Lithium battery ester electrolyte with moisture-proof function as well as preparation method and application of lithium battery ester electrolyte

Abstract

The invention discloses a lithium battery ester electrolyte with a moisture-proof function. The lithium battery ester electrolyte comprises an electrolyte and a deep eutectic solvent, the mass ratio of the deep eutectic solvent to the electrolyte is 1: (10-1000); raw materials of the deep eutectic solvent comprise urea and caprolactam. The invention also provides a preparation method and application of the lithium battery ester electrolyte with the moisture-proof function. After the lithium battery ester electrolyte disclosed by the invention is treated under a humid condition, the hydrolysis degree of lithium salt is 1t; and the electrolyte has good moisture-proof characteristic and can be compatible with the existing battery assembly, the highest first-week coulombic efficiency of the lithium metal battery containing the electrolyte treated under the wet condition is 99.1%, the capacity of the lithium metal battery after 500 cycles exceeds 130.1 mAh / g, and the capacity retention rate of the lithium metal battery after 500 cycles exceeds 87.8%.

Description

Technical Field

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

[0002] In today's era of rapid technological development, lithium batteries have become an important component of many mobile portable devices due to their significant advantages such as high energy density and long cycle life. They are widely used in new energy vehicles and large-scale smart devices. However, lithium batteries face a serious problem in the manufacturing process. Due to the limitations of the manufacturing process, trace amounts of water will inevitably remain inside the battery. The presence of trace amounts of water in the battery will cause the lithium salt LiPF 6 Hydrolysis triggers a series of side reactions, seriously affecting battery performance and service life.

[0003] To address this problem, the industry has tried a variety of methods to reduce the moisture content in batteries. Common measures include strictly controlling the humidity of the production environment and deep drying of raw materials, but these methods require a lot of professional equipment and manpower, resulting in a significant increase in production costs. Moreover, even if these strict moisture control measures are taken, moisture may still enter the battery during actual production and use, making it difficult to fundamentally prevent side reactions. Therefore, seeking a method that can reduce the intrinsic reactivity of water molecules and inhibit side reactions from the source has become the key to solving the above problems. Among them, building a stable electrolyte system has become the main way that is simple and easy to promote in industry.

[0004] As a new type of solvent system, deep eutectic solvents are formed by hydrogen bond donors and hydrogen bond acceptors through hydrogen bond interactions. They have the advantages of high safety, environmental protection, simple preparation process, and low cost. However, the application of deep eutectic solvents in lithium battery systems is currently subject to many restrictions. 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 and adhesives) is poor, which makes it difficult for deep eutectic solvents to be effectively used 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 electrolyte with moisture-proof function and a preparation method and application thereof in view of the deficiencies of the above-mentioned prior art. The lithium battery ester electrolyte has a lithium salt hydrolysis degree of <0.01% after being treated under humid conditions, has good moisture-proof properties, and is compatible with existing battery components. The lithium metal battery containing the electrolyte treated under humid conditions has a first-week coulombic efficiency of up to 99.1%, a capacity of more than 130.1 mAh / g after 500 cycles, and a capacity retention rate of more than 87.8% after 500 cycles.

[0006] Compared with the prior art, the present invention has the following advantages:

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

[0008] 2. Preferably, the lithium battery ester electrolyte of the present invention can be moisture-proof only by adding a deep eutectic solvent therein, and a high-performance battery with good cycle stability can be obtained by using it as the electrolyte 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, has a self-discharge rate of <2% / month, and has high electrochemical stability.

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

[0011] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 The deep eutectic solvent of the present invention inhibits the reaction between water and PF 6 - Schematic diagram of the reaction mechanism;

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

[0014] Figure 3 Schematic diagram showing the performance comparison of lithium metal batteries of Example 7 and Comparative Example 1;

[0015] Figure 4 The lithium metal battery of Example 7 deposited 1 mA h cm after one cycle. -2 TEM image of lithium morphology;

[0016] Figure 5 Comparative Example 1: Li metal battery deposited 1 mA h cm after one cycle -2 TEM image of lithium morphology;

[0017] Figure 6 Schematic diagram of the morphology comparison of the ester electrolytes of Example 11 and Comparative Example 1 after treatment with 14000 ppm water for 48 h. DETAILED DESCRIPTION

[0018] The technical solution will be described clearly and completely below in conjunction 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. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work are within 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 there may be three relationships. For example, A and / or B can represent: A exists alone, B exists alone, and A and B exist at the same time. A and B can be singular or plural.

[0020] In the following description, the terms "include", "comprising", "having" and "containing" are open-ended terms, meaning including 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 order of serial numbers does not mean the order of execution, some or all of the steps can be executed in parallel or sequentially, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0022] Those skilled in the art will appreciate that the numerical ranges in the embodiments of the present application are to be understood as each intermediate value between the upper and lower limits of the scope also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present application. The upper and lower limits of these smaller ranges may be independently included or excluded in the scope.

[0023] Unless otherwise specified, the technical / scientific terms used herein have the same meanings as those generally understood by those skilled 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 may 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 the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0024] The technical principle adopted by the present invention is that the deep eutectic solvent prepared with urea and caprolactam as raw materials is used as an additive for lithium battery ester electrolyte. The deep eutectic solvent can form a stable cluster structure with water to inhibit the LiPF 6The hydrolysis of water molecules improves the moisture resistance of lithium battery ester electrolytes. On the one hand, the donor interaction around water molecules causes steric hindrance, preventing water molecules from contacting the electrolyte; on the other hand, the formed hydrogen bonds change the stability of the OH covalent bonds and the electronic structure of water molecules, and the intrinsic reactivity of water molecules is reduced, making it unable to react with LiPF 6 The schematic diagram of the principle of preventing more side reactions and improving the moisture resistance of the electrolyte is as follows: Figure 1 As shown; in addition, the deep eutectic solvent has good compatibility with the lithium battery system, and the assembled lithium metal battery exhibits high battery performance.

[0025] Some embodiments provide a lithium battery ester electrolyte with moisture-proof function, comprising an electrolyte and a deep eutectic solvent; the mass ratio of the deep eutectic solvent to the electrolyte is 1:(10~1000); the deep eutectic solvent raw materials include urea and caprolactam.

[0026] A deep eutectic solvent composed of urea and caprolactam is used as an additive to optimize the solvation structure of lithium battery ester electrolytes and improve the moisture resistance of lithium metal ester electrolytes.

[0027] In some preferred embodiments, the molar ratio of urea to caprolactam is 1:(1-4); more 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); more 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 resulting deep eutectic solvent has a high dielectric constant, which can optimize the solvation structure of lithium battery ester electrolytes and promote the LiPF 6 By further limiting the mass ratio of deep eutectic solvent to electrolyte to 1: (10-100), the strong hydrogen bonding between deep eutectic solvents combines with water in the electrolyte, and the water in the electrolyte is quenched, thus inhibiting the LiPF reaction caused by water. 6 Hydrolysis. During the experiment, the inventors found that the ratio of deep eutectic solvent to electrolyte in lithium battery ester electrolyte affects the performance of electrolyte and battery. When the content of deep eutectic solvent in lithium battery ester electrolyte is too high, for example, the mass ratio of deep eutectic solvent to electrolyte is 1:5, the degree of hydrolysis of lithium salt increases significantly, and the cycle life and capacity retention ability are greatly reduced. When the content of deep eutectic solvent in lithium battery ester electrolyte is too low, for example, the mass ratio of deep eutectic solvent to electrolyte is 1:2000, the hydrolysis of lithium salt is obvious, the battery cannot operate, and there is no charge and 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 heating the mixture at a constant temperature of 60 to 80° C. for 12 to 24 hours.

[0030] In some embodiments, the electrolyte material includes a lithium salt, an ester solvent and an additive component; the lithium salt includes lithium hexafluorophosphate (LiPF 6 ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(oxalatoborate) (LiBOB); the ester solvent is one or more of ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC); the added component is fluoroethylene carbonate (FEC) and / or vinylene carbonate (VC).

[0031] In some preferred embodiments, the electrolyte is a mixture of a lithium salt, an ester solvent and an additive, wherein the lithium salt is lithium hexafluorophosphate (LiPF 6 ), the ester solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1, and the added component is fluoroethylene carbonate (FEC); in the electrolyte, the concentration of lithium salt is 1 M, and the mass percentage of the added component is 5%.

[0032] Preferably, the electrolyte is a mixture of lithium hexafluorophosphate, ethylene carbonate, dimethyl carbonate and fluoroethylene carbonate in a volume ratio of 1:1, which is conducive to the reaction with the deep eutectic solvent to further achieve solvation structure adjustment and form a passivation layer on the negative electrode surface.

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

[0034] By limiting the mixing temperature to 30-50°C and the holding 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 electrolyte.

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

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

[0037] The present invention has been subjected to a series of experiments before the application is filed. A part of the experimental results are listed here to further describe the invention in detail, and the following is a detailed description in conjunction with the embodiments.

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

[0039] Example 1

[0040] The present embodiment provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, wherein 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 embodiment also provides a method for preparing the above-mentioned lithium battery ester electrolyte with moisture-proof function, comprising:

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

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

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

[0045] Step 2: providing an electrolyte, specifically comprising: adding lithium hexafluorophosphate (LiPF 6 ), fluoroethylene carbonate (FEC) and an ester solvent are mixed to obtain an electrolyte; the ester solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1; in the electrolyte, the concentration of lithium hexafluorophosphate is 1M, and the mass percentage of fluoroethylene carbonate is 5%;

[0046] Step 3: Mix the deep eutectic solvent and the electrolyte at 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 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

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

[0048] This embodiment also provides a method for assembling the above-mentioned lithium metal battery, comprising:

[0049] Step 1: LiFePO 4 , polyvinylidene fluoride (PVDF) and conductive carbon black (SuperP) are mixed to obtain a mixed powder, N-methylpyrrolidone (NMP) is added to the mixed powder, and a stirrer is used to stir to prepare a uniform slurry; the polyvinylidene fluoride has a molecular weight of 700,000 and 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: coating the slurry on an aluminum current collector, drying it in a vacuum oven at 60° C. for 12 h, taking it out and pressing it with a tablet press at 12 MPa to obtain a sheet, and cutting the sheet with a cutting machine into pieces with 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-mentioned lithium battery ester electrolyte as the electrolyte; assemble the battery in the order of positive electrode shell, positive electrode sheet, separator, negative electrode sheet, nickel foam and negative electrode shell; the positive electrode shell and the negative electrode shell are both model 2032, purchased from Henan Pengxiang Yunda Company; the polypropylene film is Celgard2500 polypropylene film.

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

[0053] Embodiments 2 to 20

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

[0055] Examples 2 to 16 also 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 electrolyte and the lithium metal battery of Examples 2 to 16 are shown in Tables 1 and 2.

[0056] Examples 17 to 20 respectively provide a lithium battery ester electrolyte and a preparation method thereof, wherein the 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 provide a lithium metal battery and an assembly method thereof, which are the same as those of Example 1. The performance of the lithium battery ester electrolyte and the lithium metal battery of Examples 17 to 20 are shown in Table 4.

[0058] Embodiment 21

[0059] The present embodiment provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, wherein 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 embodiment also provides a method for preparing the above-mentioned lithium battery ester electrolyte with moisture-proof function, comprising:

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

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

[0063] Step 102, putting the mixed powder into a glass beaker, stirring at a constant temperature of 60° C. for 24 h, to obtain a clear and transparent deep eutectic solvent;

[0064] Step 2, providing an electrolyte, specifically comprising: mixing 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 1M, and the mass percentage of fluoroethylene carbonate is 5%;

[0065] Step 3: Mix the deep eutectic solvent and the electrolyte at 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 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

[0066] After the lithium battery ester electrolyte of this embodiment is placed under an ambient air humidity of 35% for 48 hours, the degree of hydrolysis of lithium salt is less than 0.01%.

[0067] Embodiment 22

[0068] The present embodiment provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, wherein 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 embodiment also provides a method for preparing the above-mentioned lithium battery ester electrolyte with moisture-proof function, comprising:

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

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

[0072] Step 102, putting the mixed powder into a glass beaker, stirring at a constant temperature of 60° C. for 24 h, to obtain a clear and transparent deep eutectic solvent;

[0073] Step 2, providing an electrolyte, specifically comprising: mixing 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 1M, and the mass percentage of vinylene carbonate is 5%;

[0074] Step 3: Mix the deep eutectic solvent and the electrolyte at 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 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

[0075] After the lithium battery ester electrolyte of this embodiment is placed under an ambient air humidity of 35% for 48 hours, the degree of hydrolysis of lithium salt is less than 0.01%.

[0076] Embodiment 23

[0077] The present embodiment provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, wherein 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 embodiment also provides a method for preparing the above-mentioned lithium battery ester electrolyte with moisture-proof function, comprising:

[0079] Step 1: providing a deep eutectic solvent, specifically including:

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

[0081] Step 102, putting the mixed powder into a glass beaker, stirring at a constant temperature of 60° C. for 24 h, to obtain a clear and transparent deep eutectic solvent;

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

[0083] Step 3: Mix the deep eutectic solvent and the electrolyte at 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 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

[0084] After the lithium battery ester electrolyte of this embodiment is placed under an ambient air humidity of 35% for 48 hours, the degree of hydrolysis of lithium salt is less than 0.01%.

[0085] Embodiment 24

[0086] The present embodiment provides a lithium battery ester electrolyte with moisture-proof function, including an electrolyte and a deep eutectic solvent, wherein 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.

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

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

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

[0090] Step 102, putting the mixed powder into a glass beaker, stirring at a constant temperature of 60° C. for 24 h, to obtain a clear and transparent deep eutectic solvent;

[0091] Step 2, providing an electrolyte, specifically comprising: mixing a lithium salt, an additive component and an ester solvent to obtain an electrolyte; the ester solvent is dimethyl carbonate; in the electrolyte, the concentration of the lithium salt is 1M, the lithium salt is lithium bis(oxalatoborate) and lithium bis(fluorosulfonyl)imide in 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 in a mass ratio of 1:2;

[0092] Step 3: Mix the deep eutectic solvent and the electrolyte at 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 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

[0093] After the lithium battery ester electrolyte of this embodiment is placed under an ambient air humidity of 35% for 48 hours, the degree of hydrolysis of lithium salt is less than 0.01%.

[0094] Comparative Example 1

[0095] This comparative example provides an ester electrolyte without adding a deep eutectic solvent, and the preparation method comprises:

[0096] Step 1: providing an electrolyte, specifically comprising: adding lithium hexafluorophosphate (LiPF 6 ), fluoroethylene carbonate (FEC) and an ester solvent are mixed to obtain an electrolyte; the ester solvent is ethylene carbonate (EC) and dimethyl carbonate (DMC) in a volume ratio of 1:1; in the electrolyte, the concentration of lithium hexafluorophosphate is 1M, and the mass percentage of fluoroethylene carbonate is 5%;

[0097] Step 2: The electrolyte is placed in a clean aluminum bottle and stored at 50° C. for 3 hours to obtain an ester electrolyte.

[0098] Comparative Examples 2 to 15

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

[0100] Performance Testing

[0101] 1. Moisture resistance test:

[0102] The aluminum bottle containing the lithium battery ester electrolyte is placed in an open state in an air environment for a preset time, or ultrapure water is added to the aluminum bottle containing the lithium battery ester electrolyte and then placed in an open state for a preset time to obtain a hydrolyzate after humid condition treatment. The hydrolyzate after humid condition treatment is sampled to determine the degree of hydrolysis of the lithium salt. The method for determining the degree of hydrolysis is nuclear magnetic resonance phosphorus spectroscopy (31 P-NMR), the electrolyte is tested by nuclear magnetic resonance phosphorus spectrum, the relative content of different phosphorus species is calculated by peak area integration, and the degree of lithium salt hydrolysis is determined; the preset time is 24~72 h, and the ambient humidity in the air environment is 30%~60%.

[0103] 2.Battery performance test:

[0104] The performance of the lithium metal battery was measured using a Xinwei battery tester. The electrolyte in the lithium metal battery was the hydrolyzate 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 self-discharge rate η is calculated as η = (C 1 -C 2 ) / C 1 ×100%, where C 1 is the initial capacity, C 2 To store the remaining capacity for 30 days.

[0107] Performance Evaluation

[0108] In Tables 1 and 2, the electrolyte in the electrolyte characteristics is the electrolyte treated under humid conditions, which is the electrolyte treated under humid conditions obtained by leaving an aluminum bottle containing a lithium battery ester electrolyte open in an air environment for a preset time, wherein the preset time is 48 hours and the air environment humidity is 35%; the lithium metal battery in the battery performance is a lithium metal battery assembled using the corresponding electrolyte treated under humid conditions as the electrolyte.

[0109] According to Table 1 and Table 2, the ester electrolyte of Comparative Example 1 without adding deep eutectic solvent has a lithium salt hydrolysis degree of >0.01% after being treated under humid conditions, a light yellow color, a pH of 1, and the battery cannot operate, and has no charge and discharge capacity. The lithium salt hydrolysis degree of Example 1 lithium battery ester electrolyte after being treated under humid conditions is <0.01%, the color is transparent, the pH is 5, the first week coulomb efficiency 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, it can be seen that the introduction of deep eutectic solvents into ester electrolytes can effectively inhibit lithium salt hydrolysis and improve battery performance.

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

[0111] Table 2 Preparation parameters and performance parameters of Examples 9 to 16 and Comparative Examples 10 to 15

[0112] From Table 1 and Table 2, it can be seen that the electrolyte characteristics and battery performance are different after humid condition treatment when the mass ratio of deep eutectic solvent to electrolyte is 1:2000, such as Comparative Examples 2 to 6, the degree of hydrolysis of lithium salt in the electrolyte after humid condition treatment is greater than 0.01%, and the lithium metal batteries cannot operate and have no charge and discharge capacity; when the molar ratio of urea to caprolactam in the deep eutectic solvent is 2:1 or 1:5, the degree of hydrolysis of lithium salt in the electrolyte after humid condition treatment is greater than 0.01%, and the lithium metal batteries cannot operate and have no charge and discharge capacity, indicating that when the amount of deep eutectic solvent added is too low, the molar ratio of urea to caprolactam is too high or too low, the degree of hydrolysis of lithium salt cannot be reduced, the pH of the electrolyte is low, the battery performance is poor, and it is even difficult to operate.

[0113] Based on Examples 1 to 16, it can be seen that when the mass ratio of the deep eutectic solvent to the electrolyte is 1: (10 to 1000), and the molar ratio of urea to caprolactam is 1: (1 to 4), under the same test conditions, the degree of lithium salt hydrolysis is <0.01%, and the first week coulombic efficiency is >87%, indicating that the moisture resistance of the electrolyte and the battery performance are improved; when the mass ratio of the deep eutectic solvent to the electrolyte is 1: (10 to 1000), and the molar ratio of urea to caprolactam is 1: (1 to 3), under the same test conditions, the first week coulombic efficiency is >91%, the capacity after 500 cycles exceeds 98%, and the capacity retention rate after 500 cycles exceeds 65%, among which the highest 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 first week coulombic efficiency is 99.1%, and the capacity after 500 cycles exceeds 130.1 mAh / g, the capacity retention rate after 500 cycles exceeds 87.8%; Example 11 The degree of hydrolysis of the electrolyte lithium salt after moisture treatment is shown in the nuclear magnetic resonance phosphorus spectrum. Figure 2 As shown, according to Figure 2 It can be seen that only LiPF 6 The signal peak has a peak area that remains essentially unchanged after integration, indicating that the lithium salt ester electrolyte of Example 11 undergoes almost no hydrolysis reaction.

[0114] Based on Comparative Examples 8 to 9 and Examples 5 to 8, when the mass ratio of the deep eutectic solvent to the electrolyte is determined, the molar ratio of urea to caprolactam is different, and the performance of the lithium metal battery is different, which is manifested as the battery performance first increases and then decreases with the increase of the molar ratio. When the molar ratio is 1:3, the first week coulomb efficiency is raised to a maximum value of 98.9%, the capacity of 500 cycles is increased to 128.7 mAh / g, and the capacity retention rate of 500 cycles is 86.4%. When the molar ratio is increased to 1:4, the battery performance decreases slightly, and the first week coulomb efficiency is still maintained at more than 94%, the capacity of 500 cycles is more than 109 mAh / g, and the capacity retention rate of 500 cycles is more than 72%. However, when the molar ratio continues to increase to 1:5, the battery performance decreases rapidly. At a rate of 1C, the battery cannot operate and has no charge and discharge capacity, indicating that the composition of the deep eutectic solvent affects the performance of the lithium metal battery. Within a certain range, when the caprolactam content in the deep eutectic solvent increases, it is more conducive to improving the battery performance of the lithium metal battery.

[0115] Figure 3 : is a comparison chart of the cycle performance of the lithium metal battery of Comparative Example 1 and the lithium metal battery of Example 7, wherein the electrolytes in the lithium metal batteries are all electrolytes treated according to 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 rate of 1C and has no charge and discharge capacity, while the lithium metal battery of Example 7 still maintains high capacity and high coulombic efficiency after 500 cycles.

[0116] Figure 4 The deposition rate of 1 mA h cm after one cycle of the lithium metal battery in Example 7 -2 TEM image after lithium, Figure 5 The lithium metal battery of Comparative Example 1 deposited 1 mA h cm after one cycle. -2 TEM images after lithium, where the electrolytes in the lithium metal batteries are all treated according to the humid conditions in Table 1. Figure 4 and Figure 5 visible, Figure 5 Obvious crystals appear in the lithium metal battery, which are lithium dendrites, indicating that the lithium battery deposition mechanism inside the lithium metal battery of Comparative Example 1 is abnormal.

[0117] Table 3 Experimental parameters of Examples 17 to 20

[0118] Table 4 Performance evaluation results of Examples 17 to 20

[0119] It can be seen from Tables 3 and 4 that for the same molar ratio of urea to caprolactam and 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.

[0120] The test conditions and results of further performance testing of the electrolyte of Example 11 are shown in Table 5.

[0121] Table 5 Further performance test results of electrolyte and lithium metal battery of Example 11

[0122] In Table 5, the electrolyte in the electrolyte characteristics is the electrolyte treated under wet conditions, which is the electrolyte treated under wet conditions obtained by leaving an aluminum bottle containing a lithium battery ester electrolyte open in an air environment for a preset time, or the electrolyte treated under wet conditions obtained by adding ultrapure water to an aluminum bottle containing a lithium battery ester electrolyte and then leaving it open for a preset time, wherein the ambient humidity, the amount of water added and the preset time are all as shown in Table 5; the battery in the battery performance is a lithium metal battery assembled using the corresponding electrolyte treated under wet conditions as the electrolyte.

[0123] According to Table 5, when the humidity of the test environment increases, the degree of lithium salt hydrolysis of <0.01% and high battery performance can still be maintained, indicating that the lithium salt ester electrolyte of the present invention has excellent moisture resistance. The lithium salt ester electrolyte is further affected by adding water. Within 72 hours of adding 100,000 ppm ultrapure water, the degree of lithium salt hydrolysis can still be kept below 0.01%. When the amount of added water is ≥50,000 ppm, the lithium metal battery will show a significant decrease in battery performance, indicating that the lithium salt ester electrolyte of the present invention can cope with the influence of moisture in external working conditions and can operate in a relatively high humidity environment.

[0124] The ester electrolytes of Example 11 and Comparative Example 1 were treated with 14000 ppm water for 48 h and then sampled and photographed. The morphology is as follows: Figure 6 As shown, according to Figure 6 It can be seen that the color of the lithium salt ester electrolyte in Example 11 after being treated with 14000 ppm water for 48 h is still clear and transparent, while the lithium salt of the ester electrolyte in Comparative Example 1 is severely hydrolyzed and the color is dark black, indicating that the addition of deep eutectic solvent can effectively inhibit the hydrolysis of lithium salt.

[0125] A lithium metal battery is assembled using the lithium battery ester electrolyte of Example 11 as the electrolyte, wherein both the positive electrode shell and the negative electrode shell are selected to have a moisture-proof grade as low as IP54. The lithium metal battery storage stability test results show that when the shell has low moisture-proofness, the self-discharge rate of the lithium metal battery of the present invention is less than 2% / month after one year of storage, indicating that the lithium battery ester electrolyte of the present invention is compatible with multiple types of shells and can be stored for a long time in a high humidity environment. The lithium metal battery assembled with the lithium battery ester electrolyte 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.

2. The moisture-proof lithium battery ester electrolyte according to claim 1, characterized in that: The molar ratio of urea to caprolactam is 1:(1-4).

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

4. The moisture-proof lithium battery ester electrolyte according to claim 1, characterized in that: The mass ratio of the deep eutectic solvent to the electrolyte is 1:(10-100).

5. The moisture-proof lithium battery ester electrolyte according to claim 4, characterized in that: The mass ratio of the deep eutectic solvent to the electrolyte is 1:(20-100).

6. The moisture-proof lithium battery ester electrolyte according to claim 1, characterized in that: The preparation method of the deep eutectic solvent comprises: mixing urea and caprolactam and heating them at a constant temperature of 60 to 80° C. for 12 to 24 hours.

7. The moisture-proof lithium battery ester electrolyte according to claim 1, characterized in that: The materials of the electrolyte include lithium salt, ester solvent and additive components; the lithium salt includes one or more of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide and lithium bis(oxalatoborate); 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.

8. A method for preparing the moisture-proof lithium battery ester electrolyte according to claim 1, characterized in that: include: The deep eutectic solvent is mixed with the electrolyte according to a preset mass ratio, and kept warm at 30-50° C. for 3-6 hours to obtain a lithium battery ester electrolyte with moisture-proof function.

9. A use of the moisture-proof lithium battery ester electrolyte as claimed in claim 1 in a lithium battery, characterized in that: include: The lithium battery ester electrolyte with moisture-proof function is used as the electrolyte, and the lithium sheet is used as the negative electrode.

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