Metal lithium negative electrode, lithium secondary battery, and vehicle

Abstract

The application discloses a metal lithium negative electrode, a lithium secondary battery and a vehicle, and a structure formula of a polymer protective film is as follows: The polymer protective film disclosed by the application has good binding force with metal lithium and good ion conduction capacity, thereby effectively preventing the metal lithium from contacting the electrolyte while ensuring effective conduction of lithium ions, making the lithium ions uniformly distributed and avoiding generation of lithium dendrites, and being beneficial to reducing interface impedance between the polymer and the metal lithium and improving cycle performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application generally relates to the technical field of lithium battery, and in particular to a metal lithium negative electrode, a lithium secondary battery and a vehicle. BACKGROUND

[0002] Lithium metal has a very high theoretical specific capacity (3860 mAh / g) and a very low electrochemical potential (-3.040 V vs. vs Standard Hydrogen Electrode), which is an ideal negative electrode material for lithium secondary batteries. However, there are two main obstacles that limit the further development of lithium secondary batteries: (1) lithium secondary batteries are prone to lithium dendrites during the cycling process, which can cause short circuits; (2) the large surface area and high activity of lithium dendrites can cause violent reactions with the electrolyte, leading to the continuous reformation of the SEI film on the surface of the metal lithium, consumption of the electrolyte and active lithium, and also leading to a decrease in the cycle efficiency and a shortening of the cycle life of the battery.

[0003] Most existing researches focus on adding functional additives, such as vinylene carbonate, etc., to the electrolyte, and using the functional additives to form a protective film by adsorption, decomposition, polymerization, etc. with the metal lithium to improve the cycle performance of the lithium secondary battery. However, the protective strength of the protective film formed in situ during the operation of the battery is weak and does not completely cover the metal lithium, which cannot completely prevent the violent and continuous reformation of the SEI film caused by the change in the morphology of the lithium metal during the lithium deposition / dissolution process, thereby failing to effectively improve the cycle performance of the lithium secondary battery. SUMMARY

[0004] In view of the above-mentioned defects or shortcomings in the prior art, it is desirable to provide a polymer protective film, a metal lithium negative electrode, a lithium secondary battery and a vehicle, which can realize good combination with the negative electrode metal lithium, avoid contact between the metal lithium and the electrolyte, and at the same time make the distribution of lithium ions uniform, inhibit the generation of lithium dendrites, and thus improve the cycle performance of the battery.

[0005] In a first aspect, the present application provides a polymer protective film, and the structural formula of the polymer protective film is as follows:

[0006]

[0007] wherein R1 is selected from -(CH2) f -, -CH2-Y1-CH2- or -CH2-(CH2Y2CH2) j -CH2-, f is an integer from 0 to 6, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 4;

[0008] R2 is selected from fluorine, C1-C6 alkyl, phenyl, naphthyl or cyclohexane;

[0009] R3, R4, R5, R6 are each independently selected from a hydrogen or a fluorine atom, and at least one of R3, R4, R5, R6 is a fluorine atom;

[0010] R7, R8 are each independently selected from a hydrogen or a lithium atom, and at least one of R7, R8 is a hydrogen atom;

[0011] n1, n2, n3 are the molar ratios of the respective segments to the entire polymer, n1, n2, n3 are each independently any decimal number between 0 and 1, and n1 + n2 + n3 equals 1.0, n1 : n2 : n3 = 1 : (0.4 to 2.5) : (0 to 3); m is an integer from 1 to 3;

[0012] -(CH2) f -, -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j -CH2-, C1-C6 alkyl, phenyl, naphthyl, cyclohexyl, the hydrogen atoms in the above groups can be partially or totally substituted by a substituent.

[0013] As an optional solution, 0.2 ≤ n1 ≤ 0.5, 0.2 ≤ n2 ≤ 0.5, 0 < n3 ≤ 0.6.

[0014] As an optional solution, 0.3 ≤ n1 ≤ 0.45, 0.3 ≤ n2 ≤ 0.5, 0.05 ≤ n3 ≤ 0.4.

[0015] As an optional solution, n1 : n2 : n3 = 1 : (0.67 to 1.67) : (0.11 to 1.33).

[0016] As an optional solution, the substituent is selected from a halogen, a hydroxyl group, an amine group, a carbonyl group, a cyano group, a C1-C6 alkoxy group, a C1-C6 alkyl group, a C6-C 12 aryl group, or a C6-C 12 cycloalkyl group.

[0017] As an optional solution, the halogen is selected from one of fluorine, chlorine, and bromine; the amine group is selected from a C1-C6 primary amine, a C1-C6 alkyl-substituted secondary amine, or a tertiary amine; the C1-C6 alkoxy group is selected from a methoxy group or an ethoxy group; the C1-C6 alkyl group is selected from a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, or a tert-butyl group; the C6-C 12 aryl group is selected from a phenyl group, a naphthyl group, or a biphenyl group; the C6-C 12 cycloalkyl group is selected from a cyclohexyl group or a bicyclohexyl group.

[0018] As an optional solution, the molecular weight of the polymer protective film is from 10,000 to 500,000.

[0019] In a second aspect, the present application provides a metal lithium negative electrode, comprising a metal lithium sheet and the polymer protective film of the first aspect, the polymer protective film being arranged on the metal lithium sheet.

[0020] In a third aspect, the present application provides a lithium secondary battery, comprising the metal lithium negative electrode of the second aspect.

[0021] In a fourth aspect, the present application provides a vehicle, comprising the metal lithium secondary battery of the third aspect.

[0022] The polymer protective film provided by the present application comprises a single-ion conductor chain segment, a fluorinated ethoxy chain segment and a carboxylic acid / carboxylate lithium chain segment. The single-ion conductor chain segment is used for conducting lithium ions, which is conducive to increasing the number of lithium ion migration and making the distribution of lithium ions uniform, thereby preventing the generation of lithium dendrites and improving the cycle performance and safety performance of the battery. The fluorinated ethoxy chain segment is used for blocking the electrolyte to prevent the contact between the electrolyte and the lithium metal, thereby reducing the occurrence of side reactions. Meanwhile, the chain segment can also be used for conducting lithium ions, improving the electrical conductivity of the polymer, reducing the concentration polarization and further preventing the generation of lithium dendrites. The carboxylic acid / carboxylate lithium chain segment can form a strong action with the lithium metal, which can prevent the polymer and the lithium metal from being separated from contact and can also reduce the interface impedance between the polymer and the lithium metal, thereby improving the performance of the battery. DETAILED DESCRIPTION

[0023] The present application will be further described in detail below with reference to the embodiments. It can be understood that the specific embodiments described herein are only used to explain the related application, but not to limit the application. In addition, it should be noted that only the parts related to the application are shown in the embodiments for the convenience of description.

[0024] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the embodiments.

[0025] The embodiments of the present application provide a polymer protective film, and the structural formula of the polymer protective film is as follows:

[0026]

[0027] wherein R1 is selected from -(CH2) f -CH2-Y1-CH2- or -CH2-(CH2Y2CH2) j -CH2-, f is an integer from 0 to 6, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 4;

[0028] R2 is selected from fluorine, C1-C6 alkyl, phenyl, naphthyl and cyclohexane;

[0029] R3, R4, R5, and R6 are each independently selected from a hydrogen atom or a fluorine atom, and at least one of R3, R4, R5, and R6 is a fluorine atom;

[0030] R7 and R8 are each independently selected from a hydrogen atom or a lithium atom, and at least one of R7 and R8 is a hydrogen atom;

[0031] n1, n2, and n3 are the molar ratios of the corresponding segments in the whole polymer. n1, n2, and n3 are each independently any decimal number between 0 and 1, and n1 + n2 + n3 equals 1.0, n1:n2:n3 = 1:(0.4 - 2.5):(0 - 3); m is an integer from 1 to 3;

[0032] -(CH2) f -, -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j The hydrogen atoms in -, -CH2-, C1-C6 alkyl, phenyl, naphthyl, and cyclohexyl can be partially or completely substituted by substituents.

[0033] Among them, controlling the values of n1, n2, and n3 is beneficial to regulating the ionic conductivity of the polymer protective film, the ability to block the electrolyte, and the binding force with the lithium metal negative electrode.

[0034] Compared with the traditional method of generating an interfacial protective film by adding functional additives to the electrolyte, the polymer protective film in the embodiments of the present invention can combine with lithium metal to form a protective film on the surface of the lithium metal negative electrode, and can more reliably isolate the lithium metal negative electrode and the electrolyte.

[0035] In the polymer protective film of the embodiments of the present invention, the single-ion conductor segment can only conduct lithium ions, which can increase the lithium ion transference number and make the lithium ion distribution uniform, thereby preventing the generation of lithium dendrites and improving the cycle performance and safety performance of the battery; the fluorinated ethoxy segment can block the electrolyte to prevent the contact between the electrolyte and lithium metal, thereby reducing the occurrence of side reactions. This segment can also conduct lithium ions, improve the conductivity of the polymer, reduce the concentration polarization, and further prevent the generation of lithium dendrites; the carboxylic acid / carboxylate lithium segment can form a strong interaction with lithium metal. On the one hand, it can prevent the polymer from detaching from the lithium metal, and on the other hand, it can also reduce the interfacial impedance between the polymer and the lithium metal and improve the battery performance.

[0036] Furthermore, 0.2 ≤ n1 ≤ 0.5, 0.2 ≤ n2 ≤ 0.5; 0 < n3 ≤ 0.6; for example, n1 and n2 are 0.2, 0.3, 0.4, 0.5, etc.; n3 is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, etc.

[0037] Preferably, 0.3≤n1≤0.45, 0.3≤n2≤0.5; 0.05≤n3≤0.4. The value range of n1, n2 and n3 disclosed in the embodiment is advantageous to adjust the content of the single-ion conductor segment, the fluoro-ethoxy segment and the carboxylic acid / carboxylate segment, and further control the ability of the polymer protective film to conduct lithium ions, the ability to block electrolyte and prevent the polymer from falling off from the metal lithium, so that the polymer protective film has the optimal performance and improves the cycle performance of the lithium secondary battery.

[0038] Further, n1:n2:n3=1:(0.67~1.67):(0.11~1.33). The ratio of n1, n2 and n3 disclosed in the embodiment makes the single-ion conductor segment, the fluoro-ethoxy segment and the carboxylic acid / carboxylate segment in the polymer protective film have a suitable ratio, and further ensures that the polymer protective film has good lithium ion conduction ability, ensures uniform distribution of lithium ions, blocks the contact of electrolyte and metal lithium, reduces the occurrence of side reactions, prevents the formation of lithium dendrites, and is also advantageous to firmly combine the polymer protective film and the metal lithium negative electrode, so that the interface impedance between the polymer and the metal lithium is low.

[0039] Further, the substituent is selected from halogen, hydroxyl, amine group, carbonyl, cyano, C1-C6 alkoxy, C1-C6 alkyl, C6-C 12 aryl or C6-C 12 cycloalkyl. The substituent is advantageous to improve the lithium ion conduction ability of the single-ion conductor segment, and is advantageous to improve the lithium ion transference number, so that the lithium ions are uniformly distributed and the generation of lithium dendrites is more effectively prevented.

[0040] As a preferred embodiment, the halogen is selected from one of fluorine, chlorine and bromine; the amine group is selected from C1-C6 primary amine, C1-C6 alkyl-substituted secondary amine or tertiary amine; the C1-C6 alkoxy is selected from methoxy or ethoxy; the C1-C6 alkyl is selected from methyl, ethyl, propyl, isopropyl, butyl or tert-butyl; the C6-C 12 aryl is selected from phenyl, naphthyl or biphenyl; and the C6-C 12 cycloalkyl is selected from cyclohexyl or bicyclohexyl.

[0041] Further, the polymer protective film has a molecular weight of 10,000-500,000. For example, the polymer protective film can have a molecular weight of 10,000, 15,000, 20,000, 30,000, 50,000, 100,000, 180,000, 250,000, 300,000, 360,000, 400,000, 430,000, 480,000, 500,000, or the like. Preferably, the polymer protective film has a molecular weight of 50,000-500,000. The present embodiment does not limit the specific molecular weight. The polymer disclosed in the present embodiment has good adhesion to the lithium metal negative electrode, can effectively conduct lithium ions, prevent the lithium metal from contacting the electrolyte, prevent the generation of lithium dendrites, and thus is conducive to improving the cycle performance of the lithium secondary battery.

[0042] In summary, the polymer protective film of the present application has good adhesion to the lithium metal and good ion conductivity, effectively prevents the lithium metal from contacting the electrolyte, ensures the effective conduction of lithium ions, makes the lithium ions uniformly distributed, avoids the generation of lithium dendrites, is conducive to reducing the interfacial impedance between the polymer and the lithium metal, and improves the battery performance.

[0043] Further, by controlling the content of each segment, the single-ion conductor segment, the fluoroethoxy segment, and the carboxylic acid / carboxylate lithium segment in the polymer protective film have a suitable ratio, thereby improving the cycle performance of the battery.

[0044] In a second aspect, the present embodiment provides a lithium metal negative electrode, which comprises a lithium metal sheet and the polymer protective film of the first aspect, and the polymer protective film is arranged on the lithium metal sheet. Those skilled in the art can understand that the lithium battery negative electrode has all the features and advantages of the polymer protective film described above, and will not be described in detail here.

[0045] In a specific embodiment, the lithium metal negative electrode is prepared by the following process:

[0046] The polymer is dissolved in an organic solvent to obtain a mixed solution;

[0047] The mixed solution is coated on the lithium metal sheet, dried at room temperature, and then placed in a vacuum oven for drying to obtain the lithium metal negative electrode.

[0048] The organic solvent can be selected from any one of toluene, xylene, mesitylene, n-pentane, n-hexane, n-heptane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, diethyl ether, propyl ether, butyl ether, dimethyl ether of ethylene glycol, diethyl ether of ethylene glycol, methyl ethyl ether of ethylene glycol, acetone, butanone, ethyl acetate, butyl acetate, ethyl propionate, butyl propionate, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methyl pyrrolidone.

[0049] The coating method includes drop coating, blade coating, spin coating or spray coating, as long as the polymer protective film can be uniformly coated on the lithium metal sheet, which is not limited in the embodiment.

[0050] The thickness of the protective film is 1 μm-10 μm. For example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm or 10 μm. The thickness of the protective film in the embodiment is advantageous to ensure that the lithium metal negative electrode has high activity, while avoiding direct contact of the lithium metal with the electrolyte, thereby avoiding the generation of lithium dendrites and improving the cycle performance of the battery.

[0051] In order to ensure that the polymer protective film can be firmly combined on the lithium metal sheet, the drying temperature is 25°C-50°C, and the time is 0.5h-4h. The specific reaction temperature and time are not limited in the embodiment. The temperature and time range disclosed in the embodiment are advantageous to ensure that the polymer is firmly combined on the lithium metal sheet, while avoiding affecting the activity of the lithium metal negative electrode and the problem of decomposition of the polymer.

[0052] In a third aspect, the embodiment of the present application provides a lithium secondary battery comprising the lithium metal negative electrode of the second aspect. Those skilled in the art can understand that the lithium secondary battery has all the features and advantages of the polymer protective film described above, which will not be described in detail here. In general, the lithium secondary battery of the embodiment has good specific capacity and cycle stability.

[0053] In a specific embodiment, the lithium secondary battery further comprises a positive electrode, a separator and an electrolyte. The positive electrode comprises a positive electrode current collector and an active material layer on the positive electrode current collector, and the active material layer comprises a positive electrode active material, a binder and a conductive agent. The positive electrode active material can be selected from lithium cobaltate (LiCoO2), lithium nickelate (LiNiO2), lithium iron phosphate (LiFePO4), lithium cobalt phosphate (LiCoPO4), lithium manganese phosphate (LiMnPO4), lithium nickel phosphate (LiNiPO4), lithium manganate (LiMnO2), binary material LiNi x A (1-x) O2 (wherein A is selected from one of Co and Mn, 0 (1-m-n) O2 (wherein B and C are independently selected from at least one of Co, Al and Mn, and B and C are not the same, 0

[0054] The separator can be any separator material used in existing lithium secondary batteries, and specifically can be polyethylene, polypropylene, polyvinylidene fluoride and their multilayer composite films.

[0055] The electrolyte includes an organic solvent, a lithium salt, and an additive. The organic solvent can be selected from at least one of ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, 1,4-dioxane, tetrahydrofuran, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methyl ethyl carbonate, methyl formate, ethyl formate, ethyl propionate, propyl propionate, methyl butyrate, ethyl acetate, anhydride, N-methyl pyrrolidone, N-methyl formamide, N-methyl acetamide, acetonitrile, sulfolane, dimethyl sulfoxide, ethylene sulfite, propylene sulfite, dimethyl sulfide, diethyl sulfite, dimethyl sulfite, fluorine-containing cyclic organic ester, sulfur-containing cyclic organic ester; the lithium salt is selected from at least one of organic lithium salt, inorganic lithium salt, such as LiPF6, LiBF4, LiTFSI, LiFSI, LiClO4, LiAsF6, LiBOB, LiDFOB, LiTFOP; the additive can be selected from at least one of vinylene carbonate, fluorinated carbonate, difluorinated vinylene carbonate, fluorinated vinylene carbonate, vinylene ethylene carbonate, vinyl sulfite, methanedi sulfonate, 1,3-propane sulfonic acid endolactone, 1,3-propylene sulfonic acid endolactone, vinyl sulfate, lithium difluorophosphate, lithium difluorobisoxalate phosphate, lithium tetrafluoroxalate phosphate.

[0056] In a fourth aspect, the present application provides a vehicle including the lithium secondary battery of the third aspect. For example, a plurality of battery packs composed of the lithium secondary battery described above can be included. Thus, the vehicle has all the features and advantages of the lithium secondary battery described above, which will not be repeated here.

[0057] The present application will be described below through specific examples, and it should be noted that the specific examples below are only for illustrative purposes, and do not limit the scope of the present application in any way.

[0058] Example 1

[0059] (1) Preparation of metal lithium negative electrode:

[0060] The polymer I is dissolved in 1,4-dioxane to obtain a mixed solution. The structural formula of the polymer I is as follows, and the molar ratio of the three structural units n1:n2:n3=1:1.5:0.83.

[0061]

[0062] The mixed solution is coated on the metal lithium sheet, and after air-drying at room temperature, it is placed in a vacuum oven for drying to obtain a metal lithium negative electrode.

[0063] (2) Half-cell preparation

[0064] The positive active material lithium cobaltate (LiCoO2), conductive agent carbon black (Super-P) and binder polyvinylidene fluoride (PVDF) are uniformly dispersed in solvent N-methyl pyrrolidone (NMP), and after being uniformly ground in a mortar, the slurry is coated on a copper foil by a coating machine, the thickness of the coated slurry is 100 μm; then it is dried at room temperature, cut into a disc with a diameter of 13 mm by a slicer, then the disc is placed in a vacuum drying oven at 80°C for 12 h, and taken out when the temperature drops to room temperature after drying, obtaining a positive electrode sheet;

[0065] The positive electrode sheet is transferred to an argon-filled glove box (O2 content ≤0.5 ppm, H2O ≤0.5 ppm), and the above-obtained metal lithium negative electrode sheet is used as the counter electrode, 1 mol / L LiPF6EC / DMC / DEC (v / v / v = 1 / 1 / 1) solution is used as the electrolyte, and a CR2025 type button half-cell is assembled in the glove box.

[0066] Example 2

[0067] The difference between this example and Example 1 is that:

[0068] The structural formula of polymer II is shown below, and the molar ratio of the three structural units n1:n2:n3 = 1:1.12:0.37;

[0069]

[0070] Example 3

[0071] The difference between this example and Example 1 is that:

[0072] The structural formula of polymer III is shown below, and the molar ratio of the three structural units n1:n2:n3 = 1:0.9:0.1

[0073]

[0074] Example 4

[0075] The difference between this example and Example 2 is that:

[0076] The structural formula of polymer IV is shown below, and m = 1

[0077]

[0078] Example 5

[0079] The difference between this example and Example 2 is that:

[0080] The structural formula of polymer V is shown below, and R1 is -CF2-

[0081]

[0082] Example 6

[0083] The difference between this example and Example 2 is that:

[0084] The structural formula of polymer VI is shown below, R2 is -CH3

[0085]

[0086] Example 7

[0087] The difference between this example and Example 2 is that:

[0088] The structural formula of polymer VII is shown below, R3 and R4 are both H;

[0089]

[0090] Example 8

[0091] The difference between this example and Example 2 is that:

[0092] The structural formula of polymer VIII is shown below, R7 is Li

[0093]

[0094] Comparative Example 1

[0095] The difference between this comparative example and Example 2 is that: the molar ratio of the three structural units n1:n2:n3 = 1:1:4.66

[0096]

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 2 is that: n3 = 0, n1:n2 = 1:1.12

[0099]

[0100] Comparative Example 3

[0101] The difference between this comparative example and Example 2 is that: n2 = 0, n1:n3 = 1:0.37

[0102]

[0103] Comparative Example 4

[0104] The difference between this comparative example and Example 2 is that: n1 = 0, n2:n3 = 3:1

[0105]

[0106] Comparative Example 5

[0107] The difference between this comparative example and Example 2 is that n2 = 0, n3 = 0, and the value of z1 is chosen to make the molecular weight of the polymer between 50,000 and 500,000.

[0108]

[0109] Comparative Example 6

[0110] The difference between this comparative example and Example 2 is that n1 = 0, n3 = 0, and the value of z2 is chosen to make the molecular weight of the polymer between 50,000 and 500,000.

[0111]

[0112] Comparative Example 7

[0113] The difference between this comparative example and Example 2 is that n1 = 0, n2 = 0, and the value of z3 is such that the molecular weight of the polymer is 50,000 to 500,000.

[0114]

[0115] Comparative Example 8

[0116] The difference between this comparative example and Example 2 is that the lithium sheet is not coated with a polymer protective film.

[0117] The half-cells prepared in the above examples and comparative examples were subjected to the following performance tests to characterize the electrochemical performance of the polymer protective film.

[0118] The testing procedure is as follows: Ten batteries each from the examples and comparative examples were taken and subjected to charge-discharge cycle testing at 0.5 C on a LAND CT 2001C secondary battery performance testing device at 25±1℃. The steps were as follows: rest for 5 minutes; constant current charging to 4.2 V cutoff; rest for 5 minutes; constant current discharging to 3.0 V, which constitutes one cycle. This step was repeated. During the cycle, the cycle was terminated when the battery capacity was lower than 80% of the initial discharge capacity. The number of cycles is the cycle life of the battery.

[0119] The test results are shown in Table 1:

[0120] Table 1 Performance test results of the half-cells prepared in the examples and comparative examples

[0121]

[0122] From the results shown in Table 1, the following conclusions can be drawn:

[0123] According to the test results of Examples 1-8 and Comparative Example 8, it can be seen that the half-cells of the polymer protective film assemblies of Examples 1-8 are superior to the half-cell of Comparative Example 8 in terms of the first charge-discharge capacity, the first coulombic efficiency and the cycle life. The metal lithium negative electrode of the half-cell of Comparative Example 8 is not coated with a polymer protective film, and therefore the polymer protective film of the present application is beneficial to improving the performance of the battery.

[0124] According to the test results of Examples 1-3 and Comparative Example 1, it can be seen that the half-cells of Examples 1-3 are superior to the half-cell of Comparative Example 1 in terms of the first charge-discharge capacity, the first coulombic efficiency and the cycle life. The molar ratio of the polymer single-ion conductor segment, the fluorinated ethoxy segment and the carboxylic acid / carboxylate segment of Comparative Example 1 is 1:1:4.66, in which the content of the carboxylic acid / carboxylate segment is excessive, which can react violently with lithium metal, consume a large amount of active lithium, and thus reduce the performance of the battery. Therefore, it can be seen that the molar ratio range of the single-ion conductor segment, the fluorinated ethoxy segment and the carboxylic acid / carboxylate segment disclosed in the present application is beneficial to the polymer protective film having good lithium ion conductivity and good binding force with metal lithium, so that the lithium ions are uniformly distributed, the generation of lithium dendrites is inhibited, and thus the cycle performance of the battery is improved.

[0125] According to the test results of Examples 4-8, it can be seen that different R groups have little effect on the performance of the polymer, and the polymer structures disclosed in the present application can effectively improve the performance of the battery.

[0126] According to the test results of Example 2 and Comparative Examples 2-7, it can be seen that the half-cell of Example 2 is superior to the half-cells of Comparative Examples 2-7 in terms of the first charge-discharge capacity, the first coulombic efficiency and the cycle life. The polymers of Comparative Examples 2-4 only include two of the segments, and the polymers of Comparative Examples 5-7 only include one of the segments. Therefore, it can be seen that the polymer disclosed in the present application can improve the performance of the battery due to the synergistic effect of the single-ion conductor segment, the fluorinated ethoxy segment and the carboxylic acid / carboxylate segment, which enables the polymer protective film to combine with the metal lithium negative electrode and ensures that the metal lithium negative electrode has good ion conductivity, effectively isolates the metal lithium and the electrolyte, ensures the effective conduction of lithium ions, promotes the uniform distribution of lithium ions, inhibits the generation of lithium dendrites, and is also beneficial to reducing the interfacial impedance between the polymer and the metal lithium, and improving the cycle performance of the battery.

[0127] The above description is only the preferred embodiment of the present application and the explanation of the technical principles. It should be understood by those skilled in the art that the scope of the protection of the present application is not limited to the technical solutions formed by the specific combinations of the above technical features. It should also cover other technical solutions formed by the combinations of the above technical features or their equivalent features without departing from the concept of the present application. For example, the technical solutions formed by the mutual replacements of the above features and the technical features disclosed in the present application (but not limited to) with similar functions.

Claims

1. A metal lithium negative electrode, characterized by, The metal lithium sheet and a polymer protective film are included, the polymer protective film is set on the metal lithium sheet, and the polymer protective film has a structural formula as follows: wherein R1is selected from -(CH2) f -CH2-Y1-CH2- or -CH2-(CH2Y2CH2) j -CH2-, f is an integer from 0 to 6, Y1, Y2are each independently selected from O, NH or S, j is an integer from 1 to 4; R2 is selected from fluorine, C1-C6 alkyl, phenyl, naphthyl or cyclohexane; R3, R4, R5, R6 are each independently selected from hydrogen or fluorine atom, and at least one of R3, R4, R5, R6 is fluorine atom; R7, R8 are each independently selected from hydrogen or lithium atom, and at least one of R7, R8 is hydrogen atom; n1, n2, n3 are molar ratios of corresponding segments in the whole polymer, n1, n2, n3 are each independently any decimal number between 0 and 1, and n1+n2+n3 equals to 1.0, n1:n2:n3 = 1:(0.67-1.67):(0.11-1.33); m is an integer between 1 and 3; said -(CH2) f -CH2-, said -CH2-Y1-CH2-, said -CH2-(CH2Y2CH2) j a hydrogen atom in said -CH2-, said C1-C6 alkyl, said phenyl, said naphthyl, said cyclohexane group can be partially or totally replaced by a substituent; The molecular weight of the polymer protective film is 10000-500000; said substituents are selected from the group consisting of halogen, hydroxyl, amine, carbonyl, cyano, C1-C6alkoxy, C1-C6alkyl, C6-C 12 aryl or C6-C 12 cycloalkyl.

2. The lithium metal anode of claim 1, wherein, 0.2≤n1≤0.5, 0.2≤n2≤0.5, 0 3. The lithium metal anode of claim 2, wherein, 0.3≤n1≤0.45, 0.3≤n2≤0.5, 0.05≤n3≤0.

4.

4. The lithium metal anode of claim 1, wherein, said halogen is selected from one of fluorine, chlorine, bromine; said amine group is selected from a C1-C6 primary amine, a C1-C6 alkyl substituted secondary amine or a tertiary amine; said C1-C6 alkoxy group is selected from methoxy or ethoxy; said C1-C6 alkyl group is selected from methyl, ethyl, propyl, isopropyl, butyl or tert-butyl; said C6-C 12 aryl group is selected from phenyl, naphthyl, or biphenyl; said C6-C 12 cycloalkyl group is selected from cyclohexyl or bicyclohexyl.

5. A lithium secondary battery characterized by comprising: The metal lithium negative electrode of any one of claims 1-4 is included.

6. A vehicle characterized by comprising: The lithium secondary battery of claim 5 is included.

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