Preparation method and application of lithium metal composite artificial SEI layer

Abstract

The invention belongs to the technical field of lithium batteries, and particularly relates to a preparation method and application of a lithium metal composite artificial SEI layer. Comprising the following steps: preparing a precursor solution; pretreating lithium metal; generating an artificial SEI layer in situ; carrying out post-treatment and drying; sEI layer characterization and quality control are carried out; assembling the lithium metal battery; and performing an electrochemical performance test. The lithium metal composite artificial SEI layer is rich in LiF and sulfide at the same time, the LiF and the sulfide supplement each other, a more compact and stable interface layer is provided, direct contact between an electrolyte and lithium metal is effectively prevented, side reactions are reduced, and the service life of the electrolyte is prolonged. The LiF-rich and sulfide-rich inorganic SEI component of the lithium metal composite artificial SEI layer has better mechanical strength and high ionic conductivity, growth of lithium dendrites can be effectively inhibited, uniform deposition of lithium ions can be induced, the capacity retention ratio of a LiNCM811 total battery can be remarkably improved by the lithium metal composite artificial SEI layer, and meanwhile, the cycle life is prolonged.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium batteries, and particularly relates to a preparation method and application of a lithium metal composite artificial SEI layer. Background Art

[0002] Lithium metal is considered to be one of the most suitable anode materials for electrochemical energy storage systems due to its high theoretical specific capacity (3860 mAh·g⁻²) and low negative electrochemical potential (-3.04 V vs standard hydrogen electrode). In particular, pairing a lithium metal anode with a nickel-rich LiNixMnyCo1-x-yO2 layered cathode material (such as LiNi0.8Co0.1Mn0.1O2 (NMC811)) is one of the most effective material combinations for rechargeable batteries. However, it is highly reactive with most liquid organic electrolytes, especially carbonate-based solutions that are very compatible with currently available 4V lithium-ion layered cathode materials. In carbonate-based electrolytes, the growth of lithium dendrites is often encountered during battery operation, which severely limits the practical application of lithium metal batteries (LMBs).

[0003] To prevent side reactions and maintain the interfacial stability of the lithium metal anode, a stable solid electrolyte interface (SEI) must be formed between the lithium metal and the electrolyte. In recent years, most effective strategies for improving the interfacial stability of lithium metal anodes have focused on electrolyte design, including the use of ether-based electrolytes, high-concentration electrolytes, ionic liquid electrolytes, and some electrolyte additives. Among them, a stable SEI layer is introduced by adjusting the electrolyte degradation process or by consuming specific components of the electrolyte. Nevertheless, when using carbonate-based electrolytes, redox reactions occur on the surface of the lithium metal anode. In addition, due to its high reactivity, a passivation layer is formed on the lithium surface even when stored in a low-contamination glove box. Even with the most effective electrolyte design, these passivation layers cannot be removed from the lithium surface. During spontaneous chemical reactions, the chemical composition of the passivation layer along the lithium surface changes. Therefore, the electrochemical kinetics also changes at different positions of the metal anode, generating an uneven lithium ion flux and triggering the growth of lithium dendrites. Iodic acid has been proposed and successfully removes the passivation layer through a spontaneous reaction. However, the shuttle effect of iodide ions hinders its widespread application in LMBs. If paired with a high-voltage cathode material, irreversible self-discharge behavior occurs during charging and continuously consumes lithium.

[0004] Based on this, the present invention provides a preparation method and application of a lithium metal composite artificial SEI layer to solve the above-mentioned technical problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method and application of a lithium metal composite artificial SEI layer to solve the problems raised in the above background art.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] The first aspect of the present invention:

[0008] A method for preparing a lithium metal composite artificial SEI layer is provided, comprising the following steps:

[0009] S1. Prepare a precursor solution;

[0010] S2. Pretreat the lithium metal;

[0011] S3. In-situ generate an artificial SEI layer;

[0012] S4. Post-treatment and drying;

[0013] S5. Conduct SEI layer characterization and quality control;

[0014] S6. Assemble a lithium metal battery;

[0015] S7. Conduct electrochemical performance tests.

[0016] Preferably, in step S1, fluorosulfonyl carboxylic acid (structural general formula I, wherein R 1 and R 2 are one of fluorine atoms, C 1 -C 5 fluorinated alkyl or fluorinated alkoxy) is weighed in a proportion of 0.5% by volume fraction, and the fluorosulfonyl carboxylic acid is dissolved in an organic solvent. After mixing evenly, a precursor solution is obtained. The organic solvent is one of tetrahydrofuran, ethylene glycol dimethyl ether, dimethyl carbonate or diethyl carbonate. The mixing environment is completed in a glove box under argon protection, H 2 O ≤ 0.1 ppm, O 2 ≤ 0.1 ppm.

[0017] Preferably, in step S2, a lithium foil with a purity ≥ 99.9% is taken, cut into a diameter of 14 mm, and the surface of the lithium foil is ultrasonically cleaned with a DMC or DEC inert solvent to remove surface contaminants. The lithium foil is dried in a glove box and reserved.

[0018] Preferably, in step S3, the pretreated lithium foil is completely immersed in the precursor solution prepared in S1. The reaction conditions are that the immersion time is 10 - 60 minutes and the temperature is controlled at 25 - 40 °C. The reaction steps are as follows:

[0019] The carboxylic acid group of the fluorosulfonyl carboxylic acid reacts with the native passivation layer LiOH and Li 2 CO 3 on the lithium surface to generate a soluble carboxylate salt of lithium, removing the passivation layer;

[0020] The F and S groups in fluorosulfonyl react with the exposed fresh lithium to form a composite SEI layer rich in LiF and including Li 2 S sulfide.

[0021] Preferably, in step S4, the lithium foil is taken out of the precursor solution, washed 3 to 5 times with an organic solvent such as DMC or THF to remove unreacted residues, and vacuum dried at 50 to 80 °C for 2 to 4 hours in an argon environment to ensure that the SEI layer is dense and free of solvent residues.

[0022] Preferably, in step S5, the composition of the SEI layer is analyzed by X-ray photoelectron spectroscopy to verify the distribution of LiF and sulfide, the morphology of the SEI layer is observed using a scanning electron microscope to ensure that it is uniform, dense and crack-free, and the ionic conductivity of the SEI layer is evaluated by electrochemical impedance spectroscopy.

[0023] Preferably, in step S6, the negative electrode of the battery is covered with the composite artificial SEI layer treated in step S4 on the lithium metal, and the positive electrode is selected from LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), LiMn 2 O 4 , LiFePO 4 or LiCoO 2 , preferably NCM811, and a carbonate electrolyte solution including 1M LiPF 6 dissolved in a mixed solvent with a volume ratio of EC / EMC of 3:7 is selected. The separator is a polypropylene (PP) separator. The assembly environment is completed in an argon glove box. The electrolyte addition amount is 50 μL, and the battery type is a CR2025 button cell.

[0024] Preferably, step S7 includes a symmetric cell test and a full cell test;

[0025] For the stacked current test, the steps are as follows:

[0026] The current density is 1 mA·cm -2 , the areal capacity is 1 mAh·cm -2 , and the overpotential change is observed after cycling for 200 hours;

[0027] The variable current density test is 0.5 to 5 mA·cm -2 to verify the stability of the SEI layer at high currents;

[0028] For the full cell test, the steps are as follows:

[0029] The charge-discharge voltage range is 3.0 to 4.3 V. After activation 3 times at 0.1 C, a 1 C rate cycling test is performed;

[0030] The rate performance was tested at 0.05C - 5C to evaluate the capacity retention at high rates.

[0031] Application of the method for preparing a lithium metal composite artificial SEI layer in the assembly of a lithium metal battery.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] The lithium metal composite artificial SEI layer of the present invention is rich in both LiF and sulfides, which complement each other to provide a denser and more stable interfacial layer, effectively preventing the direct contact between the electrolyte and the lithium metal and reducing the occurrence of side reactions. The LiF-rich and sulfide inorganic SEI components of the lithium metal composite artificial SEI layer of the present invention have better mechanical strength and high ionic conductivity, can effectively inhibit the growth of lithium dendrites, and induce the uniform deposition of lithium ions. The lithium metal composite artificial SEI layer of the present invention can significantly improve the cycle performance of Li||Li symmetric batteries, can significantly improve the capacity retention of Li||NCM811 full batteries, and at the same time improve the cycle life. Description of the Drawings

[0034] Figure 1 Cycling performance graphs of Li||Li symmetric batteries using control examples and examples;

[0035] Figure 2 Cycling performance graphs of variable current density Li||Li symmetric batteries using control examples and examples;

[0036] Figure 3 Cycling performance graphs of Li||NCM811 full batteries using control examples and examples at a rate of 1C;

[0037] Figure 4 Rate performance graphs of Li||NCM811 full batteries using control examples and examples at a rate of 1C;

[0038] Figure 5 X-ray photoelectron spectroscopy (XPS) results of different depths of the lithium metal composite artificial SEI layer of the control example; a - b are XPS graphs of the surface layer of the lithium metal negative electrode, and c - d are XPS graphs of the lithium metal negative electrode at 9 nm;

[0039] Figure 6 Schematic structural formula diagram of the fluorosulfonyl carboxylic acid taken in the present invention. Detailed Embodiments

[0040] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0041] Embodiment

[0042] Please refer to Figures 1 to 6 , the present invention provides a method for preparing a lithium metal composite artificial SEI layer, including the following steps:

[0043] S1. Preparation of the precursor solution;

[0044] Weigh fluorosulfonyl carboxylic acid (structural general formula I, please refer to Figure 6 , where R 1 and R 2 are one of fluorine atoms, C 1 -C 5 fluoroalkyl or fluoroalkoxy) in a proportion of 0.5% by volume;

[0045] Dissolve the fluorosulfonyl carboxylic acid in an organic solvent, and after mixing evenly, obtain the precursor solution;

[0046] Selection of organic solvent: one of tetrahydrofuran (THF), ethylene glycol dimethyl ether (DME), dimethyl carbonate (DMC) or diethyl carbonate (DEC);

[0047] Mixing environment: completed in a glove box under argon protection, H 2 O ≤ 0.1 ppm, O 2 ≤ 0.1 ppm;

[0048] S2. Pretreatment of lithium metal;

[0049] Take a lithium foil (purity ≥ 99.9%), and cut it into a diameter of 14 mm;

[0050] Use an inert solvent such as DMC or DEC to ultrasonically clean the surface of the lithium foil to remove surface contaminants;

[0051] Dry the lithium foil in the glove box for standby;

[0052] S3. In-situ generation of the artificial SEI layer;

[0053] Completely immerse the pretreated lithium foil in the precursor solution prepared in S1;

[0054] Reaction conditions: the soaking time is 10 to 60 minutes, and the temperature is controlled at 25 to 40 °C;

[0055] Reaction mechanism:

[0056] The carboxylic acid group of fluorosulfonyl carboxylic acid reacts with the native passivation layer LiOH and Li on the lithium surface 2 CO 3 to form a soluble lithium carboxylate salt, removing the passivation layer;

[0057] The F and S groups in fluorosulfonyl react with the exposed fresh lithium to form a composite SEI layer rich in LiF and including Li 2 S sulfide;

[0058] S4. Post-treatment and drying;

[0059] Take out the lithium foil from the precursor solution and wash it 3 - 5 times with DMC or THF organic solvents to remove unreacted residues;

[0060] In an argon atmosphere, vacuum dry at 50 - 80 °C for 2 - 4 hours to ensure that the SEI layer is dense and free of solvent residues;

[0061] S5. SEI layer characterization and quality control;

[0062] Please refer to Figure 5 , analyze the SEI layer composition by X-ray photoelectron spectroscopy (XPS) to verify the distribution of LiF and sulfide;

[0063] Use a scanning electron microscope to observe the morphology of the SEI layer to ensure that it is uniform, dense and crack-free;

[0064] Perform electrochemical impedance spectroscopy tests to evaluate the ionic conductivity of the SEI layer;

[0065] S6. Lithium metal battery assembly;

[0066] The negative electrode is covered with lithium metal with the composite artificial SEI layer treated in step S4;

[0067] The positive electrode is selected from LiNi 0.8 Co 0.1 Mn 0.1 O 2 (NCM811), LiMn 2 O 4 , LiFePO 4 or LiCoO 2 , preferably NCM811;

[0068] Select a carbonate electrolyte including 1M LiPF 6 dissolved in a mixed solvent of EC / EMC with a volume ratio of 3:7;

[0069] The separator is a polypropylene (PP) separator;

[0070] The assembly environment was completed in an argon glove box, and the electrolyte addition amount was 50 μL;

[0071] The battery type is CR2025 button cell;

[0072] S7. Electrochemical performance tests were carried out;

[0073] Symmetric cell test (Li||Li):

[0074] Please refer to Figure 1 , with a current density of 1 mA·cm -2 , and an areal capacity of 1 mAh·cm -2 , and the overpotential change was observed after cycling for 200 hours;

[0075] Please refer to Figure 2 , variable current density test (0.5 - 5 mA·cm -2 ), to verify the stability of the SEI layer at high current;

[0076] Full cell test (Li||NCM811):

[0077] Please refer to Figure 3 , with a charge-discharge voltage range of 3.0 - 4.3 V, after 3 activations at 0.1 C, 1 C rate cycling test;

[0078] Please refer to Figure 4 , rate performance test (0.05 C - 5 C), to evaluate the capacity retention rate at high rates.

[0079] Control example

[0080] Prepare the precursor solution of the lithium metal composite artificial SEI layer: Add fluorosulfonyl carboxylic acid with a volume fraction of 0.5%, and the rest is an organic solvent. The above process is carried out in a glove box filled with argon (H2O ≤ 0.1 ppm, O2 ≤ 0.1 ppm);

[0081] Please refer to Figure 1 , and use the cyclic performance graphs of the Li||Li symmetric cells of the control example and the example;

[0082] The assembly and testing of the battery, specifically, include the following steps:

[0083] Select a CR2025 button cell case. The battery assembly is carried out in a glove box filled with argon, with an electrolyte addition amount of 50 μl, and polypropylene is used as the separator.

[0084] For the Li||Li symmetric cell, two 14-mm-diameter lithium foils are assembled together and tested at a current density of 1 mA·cm-2 and an areal capacity of 1 mAh·cm-2;

[0085] Li||NCM811 batteries, using the prepared lithium metal anodes of the control example and the examples, with a test voltage range of 3 - 4.3V. First, charge and discharge at a current of 0.1C for 3 times, and then perform cyclic tests at a rate of 1C;

[0086] All batteries were tested on a NEWARE battery test system.

[0087] Using a carbonate-based electrolyte (1M LiPF6 dissolved in an EC / EMC solvent with a volume ratio of 3:7, this electrolyte is used in all the technical solutions of this patent), it can be found that the symmetric battery using the control example maintained a relatively low overpotential within 200h, while the symmetric battery using the example showed a serious voltage increase near 100h, indicating that this composite artificial SEI layer helps to improve the cycle life of the lithium metal anode;

[0088] Please refer to Figure 2 , the cyclic performance diagrams of the variable current density Li||Li symmetric batteries using the control example and the examples;

[0089] The symmetric battery using the control example can maintain a relatively stable voltage curve at a larger current density, and the overpotential increases relatively slowly;

[0090] While the symmetric battery using the example showed large voltage fluctuations at a current density of 2.5 mA·cm-2, further indicating that the surface of this composite artificial SEI layer has good reversibility of lithium deposition;

[0091] Please refer to Figure 3 , the cyclic performance diagrams of the Li||NCM811 full batteries using the control example and the examples at a rate of 1C;

[0092] The Li||NCM811 battery using the control example still had a capacity retention rate of 67.04% after 300 cycles, and the curve was stable. While the Li||NCM811 battery using the example showed a large capacity decay after 80 cycles, proving that the composite artificial SEI layer successfully improved the capacity retention rate and cycle life of the lithium metal battery;

[0093] Please refer to Figure 4 , the rate performance diagrams of the Li||NCM811 full batteries using the control example and the examples at a rate of 1C;

[0094] The electrochemical performances of the full batteries using the control example and the examples were tested at rates of 0.05C, 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 0.1C. It can be seen that the specific capacity of the example decayed relatively fast as the rate increased. The specific capacity of the control example at high rates of 2C and 5C was much larger than that of the example, indicating that the lithium metal anode with the composite artificial SEI layer had excellent rate performance and good stability of the artificial SEI layer;

[0095] Please refer to Figure 5 , the X-ray photoelectron spectroscopy (XPS) results of different depths of the lithium metal composite artificial SEI layer of the comparative example;

[0096] It can be seen from the XPS results that an artificial SEI layer rich in LiF / sulfide is formed on the surface of the comparative example, and the proportion of inorganic components such as LiF / sulfide is higher inside the SEI layer;

[0097] In summary, the lithium metal composite artificial SEI layer in the present invention constructs a protective layer with lithiophilic properties, which can achieve uniform deposition of lithium ions, enabling the lithium metal battery to have a longer cycle life and a higher specific capacity.

[0098] In the description of this specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0099] The preferred embodiments of the present invention disclosed above are only used to help illustrate the present invention. The preferred embodiments do not elaborate all the details, nor do they limit the invention to the specific embodiments described. Obviously, many modifications and variations can be made according to the content of this specification. These embodiments are selected and specifically described in this specification to better explain the principles and practical applications of the present invention, so that those skilled in the art can well understand and utilize the present invention. The present invention is only limited by the claims and their full scope and equivalents.

Claims

1. A method for preparing a lithium metal composite artificial SEI layer, characterized in that: The following steps are involved: S1. Prepare precursor solution; S2. Lithium metal pretreatment; S3. In situ generation of artificial SEI layer; S4. Post-processing and drying; S5. Perform SEI layer characterization and quality control; S6. Assembling lithium metal batteries; S7. Conduct electrochemical performance test.

2. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: In the step S1, fluorosulfonyl carboxylic acid is weighed at a volume fraction of 0.5%, and the fluorosulfonyl carboxylic acid is dissolved in an organic solvent, and mixed evenly to obtain a precursor solution, wherein the organic solvent is one of tetrahydrofuran, ethylene glycol dimethyl ether, dimethyl carbonate or diethyl carbonate, and the mixing environment is completed in an argon-protected glove box, and H2O≤0.1ppm, O2≤0.1ppm.

3. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: In step S2, lithium foil with a purity of ≥99.9% is taken and cut into a diameter of 14 mm. The surface of the lithium foil is ultrasonically cleaned using an inert solvent such as DMC or DEC to remove surface pollutants. The lithium foil is dried in a glove box for later use.

4. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: In step S3, the pretreated lithium foil is completely immersed in the precursor solution prepared in S1. The reaction conditions are as follows: the immersion time is 10 to 60 minutes and the temperature is controlled at 25 to 40° C. The reaction steps are: The carboxylic acid group of the fluorosulfonyl carboxylic acid reacts with the native passivation layer LiOH and Li2CO3 on the lithium surface to generate a soluble salt of lithium carboxylate, and the passivation layer is removed; The F and S groups in the fluorosulfonyl groups react with the exposed fresh lithium to generate a composite SEI layer rich in LiF and including Li2S sulfides.

5. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: In step S4, the lithium foil is taken out from the precursor solution, washed with DMC or THF organic solvent for 3 to 5 times to remove unreacted residues, and vacuum dried at 50 to 80° C. for 2 to 4 hours in an argon environment to ensure that the SEI layer is dense and free of solvent residues.

6. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: The step S5 analyzes the SEI layer composition by X-ray photoelectron spectroscopy to verify the distribution of LiF and sulfide, observes the SEI layer morphology using a scanning electron microscope to ensure that it is uniform, dense and crack-free, and performs an electrochemical impedance spectroscopy test to evaluate the ionic conductivity of the SEI layer.

7. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: In step S6, the negative electrode of the battery is covered with lithium metal using the composite artificial SEI layer treated in step S4, and the positive electrode is LiNi 0.8 Co 0.1 Mn 0.1 O2, LiMn2O4, LiFePO4 or LiCoO2, preferably NCM811, a carbonate electrolyte comprising 1MLiPF6 dissolved in a mixed solvent of EC / EMC with a volume ratio of 3:7 is selected, the diaphragm is a polypropylene diaphragm, the assembly environment is completed in an argon glove box, the electrolyte addition amount is 50μL, and the battery type is CR2025 button battery.

8. The method for preparing a lithium metal composite artificial SEI layer according to claim 1, characterized in that: The step S7 includes a symmetrical battery test and a full battery test; The stacking current test comprises the following steps: The current density was 1 mA·cm-2, the surface capacity was 1 mAh·cm-2, and the overpotential change was observed after 200 hours of cycling; The variable current density test was 0.5~5mA·cm-2 to verify the stability of the SEI layer under high current; The full battery test comprises the following steps: The charge and discharge voltage range is 3.0~4.3V, after 0.1C activation for 3 times, 1C rate cycle test; The rate performance test is 0.05C~5C to evaluate the capacity retention rate at high rates.

9. Application of the method for preparing a lithium metal composite artificial SEI layer according to any one of claims 1 to 8 in lithium metal battery assembly.

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