the conductivity of the present application decreases slightly after introducing aprotic solvents, the present application can suppress parasitic reactions between the electrolyte and the electrode by introducing aprotic solvents and controlling their content in the electrolyte, thereby significantly improving the health of the battery.

[0081] In Examples 1-4, it can be confirmed that as the mass content of aprotic solvent in the electrolyte increases, the water content, acid value, color, conductivity, etc. of the electrolyte fluctuate, and the health status of the battery using it also fluctuates. This indicates that the mass content of aprotic solvent in the electrolyte affects the solvation structure in the electrolyte, thereby affecting the stability of the electrolyte.

[0082] As can be seen from Examples 1 and 5-6, the use of different types of aprotic solvents also affects the performance of the electrolyte. By combining aprotic nonpolar solvents and aprotic polar solvents, the solvation structure in the electrolyte can be improved, the stability of the electrolyte can be enhanced, and the battery health of the battery using it can be improved.

[0083] Through Examples 1 and 9-11, the specific ratio and selection of materials in the ester solvent can be optimized to improve the solvation structure of the electrolyte. Furthermore, in conjunction with Examples 12-13, the materials used in the additives can be adjusted to improve the performance of the electrolyte.

[0084] Table 2. Test Results

[0085]

Claims

1. An electrolyte comprising an aprotic solvent and an ester solvent, wherein the mass content of the aprotic solvent in the electrolyte is less than 10%, and the mass content of the ester solvent is more than 70%; wherein The aprotic solvent includes at least one of aprotic nonpolar solvent and aprotic polar solvent.

2. The electrolyte of claim 1, wherein, The mass content of the ester solvent is 70%~80%.

3. The electrolyte as described in claim 1 or 2, wherein, The aprotic nonpolar solvent includes substituted or unsubstituted saturated alkanes.

4. The electrolyte of claim 3, wherein, The substituted saturated alkanes include at least one of 1,2-dichloroethane, chloroform, carbon tetrachloride, and phosphorus pentachloride.

5. The electrolyte of claim 3, wherein, The unsubstituted saturated alkanes include at least one of n-hexane, n-heptane, and n-octane.

6. The electrolyte of any one of claims 1-5, wherein, The aprotic polar solvent includes at least one of dimethyl sulfoxide, N,N-dimethylformamide, and N,N-dimethylacetamide.

7. The electrolyte of any one of claims 1-6, wherein, The mass ratio of the nonprotic nonpolar solvent to the nonprotic polar solvent is (0.5~1):(1~1.5).

8. The electrolyte of any one of claims 1 to 7, wherein, The mass content of the aprotic solvent in the electrolyte is 5% to 10%.

9. The electrolyte of any one of claims 1-8, wherein, The ester solvents include ethyl acetate, ethylene carbonate, and methyl ethyl carbonate.

10. The electrolyte of claim 9, wherein, According to the mass ratio, the ethyl acetate : ethylene carbonate : methyl ethyl carbonate = (4~5): (2~3): (0.5~0.8).

11. The electrolyte according to any one of claims 1-10, wherein the electrolyte further comprises additives, the additives comprising vinylene carbonate, trimethyl phosphate, fluoroethylene carbonate, and vinyl sulfite.

12. The electrolyte of claim 11, wherein, According to the mass ratio, the vinylene carbonate : trimethyl phosphate : fluoroethylene carbonate : vinyl sulfite = (2.5~4): (3.5~5): (1.5~3): (0.5~2).

13. The electrolyte as described in claim 11 or 12, wherein the additive has a mass content of 2% to 5% in the electrolyte.

14. The electrolyte according to any one of claims 1-13, wherein the electrolyte further comprises a lithium salt, and the lithium salt has a mass content of 8% to 12% in the electrolyte.

15. A lithium-ion battery, the lithium-ion battery comprising the electrolyte as described in any one of claims 1 to 14.

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