Polymer protective films, lithium metal anodes, lithium secondary batteries, and vehicles

Abstract

This application discloses a polymer protective film, a lithium metal anode, a lithium secondary battery, and a vehicle. The structural formula of the polymer protective film is as follows: Under the synergistic effect of imidazole groups and polyaniline segments, the polymer protective film of this application has good flexibility, mechanical strength, ionic conductivity, and electrical conductivity. It can effectively adapt to the expansion of lithium metal, improve lithium ion mobility and transport, make lithium ion distribution uniform, thereby inhibiting the growth of lithium dendrites and improving the cycle performance of lithium secondary batteries.

Description

Technical Field

[0001] This invention generally relates to the field of lithium battery technology, and specifically to a polymer protective film, a lithium metal anode, a lithium secondary battery, and a vehicle. Background Technology

[0002] Lithium metal possesses an extremely high theoretical specific capacity (3860 mAh / g) and an extremely low electrochemical potential (−3.040 V). vs The standard hydrogen electrode (HME) is an ideal negative electrode material for lithium secondary batteries. However, there are two main obstacles limiting the further development of lithium secondary batteries: (1) lithium dendrites are easily generated during cycling, leading to short circuits; (2) lithium metal is easily broken and pulverized during cycling, resulting in loss of active lithium and shortening the battery cycle life. Studies have shown that constructing a protective film on the surface of lithium metal can significantly reduce the side reactions between lithium metal and electrolyte, inhibit dendrite growth, and improve battery cycle life.

[0003] Currently, conventional protective films mainly include polymer films and inorganic / organic hybrid films. However, existing polymer protective films have poor mechanical strength, failing to effectively suppress lithium dendrite growth. Furthermore, existing polymer protective films exhibit poor interaction and wettability with lithium metal, hindering lithium-ion conduction at the interface. Existing organic / inorganic hybrid protective films have poor toughness; the expansion and pulverization of lithium metal can cause the protective film to crack, leading to side reactions between lithium metal and the electrolyte, thus losing its protective effect. Therefore, current conventional protective films cannot effectively improve the cycle performance of lithium-ion secondary batteries. Summary of the Invention

[0004] In view of the above-mentioned defects or deficiencies in the prior art, it is desirable to provide a polymer protective film, a lithium metal anode, a lithium secondary battery, and a vehicle, which can suppress the growth of lithium dendrites while effectively conducting lithium ions, adapting to the expansion of lithium, and thus improving the cycle performance of the lithium secondary battery.

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

[0006]

[0007] R1 is selected from one of the following: bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, perchlorate, hexafluorophosphate, hexafluoroarsenate, tetrafluoroborate, dioxaloateborate, difluorooxaloateborate, and trifluoromethanesulfonate.

[0008] R2 is selected from -(CH2). i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; where i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100;

[0009] p is the molar ratio of reduced polyaniline segments to polyaniline segments, p is any decimal between 0 and 1, m and n are the molar ratios of polyimide segments and polyaniline segments to the whole polymer, respectively, m and n are each independent decimals between 0 and 1 and m+n equals 1.0;

[0010] -(CH2) i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j The hydrogen atoms in -CH2- can be partially or completely replaced by substituents.

[0011] As an alternative, 0.5≤m≤0.95, 0.05≤n≤0.5.

[0012] As an alternative, 0.6≤m≤0.8, 0.2≤n≤0.4.

[0013] As an alternative, the substituent is selected from halogen, hydroxyl, amino, carbonyl, cyano, C1-C6 alkoxy, C1-C6 alkyl, C6-C 12 aryl or C6-C 12 cycloalkyl groups.

[0014] As an alternative, the halogen is selected from fluorine, chlorine, and bromine; the amino group is selected from C1-C6 primary amines, C1-C6 alkyl-substituted secondary or tertiary amines; the C1-C6 alkoxy group is selected from methoxy or ethoxy; the C1-C6 alkyl group is selected from methyl, ethyl, propyl, isopropyl, butyl, or tert-butyl; and the C6-C6 alkoxy group is selected from methyl, ethyl, propyl, isopropyl, butyl, or tert-butyl. 12 The aryl group is selected from phenyl, naphthyl, or biphenyl; C6-C 12 The cycloalkyl group is selected from cyclohexyl or bicyclohexyl.

[0015] As an optional option, the molecular weight of the polymer protective film is 10,000 to 500,000, preferably 100,000 to 300,000.

[0016] In a second aspect, the present invention provides a method for preparing the polymer protective film described in the first aspect, comprising the following steps:

[0017] Aniline and an initiator are dissolved in water and heated to react, yielding mixture I.

[0018] Acid and diamine-terminated compound are added to mixture I to obtain mixture II;

[0019] Under ice-water bath conditions, a mixture of formaldehyde and acetaldehyde was added dropwise to mixture II, heated and reacted to obtain mixture III;

[0020] After cooling, mixture III is added dropwise to an aqueous solution of anion exchanger to react and form a precipitate;

[0021] The precipitate is washed and dried to obtain a polymer protective film.

[0022] As an optional solution, the acid can be any one of acetic acid, hydrochloric acid, sulfuric acid, nitric acid, or phosphoric acid.

[0023] Alternatively, the diamine-terminated compound may be selected from substituted or unsubstituted NH2-R2-NH2, wherein R2 is selected from -(CH2). i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100.

[0024] As an optional approach, the temperature for heating the reaction is 40℃~100℃, and the reaction time is 0.5h~12h.

[0025] As an optional option, the anion exchanger is any one of lithium bis(trifluoromethanesulfonate)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium di(oxalate)borate, lithium di(fluorooxalate)borate, or lithium trifluoromethanesulfonate.

[0026] Thirdly, the present invention provides a lithium metal anode, comprising a lithium metal sheet and a polymer protective film as described in the first aspect, wherein the polymer protective film is disposed on the lithium metal sheet.

[0027] Fourthly, the present invention provides a lithium secondary battery, comprising a metallic lithium anode as described in the third aspect.

[0028] Fifthly, the present invention provides a vehicle comprising the lithium secondary battery of the fourth aspect.

[0029] The polymer protective film provided in this application includes imidazole groups and polyaniline segments. The imidazole groups can immobilize anions, improve lithium-ion mobility, reduce lithium dendrite formation, and simultaneously promote lithium-ion transport, thereby increasing the conductivity of the protective film. The polyaniline segments are rigid polymers, which helps improve the mechanical strength of the protective film. Furthermore, polyaniline can form a negative electrode with a metal anode, creating a gradient of conductivity. When lithium ions pass through the polymer layer, they are evenly distributed due to the electric field, preventing lithium ion aggregation and deposition, thus inhibiting lithium dendrite growth. Detailed Implementation

[0030] The present application will now be described in further detail with reference to the embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention. Furthermore, it should be noted that, for ease of description, only the parts relevant to the invention are shown in the embodiments.

[0031] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present application will now be described in detail with reference to the embodiments.

[0032] An embodiment of the present invention provides a polymer protective film, the structural formula of which is as follows:

[0033]

[0034] R1 is selected from one of the following: bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, perchlorate, hexafluorophosphate, hexafluoroarsenate, tetrafluoroborate, dioxaloateborate, difluorooxaloateborate, and trifluoromethanesulfonate.

[0035] R2 is selected from -(CH2). i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; where i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100;

[0036] The polyaniline segment is composed of reduced polyaniline segments and oxidized polyaniline segments, where p is the molar ratio of reduced polyaniline segments to polyaniline segments, 1-p is the molar ratio of oxidized polyaniline segments to polyaniline segments, and p is any decimal between 0 and 1; m and n are the molar ratios of the entire polymer polyimide segments and polyaniline segments, respectively, and m and n are each independent decimals between 0 and 1, with m+n equal to 1.0;

[0037] -(CH2) i -、 , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j The hydrogen atoms in -CH2- can be partially or completely replaced by substituents.

[0038] Controlling the values ​​of m and n is beneficial for modulating the content of imidazole groups and polyaniline segments in the polymer protective film, thereby helping to control the ionic conductivity, mechanical strength, and flexibility of the polymer protective film; controlling the value of p is beneficial for modulating the content of doped groups in the polyaniline segments, thereby helping to control the conductivity of the polymer protective film.

[0039] The polymer protective film of this invention includes imidazole groups and polyaniline segments. The imidazole groups help immobilize anions, improve lithium-ion mobility, reduce lithium dendrite formation, and promote lithium-ion transport, thus increasing the conductivity of the protective film. The polyaniline segments, as a rigid polymer, enhance the mechanical strength of the protective film. Furthermore, polyaniline is a conductive polymer, enabling it to form a negative electrode with a different conductivity gradient with the metal negative electrode. Specifically, the less conductive polyaniline covers the surface of the highly conductive lithium metal negative electrode. The advantage of this gradient structure is that lithium ions can easily and uniformly distribute themselves as they pass through the conductive polymer layer due to the electric field. However, because the metal layer has better conductivity and a lower potential (conductive polymers have resistance and a higher potential), the uniformly distributed lithium ions will uniformly nucleate and deposit in the metal layer beneath the protective film, helping to prevent battery short circuits. Simultaneously, because the electric field in the conductive polymer forces a uniform distribution of lithium ions, lithium ions will not preferentially accumulate and deposit at the tips, thereby preventing the growth of lithium dendrites.

[0040] Compared to conventional protective films, the polymer protective film of this application exhibits superior flexibility, mechanical strength, ionic conductivity, and electrical conductivity. The imidazole groups and polyaniline in the polymer protective film of this application synergistically improve the performance of the lithium metal anode. While imidazole groups are used to conduct lithium ions, they cannot form anodes with different conductivity gradients with the metal anode. The polyaniline segments have poor ionic conductivity, but they can improve the mechanical strength of the protective film while simultaneously forming different conductivity gradients with the metal anode. Therefore, the imidazole groups and polyaniline segments work together to improve the cycle performance of the lithium metal anode.

[0041] Furthermore, conductive polymers have poor processability and are usually impossible to process after polymerization, making it difficult to mix with other polymers. The polymers in the embodiments of this application include imidazole groups and polyaniline segments, which improve the cycle performance of lithium metal anodes while solving the problem of poor processability of conductive polymers, making them easy to process and effectively achieving the superposition of the advantages of imidazole groups and polyaniline segments.

[0042] Furthermore, 0.5 ≤ m ≤ 0.95, 0.05 ≤ n ≤ 0.5. For example, m can be 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, etc.; n can be 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc.

[0043] Preferably, 0.6 ≤ m ≤ 0.8, 0.2 ≤ n ≤ 0.4. The ranges of m and n disclosed in the embodiments of this application are beneficial for the polymer protective film to have optimal ionic conductivity and electrical conductivity, which in turn helps to improve lithium ion mobility and transport, as well as to make lithium ion distribution uniform and suppress the formation of lithium dendrites.

[0044] Furthermore, the substituents are selected from halogens, hydroxyl groups, amino groups, carbonyl groups, cyano groups, C1-C6 alkoxy groups, C1-C6 alkyl groups, and C6-C6 alkyl groups. 12 aryl or C6-C 12 The cycloalkyl group. Substituents are beneficial for improving the ionic conductivity of the polymer, increasing the lithium-ion transference number, and improving the polymer's electrical conductivity, resulting in a more uniform distribution of lithium ions and more effectively preventing the formation of lithium dendrites.

[0045] In a preferred embodiment, the halogen is selected from fluorine, chlorine, and bromine; the amino group is selected from C1-C6 primary amines, C1-C6 alkyl-substituted secondary or tertiary amines; the C1-C6 alkoxy group is selected from methoxy or ethoxy; the C1-C6 alkyl group is selected from methyl, ethyl, propyl, isopropyl, butyl, or tert-butyl; C6-C 12 The aryl group is selected from phenyl, naphthyl, or biphenyl; C6-C 12 The cycloalkyl group is selected from cyclohexyl or bicyclohexyl.

[0046] Furthermore, the molecular weight of the polymer protective film is between 10,000 and 500,000. For example, the molecular weight of the polymer protective film can be 10,000, 15,000, 20,000, 30,000, 50,000, 100,000, 180,000, 250,000, 30,000, 360,000, 400,000, 430,000, 480,000, 500,000, etc. Preferably, the molecular weight of the polymer protective film is between 100,000 and 300,000. This application does not limit the specific molecular weight in its embodiments.

[0047] In summary, the polymer disclosed in this application, under the synergistic effect of imidazole groups and polyaniline segments, has good flexibility, mechanical strength, ionic conductivity and electrical conductivity. It can effectively adapt to the expansion of lithium metal, improve lithium ion mobility and transport, make lithium ion distribution uniform, thereby inhibiting the growth of lithium dendrites and improving the cycle performance of lithium secondary batteries.

[0048] Furthermore, by controlling the content of imidazole groups and polyaniline segments, a suitable ratio of imidazole groups and polyaniline segments in the polymer protective film can be achieved, thereby improving the cycle performance of the battery.

[0049] In a second aspect, the present invention provides a method for preparing a polymer protective film according to the first aspect, comprising the following steps:

[0050] Aniline and an initiator are dissolved in water and heated to react, yielding mixture I.

[0051] Acid and diamine-terminated compound are added to mixture I to obtain mixture II;

[0052] Under ice-water bath conditions, a mixture of formaldehyde and acetaldehyde was added dropwise to mixture II, heated and reacted to obtain mixture III;

[0053] After cooling, mixture III is added dropwise to an aqueous solution of anion exchanger to react and form a precipitate;

[0054] The precipitate is washed and dried to obtain a polymer protective film.

[0055] It should be noted that the initiator can be any persulfate or peroxide, such as potassium persulfate, sodium persulfate, ammonium persulfate, etc. Here, the initiator is used for the polymerization of aniline. Heating facilitates the polymerization of aniline, resulting in polyaniline.

[0056] The purpose of adding acid is to provide acidic conditions, while the acid radical ions serve as the balancing anions for the cations involved in the synthesis and polymerization, and the acid radical ions also facilitate ion exchange with other ions.

[0057] The diamine-terminated compound can be one or a mixture of two or more diamine-terminated compounds, wherein the diamine-terminated compounds are mixed in a certain proportion, which is not specifically limited here; in specific embodiments, the diamine-terminated compound can be NH2-R2-NH2, and R2 is selected according to the specific polymer structure.

[0058] A mixture of formaldehyde and acetaldehyde is used to react with diamine-terminated compounds under acidic conditions to generate a polymer backbone containing imidazole groups.

[0059] For example,

[0060] Aniline and potassium persulfate were dissolved in water, heated to 100°C, and reacted for 2 hours to obtain mixture I;

[0061] Add glacial acetic acid and NH2-CH2-CH2-NH2 to mixture I to obtain mixture II;

[0062] Under ice-water bath conditions, a mixture of formaldehyde and acetaldehyde was added dropwise to mixture II, heated to 100°C, and reacted for 2 hours to obtain mixture III;

[0063] After cooling, mixture III was added dropwise to an aqueous solution of lithium bis(trifluoromethanesulfonyl)imide to react and form a precipitate;

[0064] A polymer protective film is obtained by washing and drying the precipitate. Its structural formula can be represented as follows: , where R1 is TFSI and R2 is -CH2-CH2-.

[0065] Furthermore, the acid solution is any one of acetic acid, hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

[0066] Furthermore, the diamine-terminated compound is selected from substituted or unsubstituted NH2-R2-NH2, wherein R2 is selected from -(CH2)i-, , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100. For example, a diamine-terminated compound can be , , wait.

[0067] In the diamine-terminated compound, the substituents can be selected from halogens, hydroxyl groups, carbonyl groups, cyano groups, C1-C6 alkyl groups, C6-C6 alkyl groups, and C6-C6 alkyl groups. 12 aryl or C6-C 12 At least one of the cycloalkyl groups.

[0068] It should be noted that,

[0069] The halogen is selected from fluorine, chlorine, and bromine; the C1-C6 alkyl group is selected from methyl, ethyl, propyl, isopropyl, butyl, or tert-butyl; C6-C 12 The aryl group is selected from phenyl, naphthyl, or biphenyl; C6-C 12 The cycloalkyl group is selected from cyclohexyl or bicyclohexyl.

[0070] Furthermore, the heating temperature for the reaction is 40℃~100℃, and the reaction time is 0.5~12h. For example, the temperature can be 40℃, 50℃, 60℃, 70℃, 80℃, 90℃, or 100℃, etc.; and the time can be 0.5h, 1h, 2h, 3h, 4h, 6h, 7h, 9h, 11h, or 12h, etc. This application does not limit the specific reaction temperature and time in its embodiments. Specifically, under heating conditions, the reaction of the mixture of formaldehyde and acetaldehyde with mixture II is beneficial for promoting the reaction and increasing the yield of the reactants.

[0071] Furthermore, the anion exchanger is any one of lithium bis(trifluoromethanesulfonate)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium di(oxalate)borate, lithium di(fluorooxalate)borate, or lithium trifluoromethanesulfonate.

[0072] Thirdly, the present invention provides a lithium metal anode, comprising a lithium metal sheet and a polymer protective film according to the first aspect, the polymer protective film being disposed on the lithium metal sheet. Those skilled in the art will understand that this lithium battery anode possesses all the features and advantages of the polymer protective film described above, which will not be elaborated further here.

[0073] In a specific embodiment, a lithium metal anode is prepared through the following process:

[0074] The above polymer is dissolved in an organic solvent to obtain a mixture;

[0075] The mixture is coated onto a lithium metal sheet, dried at room temperature, and then dried in a vacuum oven to obtain a lithium metal anode.

[0076] The organic solvent may be selected from any one of toluene, xylene, trimethylbenzene, n-pentane, n-hexane, n-heptane, tetrahydrofuran, 1,3-dioxolane, 1,4-dioxane, diethyl ether, propyl ether, butyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, acetone, butanone, ethyl acetate, butyl acetate, ethyl propionate, butyl propionate, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, and N-methylpyrrolidone.

[0077] The coating method can include 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. This embodiment does not make specific limitations on this.

[0078] The thickness of the protective film is 1-10 μm. For example, it can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm. The thickness of the protective film in this embodiment is beneficial to ensuring that the lithium metal anode has high activity, while avoiding direct contact between the lithium metal and the electrolyte, thereby avoiding the generation of lithium dendrites and improving the cycle performance of the battery.

[0079] In order to ensure that the polymer film can be firmly bonded to the lithium metal sheet, the drying temperature is 25°C to 50°C, and the time is 0.5 h to 4 h. The specific drying temperature and time are not limited in the embodiments of the present invention. The temperature and time ranges disclosed in the embodiments of the present invention are beneficial to ensuring that the polymer is firmly bonded to the lithium metal sheet while avoiding affecting the activity of the lithium metal anode and causing problems such as polymer decomposition.

[0080] In the fourth aspect, the present invention provides a lithium secondary battery, including the lithium metal anode of the third aspect. Those skilled in the art can understand that this lithium secondary battery has all the features and advantages of the aforementioned polymer protective film, and will not be elaborated here too much. Generally speaking, the lithium secondary battery of the embodiments of the present invention has good specific capacity and cycle stability performance.

[0081] In a specific embodiment, the lithium secondary battery further includes: a positive electrode, a separator, and an electrolyte. Among them, the positive electrode includes a positive electrode current collector and an active material layer located on the positive electrode current collector. The active material layer includes 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 (where A is selected from one of Co and Mn, 0 < x < 1), ternary material LiNimBnC (1-m-n) O2 (where B and C are independently selected from at least one of Co, Al, and Mn, and B and C are different, 0 < m < 1, 0 < n < 1).

[0082] The separator can be any separator material used in existing lithium secondary batteries. Specifically, it can be polyethylene, polypropylene, polyvinylidene fluoride, and their multilayer composite films.

[0083] The electrolyte comprises an organic solvent, a lithium salt, and additives. The organic solvent may be selected from 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, acid anhydride, N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile, sulfolane, dimethyl sulfoxide, ethylene sulfite, propylene sulfite, dimethyl sulfide, diethyl sulfite, diethyl sulfite, dimethyl sulfite, 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, and other compounds containing... At least one of fluorinated cyclic organic esters and sulfur-containing cyclic organic esters; the lithium salt is selected from at least one of organic lithium salts and inorganic lithium salts, such as LiPF6, LiBF4, LiTFSI, LiFSI, LiClO4, LiAsF6, LiBOB, LiDFOB, and LiTFOP; the additive may be selected from at least one of vinylene carbonate, fluorocarbonate, difluoroethylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, vinyl sulfite, methanedisulfonate, 1,3-propanesulfonate lactone, 1,3-propenesulfonate lactone, vinyl sulfate, lithium difluorophosphate, lithium difluorobis(oxalate) phosphate, and lithium tetrafluoro(oxalate) phosphate.

[0084] Fifthly, the present invention provides a vehicle comprising the lithium secondary battery of the fourth aspect. For example, it may include a battery pack composed of multiple lithium secondary batteries as described above. Thus, the vehicle possesses all the features and advantages of the lithium secondary batteries described above, which will not be repeated here.

[0085] The present invention will be described below through specific embodiments. It should be noted that the specific embodiments below are for illustrative purposes only and do not limit the scope of the present invention in any way.

[0086] Example 1

[0087] (1) Prepare polymer I. The structural formula of polymer I is as follows:

[0088]

[0089] The specific process is as follows:

[0090] Aniline and potassium persulfate were dissolved in water, heated to 100°C, and reacted for 2 hours to obtain mixture I;

[0091] glacial acetic acid and Add to mixture I to obtain mixture II;

[0092] Under ice-water bath conditions, a mixture of formaldehyde and acetaldehyde was added dropwise to mixture II, heated to 100°C, and reacted for 2 hours to obtain mixture III;

[0093] After cooling, mixture III was added dropwise to an aqueous solution of lithium bis(trifluoromethanesulfonyl)imide to react and form a precipitate;

[0094] A polymer protective film is obtained by washing and drying the precipitate. Its structural formula can be represented as follows: .

[0095] (2) Preparation of lithium metal anode:

[0096] Polymer I was dissolved in 1,4-dioxane to obtain a mixture.

[0097] The mixture is coated onto a lithium metal sheet, dried at room temperature, and then dried in a vacuum oven to obtain a lithium metal anode.

[0098] (3) Preparation of half-cell

[0099] The positive electrode active material lithium cobalt oxide (LiCoO2), conductive agent carbon black (Super-P), and binder polyvinylidene fluoride (PVDF) were uniformly dispersed in the solvent N-methylpyrrolidone (NMP), ground uniformly in a mortar, and then the slurry was coated onto copper foil using a coating machine to a thickness of 100 μm. It was then air-dried at room temperature and cut into circular pieces with a diameter of 13 mm using a slicing machine. The circular pieces were then placed in a vacuum drying oven at 80 °C and dried for 12 h. After drying, they were removed when the temperature dropped to room temperature to obtain the positive electrode sheet.

[0100] The positive electrode was transferred to a glove box filled with argon (O2 ≤ 0.5 ppm, H2O ≤ 0.5 ppm). Using the above-mentioned lithium metal negative electrode as the counter electrode and a 1 mol / L LiPF6EC / DMC / DEC (v / v / v = 1 / 1 / 1) solution as the electrolyte, a CR2025 coin cell was assembled in the glove box.

[0101] Example 2

[0102] The difference between this embodiment and Embodiment 1 is that:

[0103] Polymer II was prepared, and the structural formula of polymer II is shown below:

[0104]

[0105] Example 3

[0106] The difference between this embodiment and Embodiment 1 is that:

[0107] Polymer III was prepared, and its structural formula is shown below:

[0108]

[0109] Example 4

[0110] The difference between this embodiment and Embodiment 1 is that:

[0111] Polymer IV was prepared, and its structural formula is shown below:

[0112]

[0113] Example 5

[0114] The difference between this embodiment and Embodiment 1 is that:

[0115] Polymer V was prepared, and the structural formula of polymer V is shown below:

[0116]

[0117] Comparative Example 1

[0118] The difference between this comparative example and Example 1 is that the polymer is a polymer containing only imidazole groups, with the following structural formula:

[0119]

[0120] The value of x determines the molecular weight of the polymer, which is between 100,000 and 300,000.

[0121] Comparative Example 2

[0122] The difference between this comparative example and Example 1 is that the polymer is a polymer containing only polyaniline segments, with the following structural formula:

[0123]

[0124] The value of y allows the polymer's molecular weight to range from 100,000 to 300,000.

[0125] Comparative Example 3

[0126] The difference between this comparative example and Example 1 is that the lithium sheet is not coated with a polymer protective film.

[0127] Comparative Example 4

[0128] The difference between this comparative example and Example 1 is that p=1, and the structural formula of the polymer is as follows:

[0129]

[0130] Comparative Example 5

[0131] The difference between this comparative example and Example 1 is that p=0, and the structural formula of the polymer is as follows:

[0132]

[0133] 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.

[0134] 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 at 25±1℃ using a LAND CT 2001C secondary battery performance testing device. 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 process was repeated, and the cycle was terminated when the battery capacity fell below 80% of the initial discharge capacity. The number of cycles is the cycle life of the battery.

[0135] The test results are shown in Table 1:

[0136] Table 1 Performance test results of the half-cells prepared in the examples and comparative examples

[0137]

[0138] The results are shown in Table 1. A simple analysis of the results follows.

[0139] Based on the test results of Examples 1-5 and Comparative Example 3, it can be concluded that the half-cells assembled with the polymer protective films of Examples 1-5 are superior to the half-cell of Comparative Example 3 in terms of initial charge-discharge capacity, initial coulombic efficiency, and cycle life. In the half-cell of Comparative Example 3, the lithium metal anode was not coated with a polymer protective film; therefore, the polymer protective film of this application embodiment is beneficial for improving battery performance.

[0140] Based on the test results of Examples 1-3 and Comparative Examples 1-2, it can be concluded that the molar ratio of polyimide segments to polyaniline segments has a significant impact on battery performance. The half-cells of Examples 1-3 outperformed the half-cells of Comparative Examples 1-2 in terms of initial charge-discharge capacity, initial coulombic efficiency, and cycle life. The polymers of Examples 1-3 contain both imidazole and polyaniline segments, while the polymer of Comparative Example 1 contains only imidazole segments and no polyaniline segments, and the polymer of Comparative Example 2 contains only polyaniline segments and no imidazole segments. Therefore, it can be analyzed that in Comparative Example 1, due to the absence of polyaniline segments, it is impossible to effectively construct a negative electrode with different conductivity gradients with the metal negative electrode. Consequently, lithium ions cannot be evenly distributed due to the electric field when passing through the polymer layer, leading to lithium ion aggregation and deposition, and the growth of lithium dendrites. In Comparative Example 2, due to the absence of polyimide segments, lithium ion conduction is ineffective, resulting in increased battery polarization. This makes the lithium metal negative electrode prone to lithium dendrite growth. Furthermore, the high rigidity of polyaniline leads to poor toughness of the polymer protective film. The expansion and pulverization of lithium metal can cause the protective film to crack, resulting in side reactions between lithium metal and electrolyte, loss of protective effect, and shortened battery cycle life. The polymer in the embodiments of this application includes polyimide segments and polyaniline. The two work together to improve lithium ion mobility and reduce lithium dendrite formation. On the other hand, they improve the mechanical strength of the protective film. Moreover, polyaniline can construct a negative electrode with different conductivity gradients with the metal negative electrode. When lithium ions pass through the polymer layer, they can be evenly distributed due to the electric field, preventing lithium ion aggregation and deposition, thus preventing the growth of lithium dendrites and improving battery performance.

[0141] Based on the test results of Examples 1 and 4 and Comparative Example 3, it can be concluded that the polymer structures disclosed in this application can all improve the performance of the battery.

[0142] Based on the test results of Example 1 and Comparative Examples 4-5, it can be concluded that Example 1 outperforms the half-cells of Comparative Examples 4 and 5 in terms of initial charge-discharge capacity, initial coulombic efficiency, and cycle life. Specifically, the polymer in Example 1 contains both reduced and oxidized polyaniline segments, while the polymer in Comparative Example 4 contains only reduced polyaniline segments, and the polymer in Comparative Example 5 contains only oxidized polyaniline segments. Therefore, the synergistic effect of the reduced and oxidized polyaniline segments in the polymer of this application embodiment gives the polymer excellent conductivity, enabling it to form a negative electrode with different conductivity gradients with the metal negative electrode. When lithium ions pass through the polymer layer, they are evenly distributed due to the electric field, preventing lithium ion aggregation and deposition, thereby preventing the growth of lithium dendrites.

[0143] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to technical solutions formed by specific combinations of the above-described technical features, but should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above features with (but not limited to) technical features with similar functions disclosed in this application.

Claims

1. A polymer protective film, characterized in that, The structural formula of the polymer protective film is as follows: R1 is selected from one of the following: bis(trifluoromethanesulfonyl)imide, bis(fluorosulfonyl)imide, perchlorate, hexafluorophosphate, hexafluoroarsenate, tetrafluoroborate, dioxaloateborate, difluorooxaloateborate, and trifluoromethanesulfonate. R2 is selected from -(CH2). i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; where i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100; p is the molar ratio of reduced polyaniline segments to polyaniline segments, p is any decimal between 0 and 1, m and n are the molar ratios of polyimide segments and polyaniline segments to the whole polymer, respectively, 0.5≤m≤0.95, 0.05≤n≤0.5, and m+n equals 1.0; The -(CH2) i - The above The above The above The above The -CH2-Y1-CH2- and the -CH2-(CH2Y2CH2) j The hydrogen atoms in -CH2- can be partially or completely replaced by substituents.

2. The polymer protective film according to claim 1, characterized in that, 0.6≤m≤0.8, 0.2≤n≤0.

4.

3. The polymer protective film according to any one of claims 1-2, characterized in that, The substituents are selected from halogens, hydroxyl groups, amino groups, carbonyl groups, cyano groups, C1-C6 alkoxy groups, C1-C6 alkyl groups, and C6-C6 alkyl groups. 12 aryl or C6-C 12 cycloalkyl groups.

4. The polymer protective film according to claim 3, characterized in that, The halogen is selected from fluorine, chlorine, and bromine; the amino group is selected from C1-C6 primary amines, C1-C6 alkyl-substituted secondary or tertiary amines; the C1-C6 alkoxy group is selected from methoxy or ethoxy; the C1-C6 alkyl group is selected from methyl, ethyl, propyl, and butyl; the C6-C... 12 The aryl group is selected from phenyl, naphthyl, or biphenyl; the C6-C 12 The cycloalkyl group is selected from cyclohexyl or bicyclohexyl.

5. The polymer protective film according to claim 4, characterized in that, The alkyl groups of C1-C6 are selected from isopropyl or tert-butyl.

6. The polymer protective film according to any one of claims 1-2, characterized in that, The molecular weight of the polymer protective film is 10,000 to 500,000.

7. The polymer protective film according to claim 6, characterized in that, The molecular weight of the polymer protective film is 100,000 to 300,000.

8. A method for preparing a polymer protective film according to any one of claims 1-7, characterized in that, Includes the following steps: Aniline and an initiator are dissolved in water and heated to react, yielding mixture I. An acid solution and a diamine-terminated compound are added to mixture I to obtain mixture II; Under ice-water bath conditions, a mixture of formaldehyde and acetaldehyde was added dropwise to mixture II, and the mixture was heated to react and obtain mixture III. After cooling the mixture III, it is added dropwise to an aqueous solution of anion exchanger to react and generate a precipitate; The precipitate is washed and dried to obtain the polymer protective film.

9. The method according to claim 8, characterized in that, The acid solution is any one of acetic acid, hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid.

10. The method according to claim 8, characterized in that, The diamine-terminated compound is selected from substituted or unsubstituted NH2-R2-NH2, wherein R2 is selected from -(CH2). i -、 , , , , -CH2-Y1-CH2-, -CH2-(CH2Y2CH2) j One of -CH2-; i is an integer from 2 to 100, Y1 and Y2 are each independently selected from O, NH or S, and j is an integer from 1 to 100.

11. The method according to claim 8, characterized in that, The heating reaction is carried out at a temperature of 40℃ to 100℃ for a time of 0.5 to 12 hours.

12. The method according to claim 8, characterized in that, The anion exchanger is any one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium perchlorate, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, or lithium trifluoromethanesulfonate.

13. A lithium metal anode, characterized in that, It includes a lithium metal sheet and a polymer protective film according to any one of claims 1-7, wherein the polymer protective film is disposed on the lithium metal sheet.

14. A lithium secondary battery, characterized in that, Including the lithium metal anode as described in claim 13.

15. A vehicle, characterized in that, Including the lithium secondary battery as described in claim 14.

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