Ester-based electrolyte for lithium battery with moisture-proof function, its preparation method and application

Abstract

The present invention discloses a lithium battery ester-based electrolyte with a moisture-proof function, which includes an electrolyte and a deep eutectic solvent; the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 1000); the raw materials of the deep eutectic solvent include urea and caprolactam. The present invention also provides a preparation method and an application of the above lithium battery ester-based electrolyte with a moisture-proof function. After being treated under humid conditions, the hydrolysis degree of the lithium salt in the lithium battery ester-based electrolyte of the present invention is <0.01%, and it has good moisture-proof characteristics and can be compatible with existing battery components. The initial Coulombic efficiency of the lithium metal battery containing the electrolyte treated under humid conditions is up to 99.1%, the capacity after 500 cycles exceeds 130.1 mAh / g, and the capacity retention rate after 500 cycles exceeds 87.8%.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a lithium battery ester-based electrolyte with moisture-proof function, its preparation method and application. Background Art

[0002] In the current era of rapid technological development, lithium batteries have become an important part of many mobile and portable devices due to their significant advantages such as high energy density and long cycle life, and are widely used in new energy vehicles and large-scale intelligent devices. However, in the manufacturing process of lithium batteries, a serious problem is faced. Due to the limitations of manufacturing processes, trace amounts of moisture will inevitably remain inside the battery. The presence of trace amounts of water in the battery will cause the hydrolysis of lithium salt LiPF6, triggering a series of side reactions, seriously affecting the battery performance and service life.

[0003] To address this problem, the industry has tried various methods to reduce the moisture content in the battery. Common measures include strictly controlling the humidity of the production environment and deeply drying the raw materials, etc. However, these methods require a large amount of professional equipment and manpower, resulting in a significant increase in production costs. Moreover, even with these strict humidity control measures, during actual production and use, moisture may still enter the battery interior, making it difficult to fundamentally prevent the occurrence of side reactions. Therefore, seeking a method that can reduce the intrinsic reactivity of water molecules and inhibit side reactions at the root has become the key to solving the above problems. Among them, constructing a stable electrolyte system has become the main method that is simple and easy to promote industrially at present.

[0004] Deep eutectic solvents, as a new type of solvent system, are formed by the hydrogen bond donor and hydrogen bond acceptor through hydrogen bond interaction, and have the advantages of high safety, environmental protection, simple preparation process and low cost. However, the application of deep eutectic solvents in the lithium battery system is currently restricted by many factors. In the process of realizing the present invention, the inventors found that there are at least the following problems in the prior art: for example, the compatibility of deep eutectic solvents with existing battery components (such as separators, binders) is poor, which makes it difficult for deep eutectic solvents to be effectively applied in lithium batteries. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a lithium battery ester-based electrolyte with moisture-proof function, its preparation method and application in view of the above-mentioned deficiencies of the prior art. After being treated under humid conditions, the hydrolysis degree of the lithium salt in this lithium battery ester-based electrolyte is <0.01%, and it has good moisture-proof characteristics. It can be compatible with existing battery components. The first-cycle Coulombic efficiency of the lithium metal battery containing the electrolyte treated under humid conditions is up to 99.1%, the capacity after 500 cycles exceeds 130.1 mAh / g, and the capacity retention rate after 500 cycles exceeds 87.8%.

[0006] The present invention has the following advantages compared with the prior art:

[0007] 1. The present invention provides a lithium battery ester electrolyte added with a deep eutectic solvent, which has good moisture-proof characteristics. Within 72 hours after adding 100,000 ppm of ultrapure water, the hydrolysis degree of lithium salt can still be maintained below 0.01%. Within 48 hours after adding less than 20,000 ppm of ultrapure water, the lithium metal battery with the electrolyte containing the added water can maintain a Coulombic efficiency of more than 97% in the first week, and the capacity retention rate after 500 cycles is > 80%, showing significantly higher performance than the lithium metal battery with traditional ester electrolytes.

[0008] 2. Preferably, the lithium battery ester electrolyte of the present invention can achieve moisture-proof by simply adding a deep eutectic solvent thereto. Using it as the electrolyte, a high-performance battery with good cycle stability can be obtained through conventional assembly methods.

[0009] 3. The present invention provides a lithium metal battery using the above-mentioned lithium battery ester electrolyte as the electrolyte, which can be stored for a long time in a high-humidity environment, with a self-discharge rate < 2% / month and high electrochemical stability.

[0010] 4. The lithium battery ester electrolyte of the present invention has low raw material costs, a simple and reasonable preparation process, and does not require high-precision and complex equipment, providing an effective solution to the problem of high water production cost of electrolytes in industry and having broad prospects for popularization and application.

[0011] The following further describes the technical solution of the present invention in detail with reference to the drawings and embodiments. Description of the Drawings

[0012] Figure 1 is a schematic diagram of the reaction mechanism of the deep eutectic solvent of the present invention for inhibiting the reaction of water with PF6 - ;

[0013] Figure 2 is the nuclear magnetic resonance phosphorus spectrum of the hydrolysis of lithium salt in the lithium salt ester electrolyte of Example 11;

[0014] Figure 3 is a schematic diagram of the performance comparison of the lithium metal batteries of Example 7 and Comparative Example 1;

[0015] Figure 4 is the TEM image of the lithium morphology after 1 cycle of the lithium metal battery of Example 7 with a deposition of 1 mA h cm -2 ;

[0016] Figure 5 is the TEM image of the lithium morphology after 1 cycle of the lithium metal battery of Comparative Example 1 with a deposition of 1 mA h cm -2 ;

[0017] Figure 6Schematic diagram of the morphology comparison after treating the ester-based electrolytes of Example 11 and Comparative Example 1 with 14000 ppm of water for 48 h. Detailed implementation manners

[0018] Next, the technical solutions will be clearly and completely described in combination with the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts belong to the scope of protection of the present application.

[0019] In the following description, the term "and / or" is used to describe the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, B exists alone, and the situation where A and B exist simultaneously. Wherein A and B can be singular or plural.

[0020] In the following description, terms such as "include", "comprise", "have" and "contain" are all open-ended terms, that is, they are meant to include but not limited to.

[0021] Those skilled in the art should understand that in the following description of the embodiments of the present application, the sequence numbers do not mean the order of execution, and some or all steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of the present application.

[0022] Those skilled in the art should understand that the numerical ranges in the embodiments of the present application should be understood as specifically disclosing each intermediate value between the upper and lower limits of the range. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present application. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0023] Unless otherwise specified, the technical / scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which this application belongs. Although this application only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of this application. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0024] Technical principle adopted by the present invention: A deep eutectic solvent prepared from urea and caprolactam is used as an additive for the ester-based electrolyte of a lithium battery. This deep eutectic solvent can form a stable cluster structure with water, inhibit the hydrolysis of LiPF6 caused by water, and improve the moisture-proof ability of the ester-based electrolyte of the lithium battery. On the one hand, the steric hindrance caused by the donor-donor interaction around water molecules prevents water molecules from contacting the electrolyte; on the other hand, the formed hydrogen bonds change the stability of the O-H covalent bond and the electronic structure of water molecules, reducing the intrinsic reactivity of water molecules and preventing them from contacting LiPF6, thus avoiding more side reactions. The schematic diagram of the principle for improving the moisture-proof ability of the electrolyte is as shown in Figure 1 shown; in addition, this deep eutectic solvent has good compatibility with the lithium battery system, and the assembled lithium metal battery exhibits high battery performance.

[0025] In some embodiments, an ester-based electrolyte for a lithium battery with a moisture-proof function is provided, including an electrolyte and a deep eutectic solvent; the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 1000); the raw materials of the deep eutectic solvent include urea and caprolactam.

[0026] The deep eutectic solvent composed of urea and caprolactam is used as an additive to optimize the solvation structure of the ester-based electrolyte of the lithium battery and achieve the improvement of the moisture-proof ability of the lithium metal ester-based electrolyte.

[0027] In some embodiments, the molar ratio of urea to caprolactam is 1:(1 - 4); further preferably, the molar ratio of urea to caprolactam is 1:(1 - 3); in some preferred embodiments, the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 100); further preferably, the mass ratio of the deep eutectic solvent to the electrolyte is 1:(20 - 100).

[0028] When the molar ratio of urea to caprolactam is 1:(1 - 4), the obtained deep eutectic solvent has a high dielectric constant, which can optimize the solvation structure of the ester-based electrolyte of the lithium battery and promote the dissociation of LiPF6 at the same time. By further limiting the mass ratio of the deep eutectic solvent to the electrolyte to 1:(10 - 100), the strong hydrogen bond interaction between the deep eutectic solvents combines with the water in the electrolyte, quenching the activity of water in the electrolyte and inhibiting the hydrolysis of LiPF6 caused by water. During the experiment, the inventor found that the ratio of the deep eutectic solvent to the electrolyte in the ester-based electrolyte of the lithium battery affects the electrolyte and battery performance. When the content of the deep eutectic solvent in the ester-based electrolyte of the lithium battery is too high, such as when the mass ratio of the deep eutectic solvent to the electrolyte is 1:5, the hydrolysis degree of lithium salt increases significantly, and the cycle life and capacity retention ability are greatly reduced. When the content of the deep eutectic solvent in the ester-based electrolyte of the lithium battery is too low, such as when the mass ratio of the deep eutectic solvent to the electrolyte is 1:2000, the hydrolysis of lithium salt is obvious, and the battery cannot operate and has no charge-discharge capacity.

[0029] In some embodiments, the preparation method of the deep eutectic solvent includes: mixing urea and caprolactam according to a preset molar ratio, and then heating them at a constant temperature of 60-80 °C for 12-24 h.

[0030] In some embodiments, the materials of the electrolyte include a lithium salt, an ester solvent, and an additive component; the lithium salt includes one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(oxalato)borate (LiBOB); the ester solvent is one or more of ethylene carbonate (EC), diethyl carbonate (DEC), and dimethyl carbonate (DMC); the additive component is fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC).

[0031] In some preferred embodiments, the electrolyte is formed by mixing a lithium salt, an ester solvent, and an additive component, wherein the lithium salt is lithium hexafluorophosphate (LiPF6), the ester solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1, and the additive component is fluoroethylene carbonate (FEC); in the electrolyte, the concentration of the lithium salt is 1 M, and the mass percentage of the additive component is 5%.

[0032] Preferably, the electrolyte is formed by mixing lithium hexafluorophosphate, ethylene carbonate and dimethyl carbonate with a volume ratio of 1:1, and fluoroethylene carbonate, which is beneficial to interact with the deep eutectic solvent to further realize the adjustment of the solvation structure and form a passivation layer on the surface of the negative electrode.

[0033] On the other hand, a preparation method of a lithium battery ester-based electrolyte with a moisture-proof function is provided, including: mixing the deep eutectic solvent and the electrolyte according to a preset mass ratio, and keeping them warm at a temperature of 30-50 °C for 3-6 h to obtain a lithium battery ester-based electrolyte with a moisture-proof function.

[0034] By limiting the mixing temperature to 30-50 °C and the heat preservation time to 3-6 h, the deep eutectic solvent can be fully dissolved in the electrolyte to obtain a uniform and stable lithium battery ester-based electrolyte.

[0035] On the other hand, an application of the above-mentioned lithium battery ester-based electrolyte with a moisture-proof function in a lithium battery is provided, including: using the lithium battery ester-based electrolyte with a moisture-proof function as the electrolyte and a lithium sheet as the negative electrode.

[0036] For a lithium metal battery using the above-mentioned lithium battery ester-based electrolyte with a moisture-proof function as the electrolyte, an inorganic layer SEI film rich in LiF is formed on the surface of the negative electrode, the electrode / electrolyte interface is more stable, the lithium metal battery has a longer lifespan and lower impedance, and the overall performance is improved.

[0037] Before the application of the present invention, a series of experiments were conducted. Now, some test results are listed to further describe the invention in detail, and the following will be described in detail in conjunction with the embodiments.

[0038] In the following embodiments, the order of preparing the deep eutectic solvent and the electrolyte is not limited; those skilled in the art can change the above order without exceeding the protection scope of the present invention; the present invention does not particularly limit the source of raw materials, and they can be obtained through commercial channels or laboratory synthesis, for example, obtained from the specific sources in the following embodiments.

[0039] Example 1

[0040] This example provides a lithium battery ester-based electrolyte with a moisture-proof function, including an electrolyte and a deep eutectic solvent, where the mass ratio of the deep eutectic solvent to the electrolyte is 1:1000, and the deep eutectic solvent is prepared from urea and caprolactam.

[0041] This example also provides a method for preparing the above-mentioned lithium battery ester-based electrolyte with a moisture-proof function, including:

[0042] Step 1. Provide a deep eutectic solvent, specifically including:

[0043] Step 101. Place urea particles and caprolactam particles in a mortar and mix and grind them for 30 min according to a molar ratio of 1:1 to obtain a dry and fine mixed powder; the particle size of the urea particles is 1.2 - 3.5 mm, and the particle size of the caprolactam is 2.5 - 4.5 mm; both urea and caprolactam are of analytical purity (AR) with a purity ≥ 99.7%;

[0044] Step 102. Put the mixed powder into a glass beaker and stir it at a constant temperature of 60 °C for 24 h to obtain a clear and transparent deep eutectic solvent;

[0045] Step 2. Provide an electrolyte, specifically including: Mix lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), and an ester solvent to obtain an electrolyte; the ester solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1; in the electrolyte, the concentration of lithium hexafluorophosphate is 1 M, and the mass percentage content of fluoroethylene carbonate is 5%;

[0046] Step 3. Mix the deep eutectic solvent and the electrolyte according to a mass ratio of 1:1000, put the mixed system into a clean aluminum bottle, seal it, and store it at 50 °C for 3 h to obtain a lithium battery ester-based electrolyte with a moisture-proof function.

[0047] This example also provides a lithium metal battery assembled with the above-mentioned lithium battery ester-based electrolyte with a moisture-proof function as the electrolyte.

[0048] This embodiment also provides an assembly method for the above lithium metal battery, including:

[0049] Step 1: Mix LiFePO4, polyvinylidene fluoride (PVDF), and conductive carbon black (SuperP) according to a mass ratio of 8:1:1 to obtain a mixed powder. Add N-methylpyrrolidone (NMP) to the mixed powder and stir it with a blender to prepare a uniform slurry. The molecular weight of the polyvinylidene fluoride is 700,000 and it is purchased from Zhengzhou Jinghong New Energy Technology Co., Ltd. The conductive carbon black is purchased from Zhengzhou Jinghong New Energy Technology Co., Ltd. The mass of N-methylpyrrolidone is 25 times the mass of the mixed powder. The aluminum current collector is carbon-coated aluminum foil.

[0050] Step 2: Coat the slurry on the aluminum current collector, dry it in a vacuum oven at 60°C for 12 h, take it out and press it with a tablet press at 12 MPa to obtain a sheet. Cut the sheet with a cutter into a diameter of 12 mm to obtain a positive electrode sheet.

[0051] Step 3: Use a lithium sheet with a diameter of 12 mm as the negative electrode sheet, a polypropylene film as the separator, and 100 mL of the above lithium battery ester-based electrolyte as the electrolyte. Assemble the battery in the order of the positive electrode case, the positive electrode sheet, the separator, the negative electrode sheet, the nickel foam, and the negative electrode case. The models of the positive electrode case and the negative electrode case are both 2032 and are purchased from Henan Pengxiang Yunda Co., Ltd. The polypropylene film is Celgard 2500 polypropylene film.

[0052] The performance parameters of the lithium battery ester-based electrolyte and the lithium metal battery in this embodiment are shown in Table 1.

[0053] Examples 2 to 20

[0054] Examples 2 to 16 respectively provide a lithium battery ester-based electrolyte and a preparation method thereof, in which the molar ratio of urea particles to caprolactam particles and the mass ratio of the deep eutectic solvent to the electrolyte are shown in Table 1 and Table 2, and the remaining preparation parameters are the same as those in Example 1.

[0055] Examples 2 to 16 also respectively provide a lithium metal battery and an assembly method thereof, which are the same as those in Example 1. The performance parameters of the lithium battery ester-based electrolyte and the lithium metal battery in Examples 2 to 16 are shown in Table 1 and Table 2.

[0056] Examples 17 to 20 respectively provide a lithium battery ester-based electrolyte and a preparation method thereof, in which the respective preparation parameters are shown in Table 3, and the remaining preparation parameters are the same as those in Example 1.

[0057] Examples 17 to 20 also respectively provide a lithium metal battery and an assembly method thereof, which are the same as those in Example 1. The performance of the lithium battery ester-based electrolyte and the lithium metal battery in Examples 17 to 20 is shown in Table 4.

[0058] Example 21

[0059] This example provides a lithium battery ester-based electrolyte with moisture-proof function, which includes an electrolyte and a deep eutectic solvent. The mass ratio of the deep eutectic solvent to the electrolyte is 1:1000, and the deep eutectic solvent is prepared from urea and caprolactam.

[0060] This example also provides a method for preparing the above-mentioned lithium battery ester-based electrolyte with moisture-proof function, including:

[0061] Step 1: Provide a deep eutectic solvent, specifically including:

[0062] Step 101: Place urea particles and caprolactam particles in a mortar and mix and grind them for 30 min according to a molar ratio of 1:1 to obtain a dry and fine mixed powder; the particle size of the urea particles is 1.2 - 3.5 mm, and the particle size of the caprolactam is 2.5 - 4.5 mm; both urea and caprolactam are of analytical purity (AR) with a purity ≥ 99.7%.

[0063] Step 102: Put the mixed powder into a glass beaker and stir it at a constant temperature of 60 °C for 24 h to obtain a clear and transparent deep eutectic solvent.

[0064] Step 2: Provide an electrolyte, specifically including: Mix lithium bis(fluorosulfonyl)imide, fluoroethylene carbonate, and an ester solvent to obtain an electrolyte; the ester solvent is ethylene carbonate; in the electrolyte, the concentration of lithium bis(fluorosulfonyl)imide is 1 M, and the mass percentage content of fluoroethylene carbonate is 5%.

[0065] Step 3: Mix the deep eutectic solvent and the electrolyte according to a mass ratio of 1:1000, put the mixed system into a clean aluminum bottle, seal it, and store it at a temperature of 50 °C for 3 h to obtain a lithium battery ester-based electrolyte with moisture-proof function.

[0066] After the lithium battery ester-based electrolyte of this example is placed in an environment with an air humidity of 35% for 48 h, the hydrolysis degree of the lithium salt < 0.01%.

[0067] Example 22

[0068] This example provides a lithium battery ester-based electrolyte with moisture-proof function, which includes an electrolyte and a deep eutectic solvent. The mass ratio of the deep eutectic solvent to the electrolyte is 1:100, and the deep eutectic solvent is prepared from urea and caprolactam.

[0069] This example also provides a method for preparing the above-mentioned lithium battery ester-based electrolyte with moisture-proof function, including:

[0070] Step 1: Provide a deep eutectic solvent, specifically including:

[0071] Step 101: Place urea particles and caprolactam particles in a mortar and mix and grind them for 30 min according to a molar ratio of 1:1 to obtain a dry and fine mixed powder; the particle size of the urea particles is 1.2 - 3.5 mm, and the particle size of the caprolactam is 2.5 - 4.5 mm; both urea and caprolactam are of analytical purity (AR) with a purity ≥ 99.7%.

[0072] Step 102: Put the mixed powder into a glass beaker and stir it at a constant temperature of 60 °C for 24 h to obtain a clear and transparent deep eutectic solvent.

[0073] Step Two: Provide an electrolyte, specifically including: Mix lithium bis(trifluoromethanesulfonyl)imide, vinylene carbonate, and an ester solvent to obtain an electrolyte; the ester solvent is diethyl carbonate; in the electrolyte, the concentration of lithium bis(trifluoromethanesulfonyl)imide is 1 M, and the mass percentage content of vinylene carbonate is 5%.

[0074] Step Three: Mix the deep eutectic solvent and the electrolyte according to a mass ratio of 1:100, put the mixed system into a clean aluminum bottle, seal it, and store it at 50 °C for 3 h to obtain a lithium battery ester electrolyte with moisture-proof function.

[0075] After the lithium battery ester electrolyte of this example is placed in an environment with an air humidity of 35% for 48 h, the hydrolysis degree of the lithium salt < 0.01%.

[0076] Example 23

[0077] This example provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, where the mass ratio of the deep eutectic solvent to the electrolyte is 1:1000, and the deep eutectic solvent is prepared from urea and caprolactam.

[0078] This example also provides a method for preparing the above-mentioned lithium battery ester electrolyte with moisture-proof function, including:

[0079] Step One: Provide a deep eutectic solvent, specifically including:

[0080] Step 101: Place urea particles and caprolactam particles in a mortar and mix and grind them for 30 min according to a molar ratio of 1:2 to obtain a dry and fine mixed powder; the particle size of the urea particles is 1.2 - 3.5 mm, and the particle size of the caprolactam is 2.5 - 4.5 mm; both urea and caprolactam are of analytical purity (AR) with a purity ≥ 99.7%.

[0081] Step 102: Put the mixed powder into a glass beaker and stir it at a constant temperature of 60 °C for 24 h to obtain a clear and transparent deep eutectic solvent.

[0082] Step 2: Provide an electrolyte solution, specifically including: mixing lithium bis(oxalato)borate, an additive component, and an ester solvent to obtain the electrolyte solution; the ester solvent is dimethyl carbonate; in the electrolyte solution, the concentration of lithium bis(oxalato)borate is 1 M, the mass percentage of the additive component is 5%, and the additive component is fluoroethylene carbonate and vinylene carbonate with a mass ratio of 1:2;

[0083] Step 3: Mix the deep eutectic solvent and the electrolyte solution according to a mass ratio of 1:1000, put the mixed system into a clean aluminum bottle, seal it, and store it at 50 °C for 3 h to obtain a lithium battery ester-based electrolyte solution with moisture-proof function.

[0084] After the lithium battery ester-based electrolyte solution of this example is placed in an environment with an air humidity of 35% for 48 h, the hydrolysis degree of lithium salt < 0.01%.

[0085] Example 24

[0086] This example provides a lithium battery ester-based electrolyte solution with moisture-proof function, including an electrolyte solution and a deep eutectic solvent, where the mass ratio of the deep eutectic solvent to the electrolyte solution is 1:1000, and the deep eutectic solvent is prepared from urea and caprolactam.

[0087] This example also provides a method for preparing the above-mentioned lithium battery ester-based electrolyte solution with moisture-proof function, including:

[0088] Step 1: Provide a deep eutectic solvent, specifically including:

[0089] Step 101: Mix and grind urea particles and caprolactam particles in a mortar according to a molar ratio of 1:3 for 30 min to obtain a dry and fine mixed powder; the particle size of urea particles is 1.2 - 3.5 mm, and the particle size of caprolactam is 2.5 - 4.5 mm; both urea and caprolactam are analytical pure (AR) with a purity ≥ 99.7%;

[0090] Step 102: Put the mixed powder into a glass beaker and stir it at a constant temperature of 60 °C for 24 h to obtain a clear and transparent deep eutectic solvent;

[0091] Step 2: Provide an electrolyte solution, specifically including: mixing a lithium salt, an additive component, and an ester solvent to obtain the electrolyte solution; the ester solvent is dimethyl carbonate; in the electrolyte solution, the concentration of the lithium salt is 1 M, the lithium salt is lithium bis(oxalato)borate and lithium bis(fluorosulfonyl)imide with a mass ratio of 1:1, the mass percentage of the additive component is 5%, and the additive component is fluoroethylene carbonate and vinylene carbonate with a mass ratio of 1:2;

[0092] Step 3: Mix the deep eutectic solvent and the electrolyte in a mass ratio of 1:1000. Load the mixed system into a clean aluminum bottle, seal it, and store it at 50 °C for 3 h to obtain a lithium battery ester-based electrolyte with moisture-proof function.

[0093] After the lithium battery ester-based electrolyte of this example is placed in an air environment with a humidity of 35% for 48 h, the hydrolysis degree of lithium salt is <0.01%.

[0094] Comparative Example 1

[0095] This comparative example provides an ester-based electrolyte without adding a deep eutectic solvent. The preparation method includes:

[0096] Step 1: Provide an electrolyte, specifically including: Mix lithium hexafluorophosphate (LiPF6), fluoroethylene carbonate (FEC), and an ester-based solvent to obtain an electrolyte; the ester-based solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) with a volume ratio of 1:1; in the electrolyte, the concentration of lithium hexafluorophosphate is 1 M, and the mass percentage content of fluoroethylene carbonate is 5%;

[0097] Step 2: Load the electrolyte into a clean aluminum bottle and store it at 50 °C for 3 h to obtain an ester-based electrolyte.

[0098] Comparative Examples 2 - 15

[0099] Comparative Examples 2 - 15 examine the effects of changing the molar ratio of urea and caprolactam, and / or the mass ratio of the deep eutectic solvent to the electrolyte on the performance of the ester-based electrolyte and lithium metal battery. The specific ratios and performance data are shown in Tables 1 and 2, and the remaining preparation parameters and assembly parameters are the same as those in Example 1.

[0100] Performance Test

[0101] 1. Moisture-proof ability test:

[0102] Place the aluminum bottle containing the lithium battery ester-based electrolyte in the air environment and open it for a preset time, or add ultrapure water to the aluminum bottle containing the lithium battery ester-based electrolyte and then open it for a preset time to obtain a hydrolyzed solution after being treated under humid conditions. Take a sample of the hydrolyzed solution after being treated under humid conditions and measure the hydrolysis degree of its lithium salt. The method for determining the hydrolysis degree is nuclear magnetic resonance phosphorus spectroscopy ( 31 P-NMR). Detect the electrolyte by nuclear magnetic resonance phosphorus spectroscopy, calculate the relative content of different phosphorus species by peak area integration, and determine the hydrolysis degree of lithium salt; the preset time is 24 - 72 h, and the environmental humidity in the air environment is 30% - 60%.

[0103] 2. Battery performance test:

[0104] The performance of the lithium metal battery was measured by a Neware battery tester. The electrolyte in the lithium metal battery was the hydrolyzed solution after being treated under the above-mentioned humid conditions; the current density during the test was 1C.

[0105] 3. Storage stability test:

[0106] The lithium metal battery was placed in an environment with a temperature of 25°C and an air humidity of 65% to test its monthly self-discharge rate; the calculation formula for the self-discharge rate η is η = (C1 - C2) / C1 × 100%, where C1 is the initial capacity and C2 is the remaining capacity after 30 days of storage.

[0107] Performance evaluation

[0108] In Table 1 and Table 2, the electrolyte in the electrolyte characteristics is the electrolyte after being treated under humid conditions, which is the electrolyte after being treated under humid conditions obtained by placing an aluminum bottle containing a lithium battery ester electrolyte in an air environment and opening it for a preset time. The preset time is 48h and the air environment humidity is 35%; in the battery performance, the lithium metal battery is the lithium metal battery assembled with the corresponding electrolyte after being treated under humid conditions as the electrolyte.

[0109] It can be seen from Table 1 and Table 2 that for the ester-based electrolyte without adding a deep eutectic solvent in Comparative Example 1, the hydrolysis degree of lithium salt after being treated under humid conditions > 0.01%, the color is light yellow, the pH is 1, the battery cannot operate, and there is no charge-discharge capacity. For Example 1, the hydrolysis degree of lithium salt in the lithium battery ester-based electrolyte after being treated under humid conditions < 0.01%, the color is transparent, the pH is 5, the Coulomb efficiency in the first week is 91.5%, the battery capacity after 500 cycles is 98.5 mAh / g, and the capacity retention rate after 500 cycles is 65.6%. By comparison, introducing a deep eutectic solvent into the ester-based electrolyte can effectively inhibit the hydrolysis of lithium salt and improve the battery performance.

[0110] Table 1 Preparation parameters and performance parameters of Examples 1 - 8 and Comparative Examples 1 - 9

[0111]

[0112] Table 2 Preparation parameters and performance parameters of Examples 9 - 16 and Comparative Examples 10 - 15

[0113]

[0114] As can be seen from Table 1 and Table 2, when the mass ratio of the deep eutectic solvent to the electrolyte is different, the characteristics of the electrolyte and the battery performance are different after treatment under humid conditions. When the mass ratio of the deep eutectic solvent to the electrolyte is 1:2000, as in Comparative Examples 2 to 6, the hydrolysis degree of the lithium salt in the electrolyte after treatment under humid conditions is greater than 0.01%, and the lithium metal batteries cannot operate and have no charge-discharge capacity. When the molar ratio of urea to caprolactam in the deep eutectic solvent is 2:1 or 1:5, the hydrolysis degree of the lithium salt in the electrolyte after treatment under humid conditions is greater than 0.01%, and the lithium metal batteries cannot operate and have no charge-discharge capacity, indicating that when the addition amount of the deep eutectic solvent is too low, or the molar ratio of urea to caprolactam is too high or too low, it is impossible to reduce the hydrolysis degree of the lithium salt, the pH of the electrolyte is low, the battery performance is poor, and even the operation is difficult.

[0115] Based on Examples 1 to 16, when the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 1000), and the molar ratio of urea to caprolactam is 1:(1 - 4), under the same test conditions, the hydrolysis degree of the lithium salt <0.01%, and the Coulombic efficiency in the first week >87%, indicating that both the moisture-proof ability of the electrolyte and the battery performance are improved. When the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 1000), and the molar ratio of urea to caprolactam is 1:(1 - 3), under the same test conditions, the Coulombic efficiency in the first week >91%, the capacity after 500 cycles exceeds 98%, and the capacity retention rate after 500 cycles exceeds 65%. Among them, the highest performance is when the mass ratio of the deep eutectic solvent to the electrolyte is 1:20, and the molar ratio of urea to caprolactam is 1:3. The highest performance is: the Coulombic efficiency in the first week is 99.1%, the capacity after 500 cycles exceeds 130.1 mAh / g, and the capacity retention rate after 500 cycles exceeds 87.8%. The nuclear magnetic resonance phosphorus spectrum of the hydrolysis degree of the lithium salt in the electrolyte after the humid treatment in Example 11 is as Figure 2 shown. According to Figure 2 it can be seen that there is only the signal peak of LiPF6 in the figure, and the peak area hardly changes after integration, indicating that the lithium salt ester electrolyte in Example 11 hardly undergoes a hydrolysis reaction.

[0116] Based on Comparative Examples 8 - 9 and Examples 5 - 8, when the mass ratio of the deep eutectic solvent to the electrolyte is determined and the molar ratio of urea to caprolactam is different, the performance of the lithium metal battery is different. It shows that as the molar ratio increases, the battery performance first increases and then decreases. When the molar ratio is 1:3, the first - week Coulombic efficiency is raised to the maximum value of 98.9%, the capacity after 500 cycles is increased to 128.7 mAh / g, and the capacity retention rate after 500 cycles is 86.4%. When the molar ratio increases to 1:4, the battery performance slightly decreases, but still remains that the first - week Coulombic efficiency exceeds 94%, the capacity after 500 cycles exceeds 109 mAh / g, and the capacity retention rate after 500 cycles exceeds 72%. However, when the molar ratio continues to increase to 1:5, the battery performance rapidly decreases. At a 1C rate, the battery cannot operate and has no charge - discharge capacity, indicating that the composition of the deep eutectic solvent affects the performance of the lithium metal battery. When the caprolactam content in the deep eutectic solvent increases within a certain range, it is more conducive to improving the battery performance of the lithium metal battery.

[0117] Figure 3 Figure showing the cycling performance comparison between the lithium metal battery of Comparative Example 1 and the lithium metal battery of Example 7. Among them, the electrolytes in the lithium metal batteries are all the electrolytes treated under the humid conditions in Table 1; according to Figure 3 It can be seen that the lithium metal battery of Comparative Example 1 cannot operate at a 1C rate and has no charge - discharge capacity, while the lithium metal battery of Example 7 still maintains high capacity and high Coulombic efficiency after 500 cycles.

[0118] Figure 4 TEM image of the lithium metal battery of Example 7 after depositing 1 mA h cm -2 lithium after 1 cycle, Figure 5 TEM image of the lithium metal battery of Comparative Example 1 after depositing 1 mA h cm -2 lithium after 1 cycle. Among them, the electrolytes in the lithium metal batteries are all the electrolytes treated under the humid conditions in Table 1. Comparing Figure 4 and Figure 5 it can be seen that Figure 5 obvious crystallization appears, which is lithium dendrites, indicating that the lithium deposition mechanism inside the lithium metal battery of Comparative Example 1 is abnormal.

[0119] Table 3 Experimental parameters of Examples 17 - 20

[0120]

[0121] Table 4 Performance evaluation results of Examples 17 - 20

[0122]

[0123] As can be seen from Table 3 and Table 4, at the same molar ratio of urea to caprolactam and the same mass ratio of deep eutectic solvent to electrolyte, the battery performance is basically the same, indicating that the method of the present invention can successfully prepare a highly moisture-proof electrolyte and a high-performance lithium metal battery.

[0124] The test conditions and results of further performance testing of the electrolyte of Example 11 are shown in Table 5.

[0125] Table 5 Further Performance Test Results of the Electrolyte of Example 11 and Lithium Metal Battery

[0126]

[0127] In Table 5, for the electrolyte characteristics, the electrolyte is the electrolyte after being treated under humid conditions. It is obtained by placing an aluminum bottle containing a lithium battery ester electrolyte in an air environment with an open mouth for a preset time, or by adding ultrapure water to an aluminum bottle containing a lithium battery ester electrolyte and then placing it with an open mouth for a preset time. The environmental humidity, the amount of added water, and the preset time are all as shown in Table 5; for the battery performance, the battery is a lithium metal battery assembled with the corresponding electrolyte after being treated under humid conditions as the electrolyte.

[0128] As can be seen from Table 5, when the test environmental humidity increases, the hydrolysis degree of the lithium salt remains <0.01% and the battery performance is high, indicating that the lithium salt ester electrolyte of the present invention has excellent moisture-proof performance. By further adding water externally to affect the lithium salt ester electrolyte, within 72 h after adding 100,000 ppm of ultrapure water, the hydrolysis degree of the lithium salt can still be maintained below 0.01%. When the externally added water amount ≥50,000 ppm, the battery performance of the lithium metal battery begins to decrease significantly, indicating that the lithium salt ester electrolyte of the present invention can cope with the influence of moisture in the external working conditions and can operate in a relatively high humidity environment.

[0129] The ester electrolytes of Example 11 and Comparative Example 1 were sampled and photographed after being treated with 14,000 ppm of water for 48 h, and the morphology is as Figure 6 shown. According to Figure 6 it can be seen that the color of the lithium salt ester electrolyte of Example 11 remains clear and transparent after being treated with 14,000 ppm of water for 48 h, while the lithium salt of the ester electrolyte of Comparative Example 1 undergoes severe hydrolysis and the color is dark black, indicating that adding a deep eutectic solvent can effectively inhibit the hydrolysis of the lithium salt.

[0130] A lithium metal battery was assembled using the ester-based electrolyte of Example 11 as the electrolyte. For the positive and negative casings, casings with a moisture protection level as low as IP54 were selected. The storage stability test results of the lithium metal battery showed that, under the condition of low moisture protection of the casing, the self-discharge rate of the lithium metal battery of the present invention was <2% / month after one year of storage. This indicates that the ester-based electrolyte of the lithium battery of the present invention is compatible with multiple types of casings and can be stored for a long time in a high-humidity environment. The lithium metal battery assembled with this ester-based electrolyte of the lithium battery has high electrochemical stability.

Claims

1. A lithium battery ester electrolyte with moisture-proof function, characterized in that, It includes an electrolyte and a deep eutectic solvent; the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 1000); the raw materials of the deep eutectic solvent include urea and caprolactam; the molar ratio of urea to caprolactam is 1:(1 - 4).

2. The ester-based electrolyte for lithium batteries with moisture-proof function according to claim 1, characterized in that, The molar ratio of urea to caprolactam is 1:(1 - 3).

3. The ester-based electrolyte for lithium batteries with moisture-proof function according to claim 1, characterized in that, The mass ratio of the deep eutectic solvent to the electrolyte is 1:(10 - 100).

4. The ester-based electrolyte for lithium batteries with moisture-proof function according to claim 3, characterized in that, The mass ratio of the deep eutectic solvent to the electrolyte is 1:(20 - 100).

5. The ester-based electrolyte for lithium batteries with moisture-proof function according to claim 1, characterized in that, The preparation method of the deep eutectic solvent includes: mixing urea and caprolactam and heating them at a constant temperature of 60 - 80 °C for 12 - 24 h.

6. The ester-based electrolyte for lithium batteries with moisture-proof function according to claim 1, characterized in that, The materials of the electrolyte include a lithium salt, an ester solvent and an additive component; the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium bis(oxalato)borate; the ester solvent is one or more of ethylene carbonate, diethyl carbonate and dimethyl carbonate; the additive component is fluoroethylene carbonate and / or vinylene carbonate.

7. A method for preparing an ester-based electrolyte for a lithium battery with moisture-proof function as described in claim 1, characterized in that, It includes: Mix the deep eutectic solvent and the electrolyte according to a preset mass ratio, and keep them warm at a temperature of 30 - 50 °C for 3 - 6 h to obtain a lithium battery ester electrolyte with moisture-proof function.

8. The application of a lithium battery ester electrolyte with moisture-proof function as described in claim 1 in a lithium battery, characterized in that, It includes: Using the lithium battery ester electrolyte with moisture-proof function as the electrolyte and using a lithium sheet as the negative electrode.

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