Negative electrode protective coating, lithium metal negative electrode and preparation method thereof, lithium metal battery and preparation method thereof

Abstract

The invention provides a negative electrode protective coating, a lithium metal negative electrode and a preparation method thereof, and a lithium metal battery and a preparation method thereof. The negative electrode protective coating is prepared from the following components in parts by weight: 1 to 10 parts of fluoro aromatic hydrocarbon, 79.9 to 93.9 parts of an ether solvent, 5 to 10 parts of a cross-linking agent, 0.1 to 0.5 part of a free radical initiator and 9.5 to 33 parts of lithium salt. The components and the content of the negative electrode protective paint with high stability to lithium are controlled in the range, so that the LUMO energy level of the negative electrode protective paint can be improved, and a negative electrode protective coating with a stable cross-linked chain structure and certain stress is formed, thereby effectively inhibiting the side reaction between a negative electrode and an electrolyte and improving the service life of the negative electrode. And the formation of a solid electrolyte interface (SEI) can be improved, and the cracking of an SEI film in a circulating process is reduced, so that dendritic crystal growth and dead lithium deposition are reduced, and the circulating performance and the circulating stability of the battery are further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and in particular to a negative electrode protective coating, a lithium metal negative electrode and a preparation method thereof, a lithium metal battery and a preparation method thereof. Background Art

[0002] With the increasing demand for high-energy-density battery systems, lithium metal batteries (3860 mAh / g) with their high theoretical specific energy have attracted considerable attention. Lithium metal has the lowest chemical potential (-3.04 V) and very high reactivity. While using lithium metal as a negative electrode material can significantly increase the battery's energy density, it also dramatically shortens the battery's cycle life. Furthermore, dendrite growth is unavoidable on the lithium metal negative electrode, resulting in poor battery cycling performance.

[0003] Currently, the main factors affecting the cycle performance of lithium metal batteries are: 1) The lithium metal anode reacts with the electrolyte to form an unstable solid electrolyte interface (SEI). During the repeated deposition and stripping process of lithium metal as the negative electrode active material, the uneven SEI film repeatedly breaks and re-forms, resulting in uneven lithium deposition sites, dendrite growth and dead lithium deposition, which reduces the cycle performance of the battery; 2) As the SEI layer breaks during the cycle, the active material contacts the electrolyte to produce byproducts, and the battery polarization increases, resulting in an increase in battery impedance, thereby accelerating battery failure. Therefore, there is an urgent need to optimize the chemical system on the lithium metal anode side. Summary of the Invention

[0004] The main purpose of the present invention is to provide a negative electrode protective coating, a lithium metal negative electrode and its preparation method, a lithium metal battery and its preparation method, so as to solve the problems in the prior art such as the high reactivity of the lithium metal negative electrode, the uneven solid electrolyte interface formed by the side reaction between the lithium metal negative electrode and the electrolyte, and the repeated rupture during the cycle, resulting in poor cycle performance of the battery.

[0005] In order to achieve the above-mentioned object, according to one aspect of the present invention, a negative electrode protective coating is provided, which comprises, in parts by weight: 1 to 10 parts of fluoroaromatic hydrocarbon, 79.9 to 93.9 parts of ether solvent, 5 to 10 parts of cross-linking agent, 0.1 to 0.5 parts of free radical initiator and 9.5 to 33 parts of lithium salt.

[0006] Furthermore, the negative electrode protective coating comprises, by weight, 2 to 9 parts of fluorinated aromatic hydrocarbon, 82 to 93 parts of ether solvent, 6 to 8 parts of cross-linking agent, 0.1 to 0.4 parts of free radical initiator and 10 to 31 parts of lithium salt.

[0007] Furthermore, the mass ratio of the fluorinated aromatic hydrocarbon to the lithium salt is 5 to 8:19 to 31; and / or the fluorinated aromatic hydrocarbon is selected from any one or more of fluorobenzene, 1,3,5-trifluorobenzene, o-difluorobenzene, m-difluorobenzene and p-difluorobenzene; and / or the negative electrode protective coating further includes 1 to 10 parts by weight of fluoroethylene carbonate; and / or the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium bistrifluoromethylsulfonyl imide, lithium bisfluorosulfonyl imide, lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium trifluoromethylsulfonate and lithium difluorophosphate, and the molar concentration of the lithium salt in the negative electrode protective coating is 1 to 2 mol / L.

[0008] Furthermore, the crosslinking agent is selected from any one or more of ethylene glycol dimethacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane triacrylate, acetoacetoxyethyl methacrylate, methyl methacrylate and ethyl acrylate; and / or the free radical initiator is selected from any one or more of azobisisobutyronitrile, cyclohexanone peroxide, azobisisoheptonitrile, tert-butyl hydroperoxide and dimethyl azobisisobutyrate; and / or the ether solvent is selected from any one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and ethylene glycol butyl ether; and / or the mass ratio of the ether solvent to the lithium salt is 77.9-84.9:19-31.

[0009] According to another aspect of the present invention, a method for preparing a lithium metal negative electrode is provided, which comprises: step S1, mixing the raw materials corresponding to the above-mentioned negative electrode protective coating to obtain a precursor solution; and step S2, coating the precursor solution on the surface of the current collector and performing a thermal polymerization reaction to obtain a lithium metal negative electrode containing a protective layer.

[0010] Furthermore, in the above step S2, the current collector is a lithium-copper composite tape; and / or the coating speed is 1 to 10 min / m, and the coating thickness is 10 to 20 μm; and / or the temperature of the thermal polymerization reaction is 60 to 80° C., and the thermal polymerization reaction time is 5 to 10 hours.

[0011] According to another aspect of the present invention, a lithium metal negative electrode is provided. The lithium metal negative electrode is prepared by the above-mentioned preparation method. The surface of the lithium metal negative electrode contains a protective layer with a thickness of 10 to 20 μm.

[0012] According to another aspect of the present invention, a method for preparing a lithium metal battery is provided, the method comprising: step S1, stacking a positive electrode, a separator, and the above-mentioned lithium metal negative electrode in sequence, injecting an electrolyte, and then encapsulating to obtain a packaged battery; and step S2, pressurizing, heat-insulating, and forming the packaged battery in sequence to obtain a lithium metal battery; wherein the pressurizing pressure is 0.41 to 1.4 MPa.

[0013] Furthermore, in the above step S2, the holding temperature is 30-40°C and the holding time is 12-24 hours; and / or, the formation process includes charging at a constant current of 0.1-0.15C to 20%-30% SOC.

[0014] According to another aspect of the present invention, a lithium-ion battery is provided. The lithium-ion battery is prepared by the above-mentioned preparation method.

[0015] Applying the technical solution of the present invention, the present application provides a negative electrode protective coating with high stability to lithium, preferably controlling its components and content within the above range, which can not only improve the LUMO energy level of the negative electrode protective coating, but also form a negative electrode protective coating with a stable cross-linked chain structure and a certain stress, thereby effectively inhibiting the side reaction between the negative electrode and the electrolyte, but also improve the formation of the solid electrolyte interface (SEI), reduce the rupture of the SEI film during the cycle, thereby reducing dendrite growth and dead lithium deposition, and thus improving the cycle performance and cycle stability of the battery. Specifically, fluoroaromatic hydrocarbons have rich fluorinated groups and can react with lithium metal negative electrodes to form uniform lithium fluoride (LiF), thereby forming a more stable SEI film and making the deposition of lithium more uniform. And the higher dielectric constant (ε=4~9) of the CF bond in the fluoroaromatic hydrocarbon can promote the depolymerization of lithium salts, thereby improving the ionic conductivity of the electrolyte and the efficiency of interfacial lithium ion migration. At the same time, the benzene ring conjugated system of fluoroaromatic hydrocarbons and the strong electron-withdrawing effect of fluorine can increase the LUMO energy level of the negative electrode protective coating (0.2-0.8 eV), making it more likely to be reduced preferentially on the negative electrode surface, forming a negative electrode protective layer, thereby effectively inhibiting the reaction between the negative electrode side plate and the electrolyte. In addition, compared with fluoroether compounds, fluoroaromatic hydrocarbons have a benzene ring conjugated structure, which makes the energy barrier required for their β-H elimination reaction higher than that of fluoroether compounds. Therefore, it is not easy to generate hydrogen fluoride (HF) through the β-H elimination reaction, slowing down the corrosion of the SEI film and reducing the consumption of active lithium, thereby improving the battery's cycle stability and the integrity of the interfacial structure. Ether solvents do not react easily with active lithium metal. Therefore, ether solvents have good stability for lithium metal negative electrodes. Ether solvents have good solubility for lithium salts and can fully dissolve lithium salts. Adding a crosslinker can promote chemical bonding between component molecules during the drying and curing process of the negative electrode protective coating, forming a network structure, thereby enhancing the mechanical strength and stability of the negative electrode protective coating. The addition of a free radical initiator can stimulate the crosslinking agent to produce free radicals, accelerating the crosslinking reaction and ensuring the rapid formation of the negative electrode protective coating. The addition of a lithium salt can maintain the electrochemical activity of the negative electrode protective coating system and promote the uniform formation of the SEI film on the negative electrode surface, thereby improving the cycle stability of the battery. DETAILED DESCRIPTION

[0016] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below with reference to the embodiments.

[0017] As analyzed in the background technology of this application, in the prior art, in order to solve the problems of high reactivity of lithium metal negative electrode, uneven solid electrolyte interface formed by side reactions between lithium metal negative electrode and electrolyte, and repeated rupture during the cycle process, resulting in poor cycle performance of the battery, in order to solve the above problems, this application provides a negative electrode protective coating, a lithium metal negative electrode and its preparation method, a lithium metal battery and its preparation method.

[0018] In a typical embodiment of the present application, a negative electrode protective coating is provided, which includes, by weight, 1 to 10 parts of fluoroaromatic hydrocarbons, 79.9 to 93.9 parts of ether solvents, 5 to 10 parts of cross-linking agents, 0.1 to 0.5 parts of free radical initiators, and 9.5 to 33 parts of lithium salts.

[0019] The present application provides a negative electrode protective coating with high stability to lithium, and its components and content are preferably controlled within the above range, which can not only improve the LUMO energy level of the negative electrode protective coating, but also form a negative electrode protective coating with a stable cross-linked chain structure and a certain stress, thereby effectively inhibiting the side reaction between the negative electrode and the electrolyte, but also improve the formation of the solid electrolyte interface (SEI), reduce the rupture of the SEI film during the cycle, thereby reducing dendrite growth and dead lithium deposition, and thus improving the cycle performance and cycle stability of the battery. Specifically, fluoroaromatic hydrocarbons have rich fluorinated groups, which can react with lithium metal negative electrodes to form uniform lithium fluoride (LiF), thereby forming a more stable SEI film and making the deposition of lithium more uniform. And the higher dielectric constant (ε=4~9) of the CF bond in the fluoroaromatic hydrocarbon can promote the depolymerization of lithium salts, thereby improving the ionic conductivity of the electrolyte and the efficiency of interfacial lithium ion migration. At the same time, the benzene ring conjugated system of fluoroaromatic hydrocarbons and the strong electron-withdrawing effect of fluorine can increase the LUMO energy level of the negative electrode protective coating (0.2-0.8 eV), making it more likely to be reduced preferentially on the negative electrode surface, forming a negative electrode protective layer, thereby effectively inhibiting the reaction between the negative electrode side plate and the electrolyte. In addition, compared with fluoroether compounds, fluoroaromatic hydrocarbons have a benzene ring conjugated structure, which makes the energy barrier required for their β-H elimination reaction higher than that of fluoroether compounds. Therefore, it is not easy to generate hydrogen fluoride (HF) through the β-H elimination reaction, slowing down the corrosion of the SEI film and reducing the consumption of active lithium, thereby improving the battery's cycle stability and the integrity of the interfacial structure. Ether solvents do not react easily with active lithium metal. Therefore, ether solvents have good stability for lithium metal negative electrodes. Ether solvents have good solubility for lithium salts and can fully dissolve lithium salts. Adding a crosslinker can promote chemical bonding between component molecules during the drying and curing process of the negative electrode protective coating, forming a network structure, thereby enhancing the mechanical strength and stability of the negative electrode protective coating. The addition of a free radical initiator can stimulate the crosslinking agent to produce free radicals, accelerating the crosslinking reaction and ensuring the rapid formation of the negative electrode protective coating. The addition of a lithium salt can maintain the electrochemical activity of the negative electrode protective coating system and promote the uniform formation of the SEI film on the negative electrode surface, thereby improving the cycle stability of the battery.

[0020] In addition, the weight percentage of the fluoroaromatic hydrocarbon can be 1 part, 2.5 parts, 5 parts, 7.5 parts or 10 parts. Of course, the weight percentage of the fluoroaromatic hydrocarbon can be any point value within the above range, which will not be repeated here.

[0021] The weight percentage of the ether solvent can be 79.9 parts, 80.5 parts, 85 parts, 90.5 parts or 93.9 parts. Of course, the weight percentage of the ether solvent can be any point value within the above range, which will not be repeated here.

[0022] The weight percentage of the cross-linking agent can be 5 parts, 6.5 parts, 7 parts, 8.5 parts or 10 parts. Of course, the weight percentage of the cross-linking agent can be any point value within the above range, which will not be repeated here.

[0023] The weight percentage of the free radical initiator can be 0.1 part, 0.25 part, 0.3 part, 0.45 part or 0.5 part. Of course, the weight percentage of the free radical initiator can be any point value within the above range, which will not be repeated here.

[0024] The weight percentage of the lithium salt can be 9.5 parts, 15 parts, 20 parts, 25 parts or 33 parts. Of course, the weight percentage of the lithium salt can be any point value within the above range, which will not be repeated here.

[0025] In order to further improve the protective performance of the negative electrode protective coating, in one embodiment of the present application, the above-mentioned negative electrode protective coating includes, by weight: 2 to 9 parts of fluorinated aromatic hydrocarbons, 82 to 93 parts of ether solvents, 6 to 8 parts of cross-linking agents, 0.1 to 0.4 parts of free radical initiators and 10 to 31 parts of lithium salts.

[0026] In one embodiment of the present application, the mass ratio of the fluorinated aromatic hydrocarbon to the lithium salt is 5 to 8:19 to 31; and / or the fluorinated aromatic hydrocarbon is selected from any one or more of fluorobenzene, 1,3,5-trifluorobenzene, o-difluorobenzene, m-difluorobenzene and p-difluorobenzene; and / or the negative electrode protective coating further includes 1 to 10 parts by weight of fluoroethylene carbonate; and / or the lithium salt is selected from any one or more of lithium hexafluorophosphate (LiPF6), lithium bistrifluoromethylsulfonyl imide (LiTFSI), lithium bisfluorosulfonyl imide (LiFSI), lithium difluorooxalatoborate (LiDFOB), lithium tetrafluoroborate (LiBF4), lithium trifluoromethylsulfonate (LiTFA) and lithium difluorophosphate (LiPO2F2), and the molar concentration of the lithium salt in the negative electrode protective coating is 1 to 2 mol / L.

[0027] It is preferred to control the types of fluoroaromatic hydrocarbons and lithium salts, and the mass ratio of fluoroaromatic hydrocarbons and lithium salts within the above ranges, which helps the fluoroaromatic hydrocarbons better promote the depolymerization of the lithium salt, thereby further improving the ionic conductivity of the electrolyte and the interfacial lithium ion migration efficiency.

[0028] Preferably, the fluorinated aromatic hydrocarbon is a combination of fluorobenzene and 1,3,5-trifluorobenzene, and the mass ratio of fluorobenzene to 1,3,5-trifluorobenzene is 1:1 to 4, which helps to further improve the LUMO energy level of the negative electrode protective coating, so that the fluorinated aromatic hydrocarbon is more inclined to be preferentially reduced on the negative electrode surface to form a negative electrode protective layer, thereby better inhibiting the reaction between the negative electrode side plate and the electrolyte.

[0029] Fluorobenzene has excellent interface stability building capability, and fluoroethylene carbonate has the characteristic of being easily decomposed on the negative electrode surface to form a LiF-rich SEI film. Preferably, the fluoroaromatic hydrocarbon fluorobenzene and fluoroethylene carbonate are used in combination, and the mass ratio of the fluoroaromatic hydrocarbon fluorobenzene to fluoroethylene carbonate is 1:1 to 4, which helps to build a denser, uniform and stable negative electrode interface film layer, thereby improving the controllability of lithium deposition, inhibiting dendrite growth, and thus improving the cycle performance of the battery.

[0030] The preferred lithium salt is a combination of lithium bis(trifluoromethylsulfonyl imide) and lithium bis(fluorosulfonyl imide), and the mass ratio of lithium bis(trifluoromethylsulfonyl imide) to lithium bis(fluorosulfonyl imide) is 1:1-2, which helps to further promote the uniform formation of the SEI film and enhance the lithium conductivity of the SEI film, thereby improving the cycle stability of the battery.

[0031] In one embodiment of the present application, the cross-linking agent is selected from any one or more of ethylene glycol dimethacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane triacrylate, acetoacetoxyethyl methacrylate, methyl methacrylate and ethyl acrylate, preferably ethylene glycol dimethacrylate; and / or, the free radical initiator is selected from any one or more of azobisisobutyronitrile, cyclohexanone peroxide, azobisisoheptonitrile, tert-butyl hydroperoxide and dimethyl azobisisobutyrate, preferably azobisisobutyronitrile; and / or, the ether solvent is selected from any one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and ethylene glycol butyl ether, preferably ethylene glycol dimethyl ether; and / or, the mass ratio of the ether solvent to the lithium salt is 77.9-84.9:19-31.

[0032] The types of crosslinking agents and free radical initiators are preferably within the above ranges, which helps to better crosslink the components of the negative electrode protective coating, thereby further improving the mechanical strength and stability of the negative electrode protective coating. The types of ether solvents are preferably within the above ranges, which helps to better match the lithium metal negative electrode, thereby maintaining the stability of the lithium metal negative electrode.

[0033] In another typical embodiment of the present application, a method for preparing a lithium metal negative electrode is provided, which includes: step S1, mixing the raw materials corresponding to the above-mentioned negative electrode protective coating to obtain a precursor solution; and step S2, coating the precursor solution on the surface of the current collector and performing a thermal polymerization reaction to obtain a lithium metal negative electrode containing a protective layer.

[0034] Since the lithium metal negative electrode has a very strong reactivity, the precursor solution obtained by mixing the above-mentioned negative electrode protective coating in this application has high stability to the lithium metal negative electrode, can reduce the consumption of electrode active materials, thereby forming a stable and uniform SEI film on the electrode surface, thereby improving the cycle performance and cycle life of the lithium metal battery. In step S2, the precursor solution undergoes a thermal polymerization reaction, so that the π conjugated structure of the benzene ring in the fluoroaromatic hydrocarbon in the precursor solution forms a cross-linked network in the SEI film, and the toughness of the SEI film is enhanced by intermolecular π-π stacking, so that the SEI film has good stress-strain properties, thereby reducing the rupture of the SEI film. At the same time, the protective layer formed on the surface of the negative electrode current collector can not only effectively inhibit the reaction between the negative electrode side electrode and the electrolyte, thereby improving the cycle performance and cycle stability of the battery, but also the above-mentioned protective layer has a high ionic conductivity, which can inhibit the side reaction products such as dead lithium on the positive and negative electrode sides from migrating to the positive electrode through the electrolyte.

[0035] In one embodiment of the present application, in the above step S2, the current collector is a lithium-copper composite tape; and / or the coating speed is 1 to 10 min / m, and the coating thickness is 10 to 20 μm; and / or the temperature of the thermal polymerization reaction is 60 to 80° C., and the thermal polymerization reaction time is 5 to 10 hours.

[0036] The above-mentioned current collector is preferably used to facilitate better composite with the protective layer. A vacuum coating device (Kejing MSK-AFA-HC100) is preferably used to apply the precursor solution to the surface of the current collector at the above-mentioned coating speed, which helps to form a uniform protective layer. The coating thickness is preferably controlled within the above-mentioned range, which helps the protective layer better protect the lithium metal negative electrode and improve the efficiency of ion conduction at the negative electrode interface, thereby improving the cycle performance of the battery.

[0037] The above preparation processes are all carried out in an environment with a protective atmosphere and an oxygen content and a water content of less than 1 ppm.

[0038] In another typical embodiment of the present application, a lithium metal negative electrode is provided. The lithium metal negative electrode is prepared by the above-mentioned preparation method. The surface of the lithium metal negative electrode contains a protective layer, and the thickness of the protective layer is 10 to 20 μm.

[0039] The lithium metal anode prepared by the above-mentioned preparation method has good cycle performance and cycle stability. At the same time, controlling the thickness of the protective layer on the surface of the lithium metal anode within the above-mentioned range can better protect the lithium metal anode and inhibit the growth of lithium dendrites, thereby improving the cycle performance of the battery.

[0040] In another typical embodiment of the present application, a method for preparing a lithium metal battery is provided, which comprises: step S1, stacking a positive electrode, a separator, and the above-mentioned lithium metal negative electrode in sequence, injecting an electrolyte, and then encapsulating to obtain a packaged battery; and step S2, pressurizing, keeping warm, and forming the packaged battery in sequence to obtain a lithium metal battery; wherein the pressurizing pressure is 0.41 to 1.4 MPa.

[0041] The present application adopts the above-mentioned lithium metal negative electrode containing a protective layer to effectively improve the cycle performance of the lithium metal battery. At the same time, combined with the scheme of pressurization before formation in step S2, the current distribution on the electrode surface can be made more uniform, thereby promoting the stable formation of SEI. Preferably, a clamp is used to pressurize the packaged battery, which can effectively reduce the contact distance between the positive and negative electrodes, promote the electrolyte to fully infiltrate the electrodes, thereby reducing the impedance of the battery, and thus extending the cycle life of the battery. Wherein, the pressurized pressure is too small to apply uniform pressure to the packaged battery, resulting in a long contact distance between the positive and negative electrodes, uneven charge distribution on the electrode surface, and aggravation of uneven lithium transmission and deposition during repeated charge and discharge. The pressurized pressure is too large, and the effect of improving the battery cycle performance is not obvious. The positive side further aggravates the powdering as the pressure increases. At the same time, the increase in pressure causes the destruction of the electrode structure, thereby affecting the uniformity of the positive and negative electrode contact distances, resulting in uneven current reaching the electrode surface speed, and excessively high local electrode surface electron concentration, which in turn aggravates the generation of uneven SEI and accelerated induced dendrites, thereby accelerating the capacity decay of the lithium metal battery and reducing the cycle life of the lithium metal battery. Therefore, the preferred pressurized pressure in this application is within the above range, which can shorten the contact distance between the positive and negative electrodes and evenly distribute the current on the electrode surface, thereby facilitating the formation of a uniform and dense SEI film. At the same time, the protective layer on the surface of the compressed lithium metal negative electrode can promote the electrolyte to infiltrate the interior of the electrode, so that the surface temperature of the electrode is evenly distributed, and the charge is fully in contact with the electrolyte, effectively alleviating the occurrence of side reactions under a small and uniform current. In summary, the synergistic effect of the above-mentioned formation scheme and the lithium metal negative electrode containing the protective layer can achieve the effect of optimizing the negative electrode structure, reducing the battery impedance, and thus increasing the long cycle performance of the lithium metal battery.

[0042] In one embodiment of the present application, in the above step S2, the insulation temperature is 30-40°C and the insulation time is 12-24 hours; and / or, the formation process includes charging at a constant current of 0.1-0.15C to 20%-30% SOC.

[0043] It is preferred to keep the electrode at the holding temperature and time within the above range while applying the above pressure, which helps to better form a uniform and dense SEI film on the electrode surface during the subsequent formation process.

[0044] In another typical embodiment of the present application, a lithium-ion battery is provided. The lithium-ion battery is prepared by the above-mentioned preparation method.

[0045] The lithium-ion battery prepared by the above preparation method has good cycle performance, cycle stability and cycle life.

[0046] The beneficial effects of the present application will be further illustrated below with reference to examples.

[0047] The following examples and comparative examples adopt the following pressure forming method, as shown in Table 1.

[0048] Pressure formation method 1: Pressurize the clamp on the packaged battery with a pressure of 0.8MPa. After the packaged battery with the pressure clamp is kept in a 30℃ incubator for 12 hours, it is charged at a constant current of 0.15C to 20% SOC for formation.

[0049] Pressure formation method 2: Pressurize the clamp on the packaged battery with a pressure of 0.41MPa. Keep the packaged battery with the pressure clamp in a 30℃ incubator for 12 hours, and then charge it at a constant current of 0.15C to 20% SOC for formation.

[0050] Pressure formation method three: pressurize the clamp on the packaged battery with a pressure of 1.4MPa. After the packaged battery with the pressure clamp is kept in a 30℃ incubator for 12 hours, it is charged at a constant current of 0.15C to 20% SOC for formation.

[0051] Pressure formation method 4: Pressurize the clamp on the packaged battery with a pressure of 2.0 MPa. After the packaged battery with the pressure clamp is kept in a 30°C incubator for 12 hours, it is charged at a constant current of 0.15C to 20% SOC for formation.

[0052] Example 1

[0053] Preparation of lithium metal anode: The raw materials of the negative electrode protective coating are 10 parts by weight of fluoroaromatic hydrocarbon, 79.9 parts of ether solvent ethylene glycol dimethyl ether, 10 parts of crosslinking agent ethylene glycol dimethacrylate, 0.1 parts of free radical initiator azobisisobutyronitrile, and 29 parts of lithium salt lithium bistrifluoromethylsulfonyl imide (LiTFSI). The molar concentration of the lithium salt is 1 mol / L. Among them, the fluoroaromatic hydrocarbon is a combination of fluorobenzene and 1,3,5-trifluorobenzene, with a mass ratio of 1:1.

[0054] In a dry argon atmosphere glove box, the raw materials for the negative electrode protective coating were mixed to obtain a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was coated on the surface of the current collector lithium-copper composite tape using a vacuum coating device (Kejing MSK-AFA-HC100). The mixture was then thermally polymerized at 60°C for 5 hours to obtain a lithium metal negative electrode with a protective layer, which was then cut. The lithium layer in the lithium-copper composite tape was 30 μm thick, the coating speed was 10 min / m, and the coating thickness was 10 μm.

[0055] Preparation of the positive electrode: The positive electrode active material, lithium iron phosphate (LFP), the conductive agent (Super P conductive carbon), and the binder, polyvinylidene fluoride, were mixed in a weight ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added and stirred in a vacuum mixer until the mixture was homogeneous to obtain a positive electrode slurry. The solid content of the positive electrode slurry was approximately 72%. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil and dried. After cold pressing, cutting, and slitting, it was dried under vacuum at 85°C for 4 hours to obtain the positive electrode.

[0056] Preparation of the separator: Polyethylene (PE) with a thickness of 15 μm was used as the separator.

[0057] Preparation of a lithium metal battery: The positive electrode, separator, and lithium metal negative electrode with a protective layer are stacked in sequence. After the tabs are welded, they are placed in an aluminum-plastic film outer packaging. After electrolyte is injected, the battery is vacuum-sealed and allowed to stand to produce a packaged battery. The packaged battery is then pressure-formed using a fixture, which sequentially involves pressurizing, maintaining temperature, and then forming the battery to produce a lithium metal battery.

[0058] Example 2

[0059] The difference from Example 1 is that, in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are 5 parts of fluorinated aromatic hydrocarbon o-difluorobenzene, 84.9 parts of ether solvent ethylene glycol dimethyl ether, 10 parts of cross-linking agent methyl methacrylate, 0.1 parts of free radical initiator azobisisobutyronitrile, and 19 parts of lithium salt lithium bis(fluorosulfonyl)imide (LiFSI), and the molar concentration of the lithium salt is 1 mol / L.

[0060] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to produce a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating device. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 20 μm, resulting in a lithium metal battery.

[0061] Example 3

[0062] The difference from Example 1 is that, in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, in parts by weight, 1 part of the fluorinated aromatic hydrocarbon m-difluorobenzene, 93.9 parts of the ether solvent ethylene glycol dimethyl ether, 5 parts of the cross-linking agent ethyl acrylate, 0.1 parts of the free radical initiator azobisisobutyronitrile, and 25 parts of the lithium salt lithium hexafluorophosphate (LiPF6), and the molar concentration of the lithium salt is 1.5 mol / L, and a lithium metal battery is finally obtained.

[0063] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to form a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating apparatus. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 10 μm, resulting in a lithium metal battery.

[0064] Example 4

[0065] The difference from Example 1 is that in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, in parts by weight, 1 part of the fluorinated aromatic hydrocarbon m-difluorobenzene, 93.5 parts of the ether solvent ethylene glycol dimethyl ether, 5 parts of the cross-linking agent ethyl acrylate, 0.5 parts of the free radical initiator azobisisobutyronitrile, and 33 parts of the lithium salt lithium hexafluorophosphate (LiPF6), and the molar concentration of the lithium salt is 2 mol / L.

[0066] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to produce a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating device. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 20 μm, resulting in a lithium metal battery.

[0067] Example 5

[0068] The difference from Example 1 is that, in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, in parts by weight, 10 parts of fluoroaromatic hydrocarbons, 79.9 parts of ether solvent ethylene glycol dimethyl ether, 10 parts of cross-linking agent methyl methacrylate, 0.1 parts of free radical initiator azobisisobutyronitrile, 9.5 parts of lithium salt lithium tetrafluoroborate and 10 parts of m-fluoroethylene carbonate (FEC), and the molar concentration of the lithium salt is 1 mol / L.

[0069] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to form a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating apparatus. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 10 μm, resulting in a lithium metal battery.

[0070] Example 6

[0071] The difference from Example 1 is that the total weight of fluoroaromatic hydrocarbons (fluorobenzene and 1,3,5-trifluorobenzene) and lithium salt lithium bistrifluoromethylsulfonyl imide is 39 parts, the mass ratio of fluoroaromatic hydrocarbons to lithium salt is 8:31, and a lithium metal battery is finally obtained.

[0072] Example 7

[0073] The difference from Example 1 is that the total weight of fluoroaromatic hydrocarbons (fluorobenzene and 1,3,5-trifluorobenzene) and lithium salt lithium bistrifluoromethylsulfonyl imide is 39 parts, the mass ratio of fluoroaromatic hydrocarbons to lithium salt is 2:37, and a lithium metal battery is finally obtained.

[0074] Example 8

[0075] The difference from Example 1 is that the total weight of the ether solvent ethylene glycol dimethyl ether and the lithium salt lithium bis(trifluoromethylsulfonyl)imide is 108.9 parts, the mass ratio of the ether solvent to the lithium salt is 77.9:31, and a lithium metal battery is finally obtained.

[0076] Example 9

[0077] The difference from Example 1 is that the total weight of the ether solvent ethylene glycol dimethyl ether and the lithium salt lithium bis(trifluoromethylsulfonyl)imide is 108.9 parts, the mass ratio of the ether solvent to the lithium salt is 93.9:15, and a lithium metal battery is finally obtained.

[0078] Example 10

[0079] The difference from Example 1 is that the fluorinated aromatic hydrocarbon is a combination of fluorobenzene and 1,3,5-trifluorobenzene, the mass ratio of fluorobenzene to 1,3,5-trifluorobenzene is 1:4, and a lithium metal battery is finally obtained.

[0080] Example 11

[0081] The difference from Example 1 is that the fluorinated aromatic hydrocarbon is a combination of fluorobenzene and 1,3,5-trifluorobenzene, the mass ratio of fluorobenzene to 1,3,5-trifluorobenzene is 1:9, and a lithium metal battery is finally obtained.

[0082] Example 12

[0083] The difference from Example 1 is that the temperature of the thermal polymerization reaction is 80° C., and a lithium metal negative electrode containing a protective layer is obtained, and finally a lithium metal battery is obtained.

[0084] Example 13

[0085] The difference from Example 1 is that the temperature of the thermal polymerization reaction is 100° C., and a lithium metal negative electrode containing a protective layer is obtained, and finally a lithium metal battery is obtained.

[0086] Comparative Example 1

[0087] The difference from Example 1 is that in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, by weight, 0.1 parts of a fluorinated aromatic hydrocarbon, 89.4 parts of an ether solvent, ethylene glycol dimethyl ether, 10 parts of a cross-linking agent, ethylene glycol dimethacrylate, 0.1 parts of a free radical initiator, azobisisobutyronitrile, and 29 parts of a lithium salt, lithium bistrifluoromethylsulfonyl imide, and the molar concentration of the lithium salt is 1 mol / L. The fluorinated aromatic hydrocarbon is a combination of fluorobenzene and 1,3,5-trifluorobenzene, and the mass ratio of the two is 1:1.

[0088] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to form a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating apparatus. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 10 μm, resulting in a lithium metal battery.

[0089] Comparative Example 2

[0090] The difference from Example 1 is that, in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are 12 parts of fluorinated aromatic hydrocarbon o-difluorobenzene, 76.9 parts of ether solvent ethylene glycol dimethyl ether, 10 parts of cross-linking agent methyl methacrylate, 0.1 parts of free radical initiator azobisisobutyronitrile, and 29 parts of lithium salt lithium bistrifluoromethylsulfonyl imide, and the molar concentration of the lithium salt is 1 mol / L.

[0091] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to produce a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating apparatus. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 50 μm, resulting in a lithium metal battery.

[0092] Comparative Example 3

[0093] The difference from Example 1 is that in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are 79.9 parts of an ether solvent ethylene glycol dimethyl ether, 10 parts of a cross-linking agent methyl methacrylate, 0.1 parts of a free radical initiator azobisisobutyronitrile, and 29 parts of a lithium salt lithium bistrifluoromethylsulfonyl imide, and the molar concentration of the lithium salt is 1 mol / L.

[0094] In a dry argon glove box, the raw materials for the negative electrode protective coating were mixed to produce a precursor solution. Under a protective atmosphere with oxygen and water contents less than 1 ppm, the precursor solution was applied to the surface of a lithium-copper composite current collector strip using a vacuum coating device. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal negative electrode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 20 μm, resulting in a lithium metal battery.

[0095] Comparative Example 4

[0096] The difference from Example 1 is that in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, by weight, 10 parts of the fluorinated aromatic hydrocarbon o-difluorobenzene, 79.9 parts of the ether solvent ethylene glycol dimethyl ether, 10 parts of the cross-linking agent methyl methacrylate, 0.1 parts of the free radical initiator azobisisobutyronitrile, and 87 parts of the lithium salt lithium bistrifluoromethylsulfonyl imide, and the molar concentration of the lithium salt is 3 mol / L. Among them, the fluorinated aromatic hydrocarbon is a combination of 1,3,5-trifluorobenzene and 1,3,5-trifluorobenzene, and the mass ratio of the two is 1:1.

[0097] In a dry argon glove box, the raw materials for the anode protective coating were mixed to produce a precursor solution. Under a protective atmosphere with oxygen and water contents below 1 ppm, the precursor solution was applied to the surface of the current collector lithium-copper composite tape using a vacuum coating machine. Thermal polymerization was then carried out at 60°C for 5 hours to produce a lithium metal anode with a protective layer, which was then cut. The coating speed was 10 min / m, and the coating thickness was 10 μm.

[0098] Preparation of a lithium metal battery: The positive electrode, separator, and lithium metal negative electrode with a protective layer are stacked in order, the tabs are welded, and the battery is placed in an aluminum-plastic film package. After injecting electrolyte, the battery is vacuum-sealed and allowed to stand to form a packaged battery. The packaged battery is then charged at a constant current of 0.15C to 20% SOC for formation without applying pressure to obtain a lithium metal battery.

[0099] Comparative Example 5

[0100] The difference from Example 1 is that the packaged battery is formed using pressure formation method 4 to obtain a lithium metal battery.

[0101] Comparative Example 6

[0102] The difference from Example 1 is that in the preparation of the lithium metal negative electrode, the raw materials of the negative electrode protective coating are, by weight, 89.9 parts of the fluorinated aromatic hydrocarbon o-difluorobenzene, 10 parts of the cross-linking agent ethylene glycol dimethacrylate, 0.1 parts of the free radical initiator azobisisobutyronitrile, and 29 parts of the lithium salt lithium bistrifluoromethylsulfonyl imide, and the molar concentration of the lithium salt is 1 mol / L. Among them, the fluorinated aromatic hydrocarbon is a combination of 1,3,5-trifluorobenzene and 1,3,5-trifluorobenzene, and the mass ratio of the two is 1:1.

[0103] In a dry argon atmosphere glove box, the raw materials of the negative electrode protective coating are mixed, but a precursor solution cannot be formed, and a lithium metal negative electrode and a lithium metal battery cannot be obtained.

[0104] Comparative Example 7

[0105] The difference from Example 1 is that the raw materials for the negative electrode protective coating are mixed in a dry argon atmosphere glove box to obtain a precursor solution. In an environment with a protective atmosphere and an oxygen and water content of less than 1 ppm, the precursor solution is coated on the surface of the current collector lithium-copper composite tape using a vacuum coating device. After thermal polymerization at 60°C for 5 hours, a lithium metal negative electrode with a protective layer is obtained and cut. The coating speed is 10 min / m, the coating thickness is 5 μm, and the final lithium metal battery is obtained.

[0106] Comparative Example 8

[0107] The difference from Example 1 is that the precursor solution is coated on the surface of the current collector lithium copper composite tape and then left to stand for 5 hours in a natural environment to obtain a lithium metal negative electrode, and finally a lithium metal battery.

[0108] Comparative Example 9

[0109] The difference from Example 1 is that the surface of the lithium metal negative electrode does not contain a protective layer, and a lithium metal battery is finally obtained.

[0110] Test method:

[0111] 1. Lithium metal battery room temperature cycle performance test

[0112] Place the lithium metal battery in a 25°C thermostat for 30 minutes to allow the lithium metal battery to reach a constant temperature. Once the lithium metal battery has reached a constant temperature, charge it at a constant current of 0.5C to 3.7V, then cycle it to a cutoff current of 0.05C. Then discharge it at a constant current of 1C to a voltage of 2.5V. This constitutes one charge-discharge cycle. Repeat the charge-discharge cycle, taking the initial discharge capacity as 100%, until the discharge capacity decays to 80%. The test is then stopped and the number of cycles recorded.

[0113] 2. Lithium metal battery rate cycle performance test

[0114] Place the lithium metal battery in a 25°C thermostat for 30 minutes to allow the lithium metal battery to reach a constant temperature. Once the lithium metal battery has reached a constant temperature, charge it at a constant current of 1C to 3.7V, then cycle it to a cutoff current of 0.05C. Then, discharge it at a constant current of 3C to a voltage of 2.5V. This constitutes one charge-discharge cycle. Using the initial discharge capacity as 100%, repeat the charge-discharge cycle until the discharge capacity decays to 80%. Stop the test and record the number of cycles.

[0115] The above test results are shown in Table 1.

[0116] Table 1

[0117]

[0118]

[0119]

[0120] As can be seen from Table 1, the test results of the lithium metal battery of the embodiment of the present application show that the combination of the lithium metal negative electrode with a protective layer and the pressure formation scheme is beneficial to the stability of the lithium metal battery electrode interface, promotes charge transfer, and thus improves the cycle and rate performance of the lithium metal battery.

[0121] Comparing the test results of Comparative Example 1 with those of Example 1, it can be seen that the mass content of fluoroaromatic hydrocarbons in Comparative Example 1 is lower, resulting in less LiF product, and thus the cycle performance of the lithium metal battery is also poor.

[0122] Comparing the test results of Comparative Example 2 with those of Example 2, it can be seen that the mass content of fluorinated aromatic hydrocarbons in Comparative Example 2 is higher, resulting in more LiF products. At the same time, the protective layer of the lithium-gold negative electrode is too thick, which increases the impedance of the battery, resulting in poor lithium deposition and cycling effect during the battery cycle.

[0123] By comparing the test results of Comparative Example 3 with those of Example 2, it can be seen that Comparative Example 3 does not contain fluorinated aromatic hydrocarbons, the viscosity of the precursor solution is relatively high, and it is impossible to construct a protective layer with a uniform and loose structure, so that the lithium metal deposition on the negative electrode surface is uneven, resulting in an increase in local overvoltage. The reaction time on the negative electrode surface is long, resulting in an increase in the side reactions of the lithium metal, causing the loss of some active lithium, and failing to achieve the effect of improving the cycle performance of the lithium metal battery.

[0124] Comparing the test results of Comparative Example 4 with those of Example 1 reveals that the molar concentration of the lithium salt in the precursor solution in Comparative Example 4 is too high, resulting in increased viscosity and difficulty in coating. The absence of pressurization during the battery formation process leads to uneven contact distances between the positive and negative electrodes. Consequently, the lithium metal battery of Comparative Example 4 exhibits poor cycling performance.

[0125] Comparing the test results of Comparative Example 5 with those of Example 1, it can be seen that the pressurization pressure in Comparative Example 5 is too high, resulting in damage to the electrode structure and uneven reaction, thereby failing to effectively improve the cycle and rate performance of the lithium metal battery.

[0126] By comparing the test results of Comparative Example 6 with those of Example 1, it can be seen that the precursor solution in Comparative Example 6 contains only fluoroaromatic hydrocarbons and does not contain ether solvents, and thus a precursor solution cannot be formed. Since lithium salts cannot be dissolved in fluoroaromatic hydrocarbons, the ether solvents that dissolve lithium salts have a critical effect on the construction of the protective layer and the cycle performance of the lithium metal battery.

[0127] Comparing the test results of Comparative Example 7 with those of Example 1, it can be seen that the thickness of the protective layer on the surface of the lithium metal negative electrode in Comparative Example 7 is thinner, resulting in a poor effect of inhibiting lithium dendrites, and thus a poor cycle life of the lithium metal negative electrode.

[0128] Comparing the test results of Comparative Example 8 with those of Example 1, it can be seen that in Comparative Example 8, the precursor solution was not subjected to thermal polymerization reaction after coating, and the obtained lithium metal negative electrode did not form a protective layer, resulting in poor cycle performance of the lithium metal battery.

[0129] From the above description, it can be seen that the above embodiments of the present invention achieve the following technical effects:

[0130] The present application provides a negative electrode protective coating with high stability to lithium, and its components and content are preferably controlled within the above range, which can not only improve the LUMO energy level of the negative electrode protective coating, but also form a negative electrode protective coating with a stable cross-linked chain structure and a certain stress, thereby effectively inhibiting the side reaction between the negative electrode and the electrolyte, but also improve the formation of the solid electrolyte interface (SEI), reduce the rupture of the SEI film during the cycle, thereby reducing dendrite growth and dead lithium deposition, and thus improving the cycle performance and cycle stability of the battery. Specifically, fluoroaromatic hydrocarbons have rich fluorinated groups, which can react with lithium metal negative electrodes to form uniform lithium fluoride (LiF), thereby forming a more stable SEI film and making the deposition of lithium more uniform. And the higher dielectric constant (ε=4~9) of the CF bond in the fluoroaromatic hydrocarbon can promote the depolymerization of lithium salts, thereby improving the ionic conductivity of the electrolyte and the efficiency of interfacial lithium ion migration. At the same time, the benzene ring conjugated system of fluoroaromatic hydrocarbons and the strong electron-withdrawing effect of fluorine can increase the LUMO energy level of the negative electrode protective coating (0.2-0.8 eV), making it more likely to be reduced preferentially on the negative electrode surface, forming a negative electrode protective layer, thereby effectively inhibiting the reaction between the negative electrode side plate and the electrolyte. In addition, compared with fluoroether compounds, fluoroaromatic hydrocarbons have a benzene ring conjugated structure, which makes the energy barrier required for their β-H elimination reaction higher than that of fluoroether compounds. Therefore, it is not easy to generate hydrogen fluoride (HF) through the β-H elimination reaction, slowing down the corrosion of the SEI film and reducing the consumption of active lithium, thereby improving the battery's cycle stability and the integrity of the interfacial structure. Ether solvents do not react easily with active lithium metal. Therefore, ether solvents have good stability for lithium metal negative electrodes. Ether solvents have good solubility for lithium salts and can fully dissolve lithium salts. Adding a crosslinker can promote chemical bonding between component molecules during the drying and curing process of the negative electrode protective coating, forming a network structure, thereby enhancing the mechanical strength and stability of the negative electrode protective coating. The addition of a free radical initiator can stimulate the crosslinking agent to produce free radicals, accelerating the crosslinking reaction and ensuring the rapid formation of the negative electrode protective coating. The addition of a lithium salt can maintain the electrochemical activity of the negative electrode protective coating system and promote the uniform formation of the SEI film on the negative electrode surface, thereby improving the cycle stability of the battery.

[0131] The above are merely embodiments of the present invention and are not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A negative electrode protective coating, characterized in that: In parts by weight, the negative electrode protective coating comprises: 1 to 10 parts of fluoroaromatic hydrocarbon; 79.9-93.9 parts of an ether solvent; 5 to 10 parts of a cross-linking agent; 0.1 to 0.5 parts of a free radical initiator; and 9.5 to 33 parts of lithium salt.

2. The negative electrode protective coating according to claim 1, characterized in that: In parts by weight, the negative electrode protective coating comprises: 2 to 9 parts of the fluoroaromatic hydrocarbon; 82-93 parts of the ether solvent; 6 to 8 parts of the cross-linking agent; 0.1 to 0.4 parts of the free radical initiator; and 10 to 31 parts of the lithium salt.

3. The negative electrode protective coating according to claim 1 or 2, characterized in that: The mass ratio of the fluorinated aromatic hydrocarbon to the lithium salt is 5-8:19-31; and / or the fluorinated aromatic hydrocarbon is selected from any one or more of fluorobenzene, 1,3,5-trifluorobenzene, o-difluorobenzene, m-difluorobenzene and p-difluorobenzene; and / or the negative electrode protective coating further includes 1-10 parts by weight of fluoroethylene carbonate; and / or the lithium salt is selected from any one or more of lithium hexafluorophosphate, lithium bistrifluoromethylsulfonyl imide, lithium bisfluorosulfonyl imide, lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium trifluoromethylsulfonate and lithium difluorophosphate, and the molar concentration of the lithium salt in the negative electrode protective coating is 1-2 mol / L.

4. The negative electrode protective coating according to any one of claims 1 to 3, characterized in that The cross-linking agent is selected from any one or more of ethylene glycol dimethacrylate, ethoxylated trimethylolpropane triacrylate, trimethylolpropane triacrylate, acetoacetoxyethyl methacrylate, methyl methacrylate and ethyl acrylate; and / or the free radical initiator is selected from any one or more of azobisisobutyronitrile, cyclohexanone peroxide, azobisisoheptonitrile, tert-butyl hydroperoxide and dimethyl azobisisobutyrate; And / or, the ether solvent is selected from any one or more of ethylene glycol dimethyl ether, ethylene glycol diethyl ether and ethylene glycol butyl ether; and / or, the mass ratio of the ether solvent to the lithium salt is 77.9-84.9:19-31.

5. A method for preparing a lithium metal negative electrode, characterized in that: The preparation method comprises: Step S1, mixing raw materials corresponding to the negative electrode protective coating according to any one of claims 1 to 4 to obtain a precursor solution; and Step S2: coating the precursor solution on the surface of the current collector and performing a thermal polymerization reaction to obtain a lithium metal negative electrode containing a protective layer.

6. The preparation method according to claim 5, characterized in that In step S2, the current collector is a lithium-copper composite tape; and / or the coating speed is 1 to 10 min / m, and the coating thickness is 10 to 20 μm; and / or the temperature of the thermal polymerization reaction is 60 to 80° C., and the thermal polymerization reaction time is 5 to 10 hours.

7. A lithium metal negative electrode, characterized in that The lithium metal negative electrode is prepared by the preparation method according to claim 5 or 6. The surface of the lithium metal negative electrode comprises a protective layer, and the thickness of the protective layer is 10 to 20 μm.

8. A method for preparing a lithium metal battery, characterized in that: The preparation method comprises: Step S1, stacking the positive electrode, the separator, and the lithium metal negative electrode according to claim 7 in sequence, injecting an electrolyte, and then encapsulating to obtain a packaged battery; and Step S2, pressurizing, heat-insulating, and forming the packaged battery in sequence to obtain a lithium metal battery; Wherein, the pressurized pressure is 0.41-1.4 MPa.

9. The preparation method according to claim 8, characterized in that In the step S2, the holding temperature is 30-40°C and the holding time is 12-24 hours; and / or the formation process includes charging at a constant current of 0.1-0.15C to 20%-30% SOC.

10. A lithium ion battery, characterized in that: The lithium-ion battery is prepared by the preparation method according to claim 8 or 9.

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