Application of crown ether-based multifunctional electrolyte additive molecules in lithium metal batteries

Abstract

The application discloses application of a crown ether-based multifunctional electrolyte additive molecule in a lithium metal battery, the crown ether-based multifunctional electrolyte additive molecule is shown in the following formula, wherein R1, R2, R3 and R4 are independently selected from H, Br, CN, F or OMe, and at least one of R1, R2, R3 and R4 is selected from CN; the adding amount of the multifunctional electrolyte additive molecule is 0.3% to 1.0% of the mass of the lithium metal battery electrolyte. The application introduces a functional group by taking a benzocrown ether as a core, reduces an interface side reaction and inhibits lithium dendrite growth through the synergistic effect of the crown ether ring and the cyano group, and further improves the cycle stability and rate performance of the lithium metal battery.

Description

Technical Field

[0001] This invention belongs to the field of secondary battery technology and relates to the application of crown ether-based multifunctional electrolyte additive molecules in lithium metal batteries. Specifically, it relates to an electrolyte additive containing a functionalized crown ether structure, its preparation method, and its application in inhibiting lithium dendrite growth and stabilizing the electrode / electrolyte interface. Background Technology

[0002] Lithium metal batteries, as a high-performance rechargeable battery system, have broad application prospects in the new energy field due to their advantages such as high specific capacity and low potential. They are expected to provide stronger and more durable power support for electric vehicles, portable electronic devices, and large-scale energy storage. However, existing technologies in the field of lithium metal batteries have several prominent problems: First, side reactions easily occur between the electrolyte and the highly active metal anode, resulting in insufficient stability of the solid electrolyte interface (SEI); second, the growth of lithium metal dendrites may cause internal short circuits, posing safety hazards; third, traditional electrolyte additives such as vinylene carbonate have limited functions and are difficult to coordinately address multiple interface challenges.

[0003] To address the aforementioned issues, existing solutions include optimizing electrolyte composition, applying solid-state electrolytes, constructing artificial SEI layers, designing three-dimensional structural materials, and using lithium-based alloys. Among these, constructing a stable SEI layer plays a crucial role in improving the performance of lithium metal batteries, as it effectively suppresses side reactions and inhibits dendrite formation. Therefore, using electrolyte additives to improve SEI characteristics has become an important research direction. Crown ethers can selectively coordinate lithium ions through their specific cavity sizes, thereby regulating ion transport behavior. However, traditional crown ethers, such as 18-crown-6, have limited functionality in organic electrolytes, only serving to protect the lithium anode, thus restricting their practical applications. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings of existing technologies by providing a multifunctional electrolyte additive molecule based on crown ethers, its preparation method, and its application. The electrolyte additive molecule is easy to synthesize and improves the cycle life and rate performance of lithium metal batteries based on solvation effect and inhibition of lithium dendrite growth.

[0005] To achieve the above objectives, the present invention is conceived as follows: Benzocrown ether is introduced as the core functional group, and its unique molecular structure design is used to regulate the lithium-ion solvation structure, reduce the desolvation energy barrier, promote uniform lithium-ion deposition, and participate in the construction of a solid electrolyte interface (SEI) film with high ionic conductivity and mechanical stability at the lithium metal electrode / electrolyte interface. Thus, through the multiple action mechanisms of the synergistic functional groups (crown ether ring, cyano group) in the molecule, the cycle stability and rate performance of lithium metal batteries are simultaneously improved.

[0006] In a first aspect, the present invention provides the application of a crown ether-based multifunctional electrolyte additive molecule in lithium metal batteries, the general formula of which is as follows:

[0007]

[0008] R1, R2, R3, and R4 are independently selected from H, Br, CN, F, or OMe, and at least one of R1, R2, R3, and R4 is selected from CN. In this structure, the complexing ability of the crown ether to the metal can induce uniform deposition of lithium ions. At the same time, the introduction of -CN enables the designed crown ether additive to preferentially form a uniform and stable interface layer on the positive electrode side before the electrolyte oxidation and decomposition.

[0009] Preferably, R1, R2, and R4 are selected from H, and R3 is selected from CN; that is, the structural formula of the crown ether-based multifunctional electrolyte additive molecule is:

[0010]

[0011] It was named 2,6-dicyano-18-benzocrown ether-6 (2CN-18C6).

[0012] This invention also provides a method for preparing 2CN-18C6, which involves a substitution reaction using N-bromosuccinimide and cuprous cyanide as reactants. The target product can be obtained through two substitution reactions. The synthesis process is simple and suitable for industrial production.

[0013] Specifically, it includes the following steps:

[0014] 18-benzocrown ether-6 was dissolved in chloroform, N-bromosuccinimide was added to react, and the intermediate product was obtained by filtration.

[0015] The intermediate product, cuprous cyanide and N,N-dimethylformamide were mixed and stirred at 152°C for 4 days. The resulting molecule was purified by recrystallization to obtain the multifunctional electrolyte additive based on crown ether.

[0016] The synthetic route for the reaction is shown below:

[0017]

[0018] Thirdly, the present invention provides a lithium metal battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; the electrolyte comprises an organic solvent, an electrolyte salt, and the above-mentioned crown ether-based multifunctional electrolyte additive molecules; wherein the content of the crown ether-based multifunctional electrolyte additive molecules is 0.3% to 1.0% of the electrolyte mass.

[0019] Preferably, the content of the crown ether-based multifunctional electrolyte additive molecules is 0.7% of the electrolyte mass.

[0020] Preferably, the organic solvent includes one or more of diethyl carbonate, acetate carbonate, methyl ethyl carbonate, and fluoroethylene carbonate, and the electrolyte salt is lithium hexafluorophosphate.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] The multifunctional additive synthesized in this invention can preferentially react simultaneously at the positive and negative electrode interfaces, constructing stable cathode electrolyte interfaces (CEI) and solid electrolyte interfaces (SEI) respectively. This provides comprehensive protection for both the high-voltage positive electrode and the highly active negative electrode, synergistically improving the overall performance of the entire battery. By precisely controlling the interfacial chemistry, side reactions between the electrolyte and the lithium metal negative electrode are effectively suppressed, constructing a stable SEI layer dominated by a highly ion-conducting inorganic phase (such as LiF), overcoming the limitation of single-function crown ether additives in existing technologies. This mechanism can inhibit lithium dendrite growth, reduce the risk of internal short circuits caused by lithium dendrites piercing the separator, and significantly improve battery safety. Attached Figure Description

[0023] The present invention will be further described by way of exemplary embodiments, which will be described in detail with reference to the accompanying drawings. These embodiments are not limiting; in these embodiments, the same reference numerals denote the same structures, wherein:

[0024] Figure 1 This is a schematic diagram of the general molecular structure formula and synthetic route of the additive of the present invention.

[0025] Figure 2 This is the 1H NMR spectrum of 2-cyano-18-benzocrown ether-6.

[0026] Figure 3 This is a schematic diagram of a lithium metal battery assembly.

[0027] Figure 4 This is a graph showing the critical current density test results for electrolytes with different concentrations of additives.

[0028] Figure 5 This is a performance comparison chart of Li||Li symmetric cells.

[0029] Figure 6 These are SEM comparison images of lithium deposition morphology, where (a) is the blank group and (b) is the additive group of the present invention.

[0030] Figure 7 This is a graph from a Li||Cu half-cell cycle test.

[0031] Figure 8This is the Tafel curve of a Li||Li symmetric cell.

[0032] Figure 9 This is a comparison chart of the rate performance of Li||LFP batteries in electrolyte containing 0.7% 2CN-18C6 and in standard electrolyte.

[0033] Figure 10 This is a comparison chart of the cycle performance curves of Li||LFP batteries at a 0.5C rate.

[0034] Figure 11 This is a comparison chart of the cycle performance curves of Li||LFP batteries at a 2.0C rate.

[0035] Figure 12 This is a performance comparison chart of Li||LCO batteries at different rates from 0.1 to 5.0C.

[0036] Figure 13 This is a graph showing the cycle performance test results of a Li||LCO battery. Detailed Implementation

[0037] The following description is intended to enable those skilled in the art to implement and use the invention, and is provided in the context of a specific application and its requirements. It will be apparent to those skilled in the art that various modifications can be made to the disclosed embodiments. Furthermore, the general principles defined in this invention can be applied to other embodiments and applications without departing from the spirit and scope of the invention. Therefore, this invention is not limited to the disclosed embodiments, but should be given the broadest scope consistent with the scope of the invention.

[0038] Example 1: Synthesis of 2,6-dicyano-18-benzocrown ether-6 (2CN-18C6)

[0039] The synthetic route for 2,6-dicyano-18-benzocrown ether-6 (2CN-18C6) is as follows: Figure 1 As shown, the specific steps are as follows:

[0040] Weigh 3.604 g (10.0 mmol) of 18-benzocrown ether-6 and dissolve it in 85 mL of chloroform in a 250 mL round-bottom flask. Set up an ice bath and protect the flask from light. Weigh 3.737 g (21 mmol) of N-bromosuccinimide in portions. Stir for 30 minutes, then remove the ice bath and set up a condenser. Reflux at 65 °C with stirring for 6 hours. After the reaction is complete, add 150 mL of isopropanol and reflux at 90 °C for 12 hours. Cool naturally and filter the mixture to remove solid impurities. Recrystallize to obtain the intermediate product. Add 1.036 g (2.0 mmol) of the intermediate product, 1.074 g (12.0 mmol) of cuprous cyanide, and fresh anhydrous DMF to a 100 mL pressure pipette equipped with a Teflon-coated magnetic stir bar. Degas under a nitrogen atmosphere by bubbling for 15 minutes. After complete sealing, the mixture was placed in an oil bath at 152 °C and stirred for 4 days. After the reaction was complete, the mixture was cooled to room temperature, carefully and slowly opened and transferred to a 500 mL round-bottom flask. A mixture of 15 g ferric chloride and 15% (100 mL) hydrochloric acid was prepared and pre-cooled. The aqueous phase containing solids was then filtered out. Finally, the product was purified by recrystallization to obtain the final white solid powder product, which is 2CN-18C6.

[0041] Figure 2 The image shows the hydrogen NMR spectrum of 2CN-18C6, which proves that 2CN-18C6 was successfully synthesized.

[0042] Example 2: Lithium metal battery application of 2,6-dicyano-18-benzocrown ether-6 (2CN-18C6):

[0043] Lithium metal battery assembly: Lithium metal battery assembly such as Figure 3 As shown, it mainly consists of five parts: positive electrode shell, lithium sheet, gasket, spring sheet, and separator. The assembly process is as follows:

[0044] (1) Cut the diaphragm into 19mm circles and put them into a glove box to dry for later use;

[0045] (2) The positive electrode shell, gasket, spring sheet, and negative electrode shell were ultrasonically cleaned three times with ethanol and water, 20 minutes each time, and then dried for later use. Simultaneously, the positive electrode material was prepared: the active material (LiFePO4), conductive agent (acetylene black), and binder (PVDF) were mixed in a weight ratio of 8:1:1, and a certain amount of 1-methyl-2-pyrrolidone (NMP) was added. The mixture was thoroughly stirred in a centrifuge for 30 minutes until a uniform slurry was formed. The slurry was coated onto aluminum foil using a 100 mm scraper. Afterward, it was dried in a vacuum oven at 60°C for 12 hours, then cut into 10 mm diameter discs. The discs were then dried and stored in a glove box for later use.

[0046] (3) Assemble the battery in the following order: positive electrode shell - positive electrode material - electrolyte - separator - electrolyte - lithium sheet - gasket - spring sheet - negative electrode shell.

[0047] Preparation of additive electrolyte: Weigh different amounts of additive 2CN-18C6 and add them to a commercial electrolyte solvent system (1M LiPF6 EC / DMC / EMC=1:1:1, 5.0 wt% FEC) to prepare an electrolyte containing the additive, wherein the additive content is 0.3 wt%, 0.5 wt%, 0.7 wt%, and 1.0 wt%, respectively.

[0048] Assemble and test lithium metal batteries: In an argon-filled glove box, use a pipette to add the prepared electrolyte to both sides of the separator, seal the battery, and test the electrochemical performance of the battery on a battery testing system.

[0049] The current density of electrolytes with different additive concentrations was tested using critical current density measurements, and the results are as follows: Figure 4 As shown, the electrolyte containing 0.7 wt% 2CN-18C6 exhibits the best performance, with a current density reaching 4.4 mA cm⁻¹. -2 It is far superior to containing 0.3wt% (3.8 mAcm) -2 ), 0.5wt% (4.0mA cm -2 ) and 1.0 wt% (3.9 mA cm -2 The electrolyte with additives, and the critical current density of the blank electrolyte battery is only 2.9 mA cm⁻¹. -2 .

[0050] In this invention, 0.7 wt% was selected as the optimal addition content for subsequent electrochemical performance testing.

[0051] Figure 5 The figure shown is at 0.5 mA cm -2 0.5 mAh cm -2 Under certain conditions, lithium-lithium symmetric battery tests were conducted to evaluate the electrochemical stability of lithium-ion intercalation and deintercalation processes after the introduction of additives. The results showed that the battery containing 2CN-18C6 electrolyte had a lower overpotential and better cycle stability. The blank electrolyte group showed a voltage surge after 150 hours, while the cycle life of the group containing 2CN-18C6 could be extended to 500 hours.

[0052] Figure 6 The figure shown is at 0.1 mA cm -2 0.1 mAh cm -2Scanning electron microscope (SEM) images after 20 cycles under the specified conditions. The lithium metal cycling material using the 2CN-18C6 electrolyte exhibits a smooth and dense morphology; while the lithium metal cycling material using the blank electrolyte has a rough surface and shows agglomeration. This indicates that the SEI derived from 2CN-18C6 plays a key role in stabilizing lithium deposition and stripping.

[0053] Figure 7 The figure shown is at 0.5 mA cm -2 0.5 mAh cm -2 The Li-Cu half-cell test results after 10 cycles under the specified conditions. Compared with the blank electrolyte (97.8%), the electrolyte with additive 2CN-18C6 has a coulombic efficiency of 98.2%, indicating that the addition of additive 2CN-18C6 enables more stable lithium deposition and stripping.

[0054] Figure 8 The diagram shows the voltage difference between -0.3V and 0.3V compared to Li. + The Tafel test curves for / Li are shown. The Tafel slope of the group containing 2CN-18C6 is slightly steeper than that of the blank group, and its exchange current density is 0.443 mA cm⁻¹. -2 This is significantly higher than that of standard electrolyte batteries. This indicates that the 2CN-18C6 enhanced battery has a higher limiting current density and faster charge transfer kinetics.

[0055] according to Figure 3 The method shown is used to prepare and test Li||LFP batteries using lithium iron phosphate as the cathode material.

[0056] Figure 9 This is a performance comparison chart of Li||LFP batteries at different rate ranges from 0.1 to 5.0 C. During cycle testing at different current densities (0.1, 0.2, 0.5, 0.7, 1.0, 2.0, and 5.0 C), the discharge capacity of the battery with 0.7wt% 2CN-18C6 showed a slight decreasing trend, at 166.4, 160.0, 152.6, 144.0, and 127.4 mAh g⁻¹, respectively. -1 The blank battery, on the other hand, exhibited a rapidly decreasing discharge capacity, at 162.9, 155.3, 148.1, 128.8, and 100.4 mAh g, respectively. -1 This further confirms that the 2CN-18C6 additive can impart high-rate performance to Li||LFP batteries by reducing interfacial side reactions and inhibiting lithium dendrite growth.

[0057] Depend on Figure 10 and Figure 11 It can be seen that at 30 °C, 2.5–4.0 V vs. Li +Within the voltage range of / Li, the cycle performance of the Li||LiFePO4 lithium metal battery of Example 1 was tested at rates of 0.5 C and 2.0 C. At a rate of 0.5 C, the battery with 0.7 wt% 2CN-18C6 added had an initial discharge specific capacity of 160.1 mAh g. -1 After 300 cycles, the discharge specific capacity is 130.9 mAh g. -1 The capacity retention rate reached 81.76%, and the coulombic efficiency remained above 99% throughout the test. The initial specific discharge capacity of the blank battery was 161.6 mAh g⁻¹. −1 However, with the increase in the number of cycles, the discharge specific capacity decreased rapidly, and after 250 cycles, its discharge specific capacity was only 106.3 mAh g. -1 Its capacity retention rate was only 65.78%. This phenomenon may be due to the formation of an unstable interface layer at the interface during cycling. This layer increases interface impedance, intensifies polarization, and leads to uneven lithium-ion deposition, ultimately resulting in the formation of lithium dendrites. These results indicate that the lithium metal battery in Example 1 with 0.7wt% 2CN-18C6 added exhibits more stable long-cycle performance compared to the blank lithium metal battery. At a 2.0C rate, the initial discharge specific capacity of the battery with 0.7wt% 2CN-18C6 added is 143.4 mAh g⁻¹. -1 After 500 cycles, its discharge specific capacity reaches as high as 137.4 mAh g. -1 The capacity retention rate reached 95.82%, and the coulombic efficiency remained above 99% throughout the test. The initial specific discharge capacity of the blank battery was 143.6 mAh g⁻¹. −1 However, with the increase in the number of cycles, the discharge specific capacity decreased rapidly, and after 300 cycles, its discharge specific capacity was only 109.1 mAh g. -1 Its capacity retention rate is only 75.97%. This excellent cycle stability is attributed to the battery's good interface compatibility and high ionic conductivity.

[0058] according to Figure 3 The method shown is used to prepare and test Li||LCO batteries using lithium cobalt oxide as the cathode material.

[0059] Figure 12This is a performance comparison chart of Li||LCO batteries at different rates from 0.1 to 5.0C. During cycle tests at different rates (0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, and 5.0 C), the discharge capacity of the battery with 0.7wt% 2CN-18C6 added showed a brief downward trend, at 186.7, 181.9, 176.7, 173.1, 169.7, 166, 155.2, and 135.1 mAh g, respectively. -1 Furthermore, its discharge capacity can reach 179.3 mAh g when returning to a low discharge rate of 0.1C. -1 This indicates that the additive-treated battery has excellent rate performance; while the battery using the reference electrolyte exhibited 172.5 mAh g⁻¹ at the initial cycle (0.1C). -1 The discharge capacity was [not specified]. When the rate was gradually increased from 0.1C to 5C and then suddenly dropped to 0.1C, the discharge capacity decreased to 105.9 mAh g. -1 And it continues to decay in subsequent cycles.

[0060] Depend on Figure 13 It can be seen that at 30 °C, 3.0–4.5 V vs. Li + Within the voltage range of / Li, the cycle performance of the Li||LCO lithium metal battery of Example 1 was tested at a rate of 1.0 C. At this rate, the battery with 0.7wt% 2CN-18C6 added had an initial discharge specific capacity of 147.4 mAh g. -1 After 300 cycles, the discharge specific capacity is 125.16 mAh g. -1 The capacity retention rate reached 85%, and the coulombic efficiency remained above 99% throughout the test. The initial specific discharge capacity of the blank battery was 154 mAh g. −1 However, with the increase in the number of cycles, the discharge specific capacity decreased rapidly, and after 300 cycles, its discharge specific capacity was only 75.53 mAh g. -1 Its capacity retention rate is only 47.75%.

[0061] In summary, the 2CN-18C6 provided by this invention can be used as a multifunctional electrolyte additive for lithium metal batteries (LMBs). The electrolyte system containing 0.7% wt of the 2CN-18C6 additive exhibits excellent electrochemical performance in lithium symmetric batteries: at 0.5 mA cm⁻¹... -2At the specified current density, the cycle life of symmetric lithium-ion batteries can reach over 500 hours. Notably, the cycle life and rate performance of Li-LFP full cells are also significantly improved. Scanning electron microscopy and lithium copper half-cell tests demonstrate that 2CN-18C6 can effectively suppress lithium dendrite growth, thereby enhancing the long-term cycle performance of lithium metal batteries.

[0062] This invention provides an important direction for developing high-efficiency electrolyte additives for next-generation lithium metal batteries. Unless otherwise specified, the raw materials and equipment used in this invention are commonly used in the field. Unless otherwise specified, the methods used in this invention are conventional methods in the field.

[0063] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. The application of crown ether-based multifunctional electrolyte additive molecules in lithium metal batteries, characterized in that, The content of the crown ether-based multifunctional electrolyte additive molecule is 0.3%~1.0% of the mass of the lithium metal battery electrolyte; the general structural formula of the multifunctional electrolyte additive molecule is as follows: ； Where R1, R2, and R4 are H, and R3 is CN.

2. The application according to claim 1, characterized in that, The multifunctional electrolyte additive molecules were prepared using the following method: 18-benzocrown ether-6 was dissolved in chloroform, N-bromosuccinimide was added to react, and the intermediate product was obtained by filtration. The intermediate product, cuprous cyanide and N,N-dimethylformamide were mixed and stirred at 152°C for 4 days. The resulting product was purified by recrystallization to obtain the multifunctional electrolyte additive molecule based on crown ether.

3. The application according to claim 2, characterized in that, The molar ratio of 18-benzocrown ether-6 to N-bromosuccinimide is 1:2.

05.

4. The application according to claim 2, characterized in that, The molar ratio of the intermediate product to cuprous cyanide is 1:

6.

5. A lithium metal battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte; characterized in that, The electrolyte comprises an organic solvent, an electrolyte salt, and a crown ether-based multifunctional electrolyte additive molecule; wherein the content of the crown ether-based multifunctional electrolyte additive molecule is 0.3% to 1.0% of the electrolyte mass; the general structural formula of the multifunctional electrolyte additive molecule is as follows: ； Where R1, R2, and R4 are H, and R3 is CN.

6. The lithium metal battery according to claim 5, characterized in that, The content of the crown ether-based multifunctional electrolyte additive molecules is 0.7% of the electrolyte mass.

7. The lithium metal battery according to claim 5, characterized in that, Organic solvents include one or more of diethyl carbonate, acetate carbonate, methyl ethyl carbonate, and fluoroethylene carbonate.

8. The lithium metal battery according to claim 5, characterized in that, The electrolyte salt is lithium hexafluorophosphate.

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