Preparation method and application of lithium metal negative electrode bismuth / flour / nitrogen hybrid composite protective layer

Abstract

The application discloses a preparation method of a lithium metal negative electrode bismuth / fluorine / nitrogen hybrid composite protective layer and application thereof, and is characterized in that bismuth trifluoromethanesulfonate and lithium nitrate are mixed and stirred in an organic solvent until completely dissolved, and then the obtained reaction solution is added dropwise to the surface of a lithium sheet, so that a bismuth / fluorine / nitrogen hybrid composite protective layer is generated in situ on the surface of the lithium sheet. x NO y Components, under the synergistic effect of all the components, a protective layer with ion conductivity, high mechanical strength and electrochemical stability is successfully constructed, the protective layer can adapt to the working conditions of large current density and large area capacity, and effectively inhibits the growth of lithium dendrites and the occurrence of lithium volume expansion effect.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal battery technology, specifically relating to a method for preparing and applying a bismuth / fluorine / nitrogen hybrid protective layer for lithium metal anodes. Background Technology

[0002] With the rapid development of consumer electronics, new energy vehicles, and energy storage systems, the market demand for high-energy-density lithium-ion batteries is gradually increasing. The energy density of lithium-ion batteries using graphite as the anode is nearing its limit, making a significant breakthrough unlikely in the short term. Lithium metal batteries, using lithium metal as the anode material, have received extensive research in recent years due to their higher energy density. However, poor cycle stability and concerning safety issues remain unresolved. Highly active lithium metal can chemically react with electrolyte components, forming a solid electrolyte interface composed of inorganic and organic substances. While a thinner solid electrolyte interface can prevent direct contact between the lithium metal anode and the electrolyte to some extent, thus reducing side reactions, poor mechanical strength during long-term cycling still leads to lithium dendrite growth and lithium volume expansion. When fresh lithium metal is exposed due to the cracking of the solid electrolyte interface, electrolyte components react with it, resulting in reduced coulombic efficiency and poor cycle stability. Inherent defects on the lithium metal surface and repeated changes in the anode / electrolyte interface during lithium insertion / stripping accelerate the formation and growth of lithium dendrites. Once the growing lithium dendrites pierce the separator, the battery will face safety issues due to short circuits. Therefore, to promote the development of high-energy-density lithium metal batteries, it is necessary to solve the complex interface problems faced by lithium metal anodes. Many strategies have been designed to address these challenges, such as electrolyte engineering, solid electrolytes, three-dimensional conductive frameworks, and artificial protective layers. Among these strategies, designing an artificial protective layer on the surface of the lithium metal anode is considered a simple and effective strategy for protecting the lithium metal anode.

[0003] Different protective layer materials possess varying physical and electrochemical properties. Some materials may exhibit better mechanical strength to suppress lithium dendrite growth, while others may demonstrate higher ionic conductivity to facilitate lithium-ion transport. Different composite material structures also provide varying shielding and support effects for lithium metal; for example, some structures are more effective at preventing side reactions, while others may be better at homogenizing lithium-ion flow. Furthermore, different protective layers exhibit varying chemical compatibility with the electrolyte and lithium metal, affecting their mode of operation and effectiveness. Some protective layers offer better resistance to electrolyte erosion and chemical reactions. They display different interfacial behaviors at the interface with lithium metal and electrolyte, including interfacial energy and interfacial resistance, which directly impact the lithium insertion / extraction process and the overall battery performance. Therefore, the rational design of artificial protective layers on the lithium metal anode surface is particularly important.

[0004] Patent CN 110071284 A designs a lithium-antimony alloy / lithium fluoride composite layer, but in ether electrolytes, even at low current density and low areal capacity (1 mA cm⁻¹), it exhibits limitations. -2 1mAh cm -2 Under the test conditions, its symmetrical battery cycle life can only reach 600h. Patent CN 109037594 A designed a self-healing organic protective layer to protect the lithium metal anode, but in an ether electrolyte containing 1wt.% LiNO3 additive, its symmetrical battery exhibits low current density and low areal capacity (1mA cm⁻¹). -2 1mAh cm -2 Under the test conditions, polarization gradually occurred after only about 300 hours of cycling. The literature "ALi3Bi / LiF interfacial layer enabling highly stable lithium metal Anode" designed a lithium metal anode protective layer containing Li3Bi and LiF. The solvent used in its preparation, methane sulfoxide, has a boiling point of 189℃, which is a high-boiling-point substance with low volatility at room temperature, leading to solvent residue issues. Furthermore, this purely inorganic composite protective layer struggles to achieve long-term cycling at high current density and large area capacity even in an ether electrolyte containing 2 wt.% LiNO3 additive, even at 10 mA cm⁻¹. -2 1mAh cm -2 Under the test conditions, the battery could only cycle for 400 hours. The literature "Rational construction Ag / LiF / sulfide rich protective layer for remarkably enhancing the stability of Li metal anodes" designed a composite protective layer containing silver / fluorine / sulfur. Its symmetrical battery, in an ether electrolyte containing 2 wt.% LiNO3 additive and at a current density of 5 mA / cm², was successfully tested. -2 , with a capacity of 5mAh cm -2 Under the test conditions, its cycle life also failed to exceed 200h.

[0005] In summary, existing artificial protective layers are limited to low current density and small cycle area capacity conditions. When the current density and cycle area capacity are increased, these protective layers often fail to provide long-term effectiveness. Therefore, the interface problem of lithium metal anodes remains unresolved. Thus, there is an urgent need to design artificial protective layers that can be scalably applied to high current density and large capacity conditions to effectively address the interface problem faced by lithium metal anodes, thereby achieving long-term stable battery cycling. Summary of the Invention

[0006] To address the complex interface issues of lithium metal anodes, this invention proposes a method for preparing a bismuth / fluorine / nitrogen hybrid composite protective layer for lithium metal anodes. This protective layer can adapt to high current density and large area capacity conditions, effectively suppressing the growth of lithium dendrites and the occurrence of lithium volume expansion effects.

[0007] To achieve its objectives, the present invention employs the following technical solution:

[0008] A method for preparing a bismuth / fluorine / nitrogen hybrid composite protective layer for lithium metal anodes, comprising the following specific steps:

[0009] Step 1: Add bismuth trifluoromethanesulfonate to an organic solvent and stir magnetically at room temperature until completely dissolved. Then add lithium nitrate and continue stirring magnetically at room temperature until completely dissolved to obtain a reaction solution.

[0010] Step 2: Under an inert gas atmosphere, the reaction solution obtained in Step 1 is dropped onto the surface of the lithium sheet, left to stand until the organic solvent has completely evaporated, then washed with organic solvent to remove excess impurities, and dried at room temperature, thus generating a bismuth / fluorine / nitrogen hybrid composite protective layer in situ on the surface of the lithium sheet.

[0011] Preferably, the organic solvent is ethylene glycol dimethyl ether. Ethylene glycol dimethyl ether is a commonly used solvent in ether electrolytes; it has a low boiling point and high volatility at room temperature, effectively mitigating the negative impacts caused by residual low-volatility solvents.

[0012] Preferably, the mass percentage of bismuth trifluoromethanesulfonate and lithium nitrate is 20-80%:80-20%. Different mass ratios of bismuth trifluoromethanesulfonate and lithium nitrate in the reaction solution result in different chemical reaction environments on the surface of the lithium metal anode, leading to differences in the structure of the protective layer and consequently, different effects.

[0013] The present invention also provides a lithium metal anode, comprising a lithium sheet and a bismuth / fluorine / nitrogen hybrid composite protective layer generated in situ on the surface of the lithium sheet.

[0014] The present invention further provides a lithium metal battery, comprising the above-mentioned lithium metal negative electrode, separator, electrolyte and positive electrode.

[0015] Preferably, the diaphragm is a dry-process polypropylene diaphragm.

[0016] Preferably, the electrolyte is an ester-based electrolyte or an ether-based electrolyte.

[0017] Preferably, the active material of the positive electrode is lithium iron phosphate or nickel cobalt aluminum.

[0018] Compared with the prior art, the beneficial effects of the present invention are reflected in:

[0019] The composite protective layer obtained by this invention is rich in Bi, Li3Bi, -CF3, LiF, and Li x NO y The composition includes: Bi, which guides uniform lithium deposition; Li3Bi, as a fast ion conductor, effectively mitigates lithium dendrite growth and lithium volume expansion; and CF3, LiF, and Li. x NO y The protective layer provides insulation and mechanical stability to the interface, ensuring long-term cycling of lithium under its protective layer. Through the synergistic effect of all components, an ionicly conductive, mechanically strong, and electrochemically stable protective layer was successfully constructed. This layer can withstand high current density and large area capacity conditions, effectively suppressing lithium dendrite growth and lithium volume expansion. When matched with commercially available nickel-cobalt-aluminum ternary cathodes or lithium iron phosphate cathodes, the batteries with the protective layer exhibited lower capacity decay, thus demonstrating promising commercial application prospects. Attached Figure Description

[0020] Figure 1 The images show scanning electron microscope (SEM) images of the surface (a) and cross-section (b) of the lithium sheet protected by the bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1. The inset in (a) is an optical photograph of the protective layer.

[0021] Figure 2 The surface elemental analysis diagram of the bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1 is shown.

[0022] Figure 3 The image shows the X-ray photoelectron spectrum of the bismuth / fluorine / nitrogen hybrid protective layer prepared in Example 1.

[0023] Figure 4 The Young's modulus diagram is shown for the bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1.

[0024] Figure 5 The symmetrical battery assembled in Example 1 using lithium sheets with and without bismuth / fluorine / nitrogen hybrid protective layers operates at 5 mA / cm². -2 5mAh cm -2 Voltage-time curve under test conditions.

[0025] Figure 6 The symmetrical battery assembled in Example 1 using lithium sheets with and without bismuth / fluorine / nitrogen hybrid protective layers operates at 5 mA / cm². -2 5mAh cm -2 Charge-discharge curves under test conditions.

[0026] Figure 7The symmetrical battery assembled in Example 1 using lithium sheets with and without bismuth / fluorine / nitrogen hybrid protective layers operates at 10 mA / cm². -2 5mAh cm -2 Voltage-time curve under test conditions.

[0027] Figure 8 The symmetrical battery assembled in Example 1 using lithium sheets with and without bismuth / fluorine / nitrogen hybrid protective layers operates at 10 mA / cm². -2 10mAh cm -2 Voltage-time curve under test conditions.

[0028] Figure 9 The symmetrical battery assembled in Example 1 using lithium sheets with and without bismuth / fluorine / nitrogen hybrid protective layers operates at 5 mA / cm². -2 5mAh cm -2 Scanning electron microscope (SEM) images after 20 cycles under test conditions, where (a) and (b) correspond to lithium sheets without protective layers, and (c) and (d) correspond to lithium sheets with protective layers.

[0029] Figure 10 The graph shows the performance of the Li / LFP full cell assembled in Example 1 from lithium sheets with bismuth / fluorine / nitrogen hybrid composite protective layers and lithium sheets without protective layers.

[0030] Figure 11 The graph shows the performance of the Li / NCA full cell assembled in Example 1 from lithium sheets with and without bismuth / fluorine / nitrogen hybrid composite protective layers.

[0031] Figure 12 The symmetrical battery assembled from lithium sheets with a bismuth / fluorine / nitrogen hybrid protective layer in Example 2 operates at 5 mA / cm². -2 5mAh cm -2 Voltage-time curve under test conditions.

[0032] Figure 13 For the symmetrical cell of Comparative Example 1 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions, with the inset showing an optical photograph of the protective layer.

[0033] Figure 14 For the symmetrical cell of Comparative Example 2 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions, with the inset showing an optical photograph of the protective layer.

[0034] Figure 15For the symmetrical cell of Comparative Example 3 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions, with the inset showing an optical photograph of the protective layer. Detailed Implementation

[0035] The technical solutions of the present invention are further described and explained below with reference to embodiments, but these embodiments are not intended to limit the scope of protection of the present invention.

[0036] Example 1

[0037] This embodiment provides a method for preparing a bismuth / fluorine / nitrogen hybrid protective layer for lithium metal anodes, including the following steps:

[0038] Step 1: Add 104 mg of bismuth trifluoromethanesulfonate to 2 mL of ethylene glycol dimethyl ether and stir magnetically at room temperature for 12 h until completely dissolved. Then add 26 mg of lithium nitrate and stir magnetically at room temperature for 12 h until completely dissolved to obtain a homogeneous reaction solution.

[0039] Step 2: In an argon-protected glove box, 50 μL of the reaction solution obtained in Step 1 is dropped onto one side of a lithium sheet with a diameter of 16 cm. The solution is allowed to stand until the organic solvent has completely evaporated. Then, it is washed with ethylene glycol dimethyl ether to remove excess impurities and dried at room temperature. This results in the in-situ formation of a bismuth / fluorine / nitrogen hybrid protective layer on the surface of the lithium sheet.

[0040] The lithium wafer protected by the bismuth / fluorine / nitrogen hybrid composite protective layer obtained in this embodiment was characterized as follows:

[0041] 1. Characterization of the morphology and composition of the protective layer

[0042] Figure 1 The image shows scanning electron microscope (SEM) images of the surface and cross-section of the lithium sheet protected by the bismuth / fluorine / nitrogen hybrid protective layer prepared in Example 1. As can be seen from the image, the protective layer on the surface of the lithium sheet exhibits a dense morphology with a thickness of 19 μm.

[0043] Figure 2 Surface elemental analysis of the bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1 shows that C, O, Bi, F, and N elements are uniformly distributed on the lithium sheet surface. The presence of these elements indicates that bismuth trifluoromethanesulfonate and lithium nitrate have undergone chemical reactions with lithium, respectively.

[0044] To further determine the composition of this protective layer, Figure 3 X-ray photoelectron spectroscopy (XPS) spectra of the bismuth / fluorine / nitrogen hybrid protective layer prepared in Example 1 are given. The results show that the main components of the protective layer are Bi, Li3Bi, -CF3, LiF, and Li x NO y .

[0045] Figure 4 The image shows the Young's modulus of the bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1. The average Young's modulus is 6.04 GPa. This high Young's modulus will help enhance the suppression of lithium dendrites and volume expansion.

[0046] 2. Half-cell performance characterization

[0047] The lithium sheet with a bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1 was used as the electrode and assembled into a symmetrical battery with an ether electrolyte and a dry-processed polypropylene membrane. A lithium sheet without the protective layer was used as a reference for comparative studies. The ether electrolyte consisted of 1 M LiTFSI dissolved in a mixed solution of 1,3-dioxolane / ethylene glycol dimethyl ether (volume ratio 1:1), with 1 wt% LiNO3 added as an additive. The electrolyte injection volume was 50 μL.

[0048] Figure 5 For two symmetrical cells at 5mA cm -2 5mAh cm -2 The voltage-time curves under the test conditions show that the symmetric battery with the bismuth / fluorine / nitrogen hybrid composite protective layer can cycle stably for more than 2000 hours under high current density and large cycle area capacity without obvious polarization, while the symmetric battery without the protective layer has already experienced severe polarization after 210 hours.

[0049] Figure 6 For two symmetrical cells at 5mAcm -2 5mAh cm -2 The charge-discharge curves under different number of cycles under test conditions show that, compared with the symmetric battery without a protective layer, the symmetric battery with a bismuth / fluorine / nitrogen hybrid protective layer exhibits a smaller charge-discharge voltage plateau, which is consistent with the lower voltage polarization.

[0050] To further verify the cycling performance of symmetric cells with bismuth / fluorine / nitrogen hybrid protective layers at higher current densities and higher cycle areal capacity. Figure 7 and Figure 8 Two symmetrical cells are given in (10 mA cm⁻¹) -2 5mAh cm -2 ) and (10mAcm -2 10mAh cm -2The voltage-time curves under the test conditions show that the symmetric battery with the bismuth / fluorine / nitrogen hybrid composite protective layer can still exhibit better cycle performance than the symmetric battery without the protective layer. This indicates that even under high current density and large cycle area capacity, the protective layer can still effectively alleviate the growth of lithium dendrites and the volume expansion effect of lithium.

[0051] Figure 9 The lithium metal anode with a bismuth / fluorine / nitrogen hybrid protective layer and the lithium metal anode without a protective layer are compared at 5 mA / cm. -2 5mAh cm -2 The scanning electron microscope images after 20 cycles under the test conditions clearly show that lithium without the protective layer exhibits more cracks and is accompanied by a large volume expansion, while the lithium surface protected by the bismuth / fluorine / nitrogen hybrid composite protective layer still maintains a relatively dense morphology. This directly demonstrates the inhibitory effect of the prepared protective layer on the growth of lithium dendrites and the volume expansion effect of lithium.

[0052] 3. Performance Characterization of Li / LFP Full Cells

[0053] The lithium sheet with a bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1 was used as the metal anode. It was then assembled with a lithium iron phosphate cathode, an ether electrolyte (1M LiTFSI dissolved in a mixed solution of 1,3-dioxolane / ethylene glycol dimethyl ether (volume ratio 1:1), with 1wt% LiNO3 added as an additive), and a dry-processed polypropylene separator to form a Li / LFP full cell. A full cell assembled with an unprotected lithium sheet as the metal anode was used as a reference for comparative research. The preparation process of the lithium iron phosphate cathode was as follows: lithium iron phosphate, carbon black, and polyvinylidene fluoride were dispersed in 1-methyl-2-pyrrolidone at a mass ratio of 8:1:1, ball-milled for 1 hour, coated onto an aluminum foil surface, vacuum dried at 80°C for 12 hours, and then punched into a 14mm diameter disc for later use.

[0054] The battery cycle test conditions are: voltage range of 2-3.8V, electrolyte injection coefficient of 0.1mL / mg.

[0055] Figure 10 The cycling performance graphs for the two types of Li / LFP full cells show that the cell protected by the protective layer maintained a capacity retention rate of up to 91.1% after 400 cycles, while the cell without the protective layer showed a faster capacity decay trend.

[0056] 4. Performance Characterization of Li / NCA Full Cells

[0057] The lithium sheet with a bismuth / fluorine / nitrogen hybrid composite protective layer prepared in Example 1 was used as the metal anode. It was then assembled with a nickel-cobalt-aluminum ternary cathode, an ester electrolyte (1M LiPF6 dissolved in ethylene carbonate / ethyl methyl carbonate / dimethyl carbonate (volume ratio 1:1:1)), and a dry-processed polypropylene separator to form a Li / NCA full cell. A full cell assembled with an unprotected lithium sheet as the metal anode was used as a reference for comparative research. The preparation process of the nickel-cobalt-aluminum ternary cathode is as follows: the positive electrode active material NCA (LiNi... 0.8 Co 0.15 Al 0.05 O2), carbon black, and polyvinylidene fluoride were dispersed in 1-methyl-2-pyrrolidone at a mass ratio of 8:1:1 and ball-milled for 1 hour. The mixture was then coated onto the surface of aluminum foil, vacuum dried at 80°C for 12 hours, and punched into discs with a diameter of 14 mm for later use.

[0058] The cycle test conditions for the full battery are: voltage range of 3-4.3V and electrolyte injection coefficient of 0.1mL / mg.

[0059] Figure 11 The performance graphs of the two Li / NCA full cells show that the capacity decay rate of the cell protected by the protective layer is significantly lower than that of the cell without the protective layer after 200 cycles. This indicates that the introduction of the bismuth / fluorine / nitrogen hybrid composite protective layer reduces the negative effects caused by interface problems.

[0060] The above results demonstrate that the introduction of the bismuth / fluorine / nitrogen hybrid composite protective layer suppresses lithium dendrite growth and lithium volume expansion under high current density and large cycle area capacity, effectively solving the complex interface problems faced by lithium metal anodes. When matched with commercially available lithium iron phosphate cathodes or nickel-cobalt-aluminum ternary cathodes, the batteries with the protective layer exhibited lower capacity decay, showing great promise for commercial applications.

[0061] Example 2

[0062] This embodiment provides a method for preparing a bismuth / fluorine / nitrogen hybrid protective layer for lithium metal anodes, including the following steps:

[0063] Step 1: Add 78 mg of bismuth trifluoromethanesulfonate to 2 mL of ethylene glycol dimethyl ether and stir magnetically at room temperature for 12 h until completely dissolved. Then add 52 mg of lithium nitrate and stir magnetically at room temperature for 12 h until completely dissolved to obtain a homogeneous reaction solution.

[0064] Step 2: In an argon-protected glove box, 50 μL of the reaction solution obtained in Step 1 is dropped onto one side of a lithium sheet with a diameter of 16 cm. The solution is allowed to stand until the organic solvent has completely evaporated. Then, it is washed with ethylene glycol dimethyl ether to remove excess impurities and dried at room temperature. This results in the in-situ formation of a bismuth / fluorine / nitrogen hybrid protective layer on the surface of the lithium sheet.

[0065] Following the same method as in Example 1, the lithium sheet with a bismuth / fluorine / nitrogen hybrid composite protective layer prepared in this example was assembled into a symmetrical battery using the electrode. Figure 12 To correspond to the symmetrical cell at 5mAcm -2 5mAh cm -2 Voltage-time curves under test conditions. The results show that by changing the mass of bismuth trifluoromethanesulfonate and lithium nitrate, the designed composite protective layer can still suppress lithium dendrite growth and lithium volume expansion under high current density and large cycle area capacity.

[0066] Comparative Example 1

[0067] The difference from Examples 1 and 2 is that the solute in the reaction solution is only bismuth trifluoromethanesulfonate, specifically:

[0068] Step 1: Add 130 mg of bismuth trifluoromethanesulfonate to 2 mL of ethylene glycol dimethyl ether and stir magnetically at room temperature for 12 h until completely dissolved to obtain a reaction solution.

[0069] Step 2: In an argon-protected glove box, 50 μL of the reaction solution obtained in Step 1 is dropped onto one side of a lithium sheet with a diameter of 16 cm. The solution is allowed to stand until the organic solvent has completely evaporated. Then, it is washed with ethylene glycol dimethyl ether to remove excess impurities and dried at room temperature. This results in the in-situ formation of a bismuth / fluorine composite protective layer on the surface of the lithium sheet.

[0070] The lithium sheet with a bismuth / fluorine composite protective layer prepared in this comparative example was assembled into a symmetrical battery using the same method as in Example 1. Figure 13 For the symmetrical cell of Comparative Example 1 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions. The embedded image shows that the protective layer has a black surface, which is the result of the reaction between bismuth trifluoromethanesulfonate and lithium. The curves show that the symmetrical battery experienced a short circuit due to its inability to withstand the test conditions of high current density and large capacity.

[0071] Comparative Example 2

[0072] The difference from Examples 1 and 2 is that the solute in the reaction solution is only lithium nitrate, specifically:

[0073] Step 1: Add 130 mg of lithium nitrate to 2 mL of ethylene glycol dimethyl ether and stir magnetically at room temperature for 12 h until completely dissolved to obtain a reaction solution.

[0074] Step 2: In an argon-protected glove box, add 50 μL of the reaction solution obtained in Step 1 to one side of a lithium sheet with a diameter of 16 cm. Let it stand until the organic solvent has completely evaporated, then wash with ethylene glycol dimethyl ether to remove excess impurities, and dry at room temperature to form a nitrogen protective layer on the surface of the lithium sheet in situ.

[0075] The nitrogen-protected lithium sheet prepared in this comparative example was assembled into a symmetrical cell using the same method as in Example 1. Figure 14 For the symmetrical cell of Comparative Example 2 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions. The embedded image shows scattered white areas on the lithium sheet surface, a result of the reaction between lithium nitrate and lithium. The curves indicate that the cycle stability of this symmetrical battery is poor, with polarization beginning to occur after 72 hours of cycling.

[0076] Comparative Example 3

[0077] The difference from Examples 1 and 2 is that the solute in the reaction solution is bismuth nitrate, specifically:

[0078] Step 1: Add 130 mg of bismuth nitrate to 2 mL of ethylene glycol dimethyl ether and stir magnetically at room temperature for 12 h until completely dissolved to obtain a reaction solution.

[0079] Step 2: In an argon-protected glove box, add 50 μL of the reaction solution obtained in Step 1 to one side of a lithium sheet with a diameter of 16 cm. Let it stand until the organic solvent has completely evaporated, then wash with ethylene glycol dimethyl ether to remove excess impurities, and dry at room temperature. This will generate a bismuth / nitrogen protective layer in situ on the lithium sheet surface.

[0080] The lithium sheet with bismuth / nitrogen protective layer prepared in this comparative example was assembled into a symmetrical battery using the same method as in Example 1. Figure 15 For the symmetrical cell of Comparative Example 3 at 5 mA cm -2 5mAh cm -2 Voltage-time curves under test conditions. The embedded image shows that the protective layer has a black surface, which is the result of the reaction between bismuth nitrate and lithium. The curves show that under high current density and large area capacity test conditions, the polarization of this symmetrical battery gradually becomes severe after 210 hours of cycling.

[0081] The above embodiments are merely preferred examples of the present invention, intended to illustrate the technical concept and features of the present invention, and are not intended to limit the implementation methods. Those skilled in the art can make changes and modifications according to the spirit and essence of the present invention, and these changes and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a bismuth / fluorine / nitrogen hybrid protective layer for lithium metal anodes, characterized in that, The steps are as follows: Step 1: Add bismuth trifluoromethanesulfonate to an organic solvent and stir magnetically at room temperature until completely dissolved. Then add lithium nitrate and continue stirring magnetically at room temperature until completely dissolved to obtain a reaction solution. Step 2: Under an inert gas atmosphere, the reaction solution obtained in Step 1 is dropped onto the surface of the lithium sheet, left to stand until the organic solvent has completely evaporated, then washed with organic solvent and dried at room temperature, thus generating a bismuth / fluorine / nitrogen hybrid composite protective layer in situ on the surface of the lithium sheet.

2. The method for preparing a bismuth / fluorine / nitrogen hybrid composite protective layer for lithium metal anodes according to claim 1, characterized in that: The organic solvent is ethylene glycol dimethyl ether.

3. The method for preparing a bismuth / fluorine / nitrogen hybrid composite protective layer for a lithium metal anode according to claim 1, characterized in that: The mass percentage of bismuth trifluoromethanesulfonate and lithium nitrate is 20-80%:80-20%.

4. A lithium metal anode, characterized in that, This includes lithium wafers and a bismuth / fluorine / nitrogen hybrid protective layer generated in situ on the surface of the lithium wafers.

5. A lithium metal battery, characterized in that, The lithium metal anode described in claim 4 is used.

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