Lithium metal negative electrode containing artificial SEI film protection layer, preparation method of lithium metal negative electrode and lithium metal battery

Abstract

The invention provides a lithium metal negative electrode containing an artificial SEI film protection layer, a preparation method of the lithium metal negative electrode and a lithium metal battery, and the preparation method comprises the following steps: obtaining a precursor solution, mixing the precursor solution and an initiator, coating the surface of metal lithium with the mixture, and carrying out a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protection layer, the precursor solution comprises a lithium salt and a monomer, and the monomer comprises one or more of a cyclic organic monomer and an inorganic monomer; the cyclic organic monomer comprises one or more of a five-membered heterocyclic compound containing one oxygen, a five-membered heterocyclic compound containing two oxygen, a six-membered heterocyclic compound containing one oxygen, a six-membered heterocyclic compound containing two oxygen, an epoxypropane derivative, maleic anhydride and a derivative thereof; the inorganic monomer comprises one or more of hexafluorocyclotriphosphazene and derivatives thereof. According to the preparation method provided by the invention, the artificial SEI film with good flexibility is formed, and the volume expansion of the lithium metal negative electrode can be relieved, so that the growth of lithium dendrites is inhibited.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium metal negative electrodes, and in particular to a lithium metal negative electrode containing an artificial SEI film protective layer, a preparation method thereof, and a lithium metal battery. Background Art

[0002] Lithium metal has a very high theoretical specific capacity (3860 mAh g -1 ) and extremely low redox potential (-3.040V vs. standard hydrogen electrode), making it a high energy density metal lithium battery (>500Wh Kg -1 ) is the best material for the negative electrode. Existing batteries use lithium metal as the negative electrode. The prepared lithium metal battery, during the battery charging process, lithium ions migrate from the positive electrode to the negative electrode and deposit to form metallic lithium, which will cause the volume of the lithium metal negative electrode to increase; during the battery discharge process, the metallic lithium is converted into lithium ions again and migrates back to the positive electrode, and the volume of the lithium metal negative electrode decreases; the deposition and stripping of the lithium metal negative electrode on the negative electrode surface during the charging and discharging process causes the volume expansion of the lithium metal negative electrode, and the volume expansion of the lithium metal negative electrode will lead to the unevenness of the lithium metal surface, and the SEI film components and surface structure generated subsequently will be uneven, which will then cause uneven deposition of lithium and the generation of lithium dendrites. Therefore, it is urgent to develop an artificial SEI film with uniform structure, good toughness and surface electronic insulation formed on the surface of the lithium metal negative electrode to inhibit the formation of lithium dendrites on the surface of the lithium metal negative electrode. Summary of the invention

[0003] In response to the technical problem that the volume expansion of existing lithium metal negative electrodes can lead to uneven lithium metal surface, which is easy to cause uneven lithium deposition and the formation of lithium dendrites, the present application provides a lithium metal negative electrode containing an artificial SEI film protective layer, a preparation method thereof, and a lithium metal battery.

[0004] In a first aspect, the present application provides a method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer, comprising the following steps: Obtaining a precursor solution, mixing the precursor solution with an initiator and coating the mixture on the surface of metallic lithium, and performing a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protective layer; The precursor solution includes a lithium salt and a monomer, wherein the monomer includes one or more of a cyclic organic monomer and an inorganic monomer; The cyclic organic monomer includes one or more of epoxy compounds containing at least one oxygen and their derivatives, maleic anhydride and their derivatives; The inorganic monomer includes one or more of hexafluorocyclotriphosphazene and its derivatives.

[0005] Preferably, obtaining the precursor solution comprises the following steps: Mixing the monomer, lithium salt and solvent uniformly to obtain the precursor solution; The molar ratio of the monomer to the lithium salt is (2-18):1.

[0006] Preferably, the heating temperature in the heating reaction is 50-200° C., and the heating time is 30 min-12 h.

[0007] Preferably, the epoxy compound containing at least one oxygen includes one or more of tetrahydrofuran and its derivatives, 1,3-dioxolane and its derivatives, 1,3-dioxane and its derivatives, tetrahydropyran and its derivatives, propylene oxide and its derivatives.

[0008] Preferably, the tetrahydrofuran derivatives include the compound shown in Structural Formula 1, the 1,3-dioxolane derivatives include the compound shown in Structural Formula 2, the tetrahydropyran derivatives include the compound shown in Structural Formula 3, the 1,3-dioxolane derivatives include the compound shown in Structural Formula 4, the maleic anhydride derivatives include the compound shown in Structural Formula 5, the propylene oxide derivatives include the compound shown in Structural Formula 6, and the hexafluorocyclotriphosphazene derivatives include the compound shown in Structural Formula 7; Wherein, R1, R2, R3, R4, R5, and R6 are independently selected from one or more of H, C1-C5 alkyl substituted or unsubstituted by halogen, C1-C5 alkenyl substituted or unsubstituted by halogen, halogen, C1-C5 ester group substituted or unsubstituted by halogen, and C1-C5 alkoxy group substituted or unsubstituted by halogen; Wherein, R7 is selected from one or more of C1~C5 alkyl substituted or unsubstituted by halogen, C1~C5 alkenyl substituted or unsubstituted by halogen, halogen, C1~C5 ester substituted or unsubstituted by halogen, C1~C5 alkoxy substituted or unsubstituted by halogen, and C1~C5 alkenyloxy substituted or unsubstituted by halogen.

[0009] Preferably, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, and lithium trifluoromethanesulfonate.

[0010] In a second aspect, the present application provides a lithium metal negative electrode containing an artificial SEI film protective layer, which is prepared by the above-mentioned method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer.

[0011] Preferably, the lithium metal negative electrode comprises lithium metal and an artificial SEI film protective layer, the artificial SEI film protective layer is arranged on the surface of the lithium metal, and the thickness of the artificial SEI film protective layer is 1-20 μm.

[0012] Preferably, the thickness of the artificial SEI film protective layer is 5-15 μm.

[0013] In a third aspect, the present application provides a lithium metal battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is the lithium metal negative electrode containing an artificial SEI film protective layer as described above.

[0014] The present application provides a method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer, wherein the precursor solution contains monomers and lithium salts, wherein the cyclic organic monomers can be polymerized in situ on the surface of lithium metal under the conditions of an initiator and heating to form an artificial SEI film with good flexibility, thereby alleviating the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be deposited more evenly on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance. The lithium salt can generate an SEI film containing inorganic components such as lithium fluoride, lithium nitride, lithium oxide, and lithium sulfate on the surface of lithium metal in situ, which can improve the ionic conductivity of the artificial SEI film on the one hand, and can also form an organic-inorganic composite film with the polymer generated by the cyclic organic monomer on the other hand, thereby enhancing the structural strength of the artificial SEI film, improving the density of the SEI film, blocking the chemical reaction between the electrolyte and the active lithium metal negative electrode, and improving the battery cycle performance. The inorganic monomers contained in the precursor solution undergo a polymerization reaction under heating conditions to form an inorganic polymer film on the surface of the lithium metal, which is an inorganic artificial SEI film. This can improve the flexibility of the artificial SEI film, alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, and allow metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance. At the same time, the inorganic polymer also has flame retardant properties, which improves the flame retardant effect of the lithium metal negative electrode. DETAILED DESCRIPTION

[0015] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0016] In order to illustrate the technical solution of the present invention, specific embodiments are provided below for illustration.

[0017] In a first aspect, the present application provides a method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer, comprising the following steps: Obtaining a precursor solution, mixing the precursor solution with an initiator and coating the mixture on the surface of metallic lithium, and performing a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protective layer; The precursor solution includes a lithium salt and a monomer, wherein the monomer includes one or more of a cyclic organic monomer and an inorganic monomer; The cyclic organic monomer includes one or more of epoxy compounds containing at least one oxygen and their derivatives, maleic anhydride and their derivatives; The inorganic monomer includes one or more of hexafluorocyclotriphosphazene and its derivatives.

[0018] Specifically, the precursor solution is directly coated on the surface of metallic lithium. Both cyclic organic monomers and inorganic monomers can undergo polymerization reactions under the conditions of an initiator and heating, and an organic artificial SEI film or an inorganic artificial SEI film is correspondingly generated on the surface of metallic lithium. The generated artificial SEI film is coated on the surface of metallic lithium and has flexibility. It can alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle to a certain extent, and avoid the uneven surface of the lithium metal due to the volume expansion of the lithium metal negative electrode, thereby allowing the metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, avoiding the uneven deposition of lithium, and further inhibiting the growth of lithium dendrites.

[0019] Epoxides containing at least one oxygen are cyclic ethers having a -CC- structure, such as ethylene oxide, propylene oxide, 1,2-butylene oxide, 1,4-butylene oxide (tetrahydrofuran), and the like.

[0020] The present application provides a method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer, wherein the precursor solution contains monomers and lithium salts, wherein the cyclic organic monomers can be polymerized in situ on the surface of lithium metal under initiator and heating conditions to form an artificial SEI film with good flexibility, thereby alleviating the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be deposited more evenly on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance. The lithium salt can generate an SEI film containing inorganic components such as lithium fluoride, lithium nitride, lithium oxide, and lithium sulfate on the surface of lithium metal in situ, which can improve the ionic conductivity of the artificial SEI film on the one hand, and can also form an organic-inorganic composite film with the polymer generated by the cyclic organic monomer on the other hand, thereby enhancing the structural strength of the artificial SEI film, improving the density of the SEI film, blocking the chemical reaction between the electrolyte and the active lithium metal negative electrode, and improving the battery cycle performance. The inorganic monomers contained in the precursor solution undergo a polymerization reaction under heating conditions to form an inorganic polymer film on the surface of lithium metal, which is an inorganic artificial SEI film. This can improve the flexibility of the artificial SEI film, alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, and allow metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance. At the same time, the inorganic polymer also has flame retardant properties, which improves the flame retardant effect of the lithium metal negative electrode.

[0021] In some embodiments, obtaining a precursor solution comprises the following steps: Mixing the monomer, lithium salt and solvent uniformly to obtain the precursor solution; The molar ratio of the monomer to the lithium salt is (2-18):1.

[0022] Specifically, the solvent plays a role in dissolving the monomer and the lithium salt, and the monomer and the lithium salt are added to the solvent and mixed to obtain a precursor solution. In the precursor solution, the molar ratio of the monomer and the lithium salt is (2~18):1, which is conducive to controlling the thickness of the formed artificial SEI film within the range of 1~20μm, has good flexibility, can effectively alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, effectively inhibit the growth of lithium dendrites, and improve the battery cycle performance.

[0023] If the molar ratio of monomer to lithium salt is lower than 2:1, the monomer content is low and the lithium salt content is high, the lithium salt is easy to precipitate, and the lithium salt forms a thicker SEI film containing inorganic components of lithium ions on the surface of lithium metal. Although the formation of lithium dendrites is inhibited, the cycle capacity retention rate of the battery is too low, affecting the normal use of the battery. If the molar ratio of monomer to lithium salt is higher than 18:1, the monomer content is too high and the lithium salt content is small, the lithium ion conductivity of the formed SEI film is reduced, the thickness is reduced, the cycle capacity retention rate of the battery is reduced, and the volume expansion of the lithium metal negative electrode during the charge and discharge cycle cannot be effectively alleviated, lithium precipitation is serious, and the battery impedance increases.

[0024] Specifically, the molar ratio of the monomer to the lithium salt may be in the following ranges: 2:1-5:1, 5:1-8:1, 8:1-12:1, or 12:1-18:1.

[0025] In some preferred embodiments, the molar ratio of the monomer to the lithium salt is (5-10):1.

[0026] When the molar ratio of the monomer to the lithium salt is within the above-mentioned preferred range, the artificial SEI film generated after the reaction has better flexibility, can better alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, is conducive to a more uniform deposition of metallic lithium on the surface of the lithium metal negative electrode, better inhibits the growth of lithium dendrites, and improves the battery cycle performance.

[0027] In some embodiments, the solvent includes one or more of a carbonate solvent, an alcohol ether solvent, a carboxylate solvent, and an ether solvent.

[0028] In some embodiments, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, fluoroethylene carbonate, and vinylene carbonate.

[0029] In some embodiments, the alcohol ether solvent includes one or more of ethylene glycol dimethyl ether and ethylene glycol diethyl ether.

[0030] In some embodiments, the carboxylate solvent includes one or more of fluorocarboxylate and carboxylate.

[0031] In some embodiments, the ether solvent includes a fluoroether.

[0032] In some embodiments, the heating temperature in the heating reaction is 50-200° C., and the heating time is 30 min-12 h.

[0033] Specifically, the heating temperature of the heating reaction is in the range of 50-200°C, which can provide reaction conditions for the polymerization reaction of cyclic organic monomers and inorganic monomers, and is also conducive to the reaction of lithium salts to form an artificial SEI film containing inorganic components of lithium ions. Controlling the heating time to 30min-12h is conducive to forming an artificial SEI film with a thickness of 1-20μm on the surface of metallic lithium.

[0034] The heating temperature of the heating reaction can be 50°C, 70°C, 80°C, 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, etc., as long as the heating temperature is within the range of 50-200°C. The heating time can be 30 min, 1.5 h, 1.0 h, 2.0 h, 3.0 h, 4.0 h, 5.0 h, 6.0 h, 9.0 h, 10 h, 12 h, etc., as long as the heating time is within the range of 30 min-12 h.

[0035] It should be noted that the specific selection of the heating temperature needs to be appropriately adjusted according to the physical properties of the selected monomer type.

[0036] In some embodiments, the at least one oxygen epoxy compound includes one or more of tetrahydrofuran and its derivatives, 1,3-dioxolane and its derivatives, 1,3-dioxane and its derivatives, tetrahydropyran and its derivatives, propylene oxide and its derivatives.

[0037] The cyclic organic monomers include the above-mentioned compounds, which are conducive to in-situ polymerization on the surface of lithium metal under heating conditions to form an artificial SEI film with good flexibility, thereby alleviating the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites.

[0038] In some embodiments, the tetrahydrofuran derivatives include compounds shown in formula 1, the 1,3-dioxolane derivatives include compounds shown in formula 2, the tetrahydropyran derivatives include compounds shown in formula 3, the 1,3-dioxolane derivatives include compounds shown in formula 4, the maleic anhydride derivatives include compounds shown in formula 5, the propylene oxide derivatives include compounds shown in formula 6, Wherein, R1, R2, R3, R4, R5 and R6 are independently selected from one or more of H, C1~C5 alkyl group which is substituted or unsubstituted by halogen, C1~C5 alkenyl group which is substituted or unsubstituted by halogen, halogen, C1~C5 ester group which is substituted or unsubstituted by halogen, and C1~C5 alkoxy group which is substituted or unsubstituted by halogen.

[0039] Specifically, the compound shown in structural formula 1 contains four carbon atoms, and R1 can be connected to any one of the carbon atoms; similarly, the compound in structural formula 2 contains three carbon atoms, and R2 can be connected to any one of the carbon atoms; the compound in structural formula 3 contains five carbon atoms, and R3 can be connected to any one of the carbon atoms; the compound in structural formula 4 contains four carbon atoms, and R4 can be connected to any one of the carbon atoms; the compound in structural formula 5 contains two carbon atoms, and R5 can be connected to any one of the carbon atoms.

[0040] The alkyl group of C1 to C5 substituted or unsubstituted by halogen includes an alkyl group of 1 to 5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or an alkyl group of 1 to 5 carbon atoms in which the hydrogen atom is not substituted by halogen. The alkyl group of 1 to 5 carbon atoms includes at least one of a linear alkyl group of 1 to 5 carbon atoms and a branched alkyl group of 1 to 5 carbon atoms, such as -CH2CH2CH3, -CH(CH3)2, methyl, ethyl, butyl, isobutyl, etc. Halogen includes elements such as F, Cl, Br, and I.

[0041] The alkenyl group having 1 to 5 carbon atoms includes at least one of a straight-chain alkenyl group having 1 to 5 carbon atoms and a branched alkenyl group having 1 to 5 carbon atoms, such as -CH=CHCH3, -CH=C(CH3)CH(CH3)-, vinyl, butenyl, pentenyl, etc.

[0042] The alkoxy group of C1 to C5 which may be substituted by halogen or not includes an alkoxy group with 1 to 5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or an alkoxy group with 1 to 5 carbon atoms in which the hydrogen atom is not substituted by halogen. The alkoxy group with 1 to 5 carbon atoms includes at least one of a linear alkoxy group with 1 to 5 carbon atoms and an alkoxy group with 1 to 5 carbon atoms containing a branch, such as methoxy, ethoxy, propoxy, butoxy, pentoxy, etc.

[0043] The ester group of C1 to C5 substituted or unsubstituted by halogen includes an ester group of 1 to 5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or an ester group of 1 to 5 carbon atoms in which the hydrogen atom is not substituted by halogen. The ester group of 1 to 5 carbon atoms includes at least one of a linear ester group of 1 to 5 carbon atoms and an ester group of 1 to 5 carbon atoms containing a branched chain, such as butyl formate, isobutyl formate, propyl acetate, etc.

[0044] In some embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, and lithium trifluoromethanesulfonate.

[0045] Specifically, the selection of the above-mentioned types of lithium salts helps the lithium salts to in situ generate a SEI film containing inorganic components containing lithium ions such as lithium fluoride, lithium nitride, lithium oxide, and lithium sulfate on the surface of lithium metal. On the one hand, it can improve the ionic conductivity of the artificial SEI film and improve the rate performance of the battery; at the same time, it can also form an organic-inorganic composite film with polymers generated from cyclic organic monomers, thereby enhancing the structural strength of the artificial SEI film, improving the density of the SEI film, blocking the chemical reaction between the electrolyte and the active lithium metal negative electrode, reducing the occurrence of side reactions, reducing battery polarization, reducing electrode impedance, and improving battery cycle performance.

[0046] In some embodiments, performing the heating reaction comprises the following steps: performing the heating reaction in an environment containing a protective gas.

[0047] Specifically, the protective gas includes nitrogen or an inert gas, wherein the inert gas includes one of helium (He), neon (Ne), argon (Ar), krypton (Kr), and xenon (Xe).

[0048] In some embodiments, the derivatives of hexafluorocyclotriphosphazene include compounds represented by structural formula 7, Wherein, R7 is selected from one or more of C1~C5 alkyl substituted or unsubstituted by halogen, C1~C5 alkenyl substituted or unsubstituted by halogen, halogen, C1~C5 ester substituted or unsubstituted by halogen, C1~C5 alkoxy substituted or unsubstituted by halogen, and C1~C5 alkenyloxy substituted or unsubstituted by halogen.

[0049] The C1-C5 alkenyloxy group includes a straight-chain alkenyloxy group having 1 to 5 carbon atoms and a branched alkenyloxy group having 1 to 5 carbon atoms, such as a propyleneoxy group, a vinyloxy group, a butyloxy group and the like.

[0050] Specifically, the use of the above-mentioned types of hexafluorocyclotriphosphazene derivatives can help to carry out polymerization reaction under heating conditions to generate inorganic polymers, namely, inorganic artificial SEI membranes, which can improve the flexibility of the artificial SEI membranes, alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, and allow metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites; at the same time, the inorganic polymer also has flame retardant properties, thereby improving the flame retardant effect of the lithium metal negative electrode.

[0051] In some embodiments, the initiator includes one or more of aluminum trifluoromethanesulfonate (Al(OTf) 3 ), azobisisobutyronitrile (AIBN), 2-hydroxy-2-methylpropiophenone (HMPP), and benzoyl peroxide (BPO).

[0052] In some embodiments, the mass of the initiator accounts for 0.1-1% of the mass of the precursor solution.

[0053] The amount of initiator added is 0.1% to 1% of the mass of the precursor solution, which can initiate polymerization reaction of the cyclic organic monomer and the inorganic monomer.

[0054] In a second aspect, the present application provides a lithium metal negative electrode containing an artificial SEI film protective layer, which is prepared by the above-mentioned method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer.

[0055] The lithium metal negative electrode provided by the present application contains an artificial SEI film protective layer, and the artificial SEI film protective layer has good flexibility, which can alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance; the artificial SEI film has high ionic conductivity; the artificial SEI film has high structural strength, which improves the density of the SEI film, blocks the chemical reaction between the electrolyte and the active lithium metal negative electrode, and improves the battery cycle performance. The inorganic polymer formed by the polymerization of inorganic monomers has flame retardant properties and improves the flame retardant effect of the battery.

[0056] In some embodiments, the lithium metal negative electrode includes lithium metal and an artificial SEI film protective layer, the artificial SEI film protective layer is disposed on the surface of the lithium metal, and the thickness of the artificial SEI film protective layer is 1 to 20 μm.

[0057] Specifically, the thickness of the artificial SEI film protective layer is in the range of 1~20μm. The artificial SEI film protective layer has moderate thickness, complete structure and good flexibility, which can alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery cycle performance.

[0058] In some preferred embodiments, the thickness of the artificial SEI film protective layer is 5-15 μm.

[0059] Specifically, the thickness of the artificial SEI film protective layer is in the range of 5 to 15 μm. The artificial SEI film protective layer has good flexibility, can better weld the volume expansion of the lithium metal negative electrode, effectively inhibit the growth of lithium dendrites, and improve the battery cycle performance while not increasing the thickness of the lithium metal negative electrode too much, and does not affect the battery energy density.

[0060] In a third aspect, the present application provides a lithium metal battery, comprising a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is the lithium metal negative electrode containing an artificial SEI film protective layer as described above.

[0061] The lithium metal battery provided by the present application contains a lithium metal negative electrode with the artificial SEI film protective layer, which reduces the reaction between the lithium metal negative electrode and the electrolyte, inhibits the formation of lithium dendrites, and improves the cycle performance of the battery. At the same time, the lithium ions contained can improve the ionic conductivity of the artificial SEI film protective layer, reduce the electrode impedance and safety performance. The inorganic polymer formed by the polymerization of inorganic monomers also has a flame retardant effect, which improves the flame retardant performance of the battery.

[0062] In some embodiments, the positive electrode includes a positive electrode active material, which is a commonly used material, such as lithium cobalt oxide, lithium manganese oxide, lithium iron phosphate, ternary materials or other new positive electrode materials. The electrolyte is also a prior art and will not be described in detail here.

[0063] The present invention is further described below by way of examples.

[0064] Example 1 A method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer comprises the following steps: The monomer is tetrahydrofuran, a cyclic organic monomer, the lithium salt is lithium hexafluorophosphate, the solvent is EC:DEC with a volume ratio of 3:7, and the initiator is aluminum trifluoromethanesulfonate Al(OTf)3.

[0065] S1: Tetrahydrofuran, lithium hexafluorophosphate and a solvent are uniformly mixed to obtain a precursor solution, wherein the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 10:1.

[0066] S2: Take 100 mL of the precursor solution, add an initiator to the precursor solution, mix well, and evenly apply it on the surface of the metallic lithium, then transfer it to an oven protected by argon inert gas, heat it at a heating temperature of 50°C for 6 hours, and obtain a lithium metal negative electrode containing an artificial SEI film protective layer; wherein the thickness of the formed artificial SEI film protective layer is 10 μm.

[0067] The mass of the added initiator is 0.2% of the mass of the precursor solution.

[0068] Example 2 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the monomers selected are inorganic monomers hexafluorocyclotriphosphazene and ethoxy (pentafluoro) cyclotriphosphazene, the heating temperature in step S2 is 80° C., and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 10 μm.

[0069] Example 3 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomer selected as tetrahydrofuran and the inorganic monomer selected as hexafluorocyclotriphosphazene and ethoxy (pentafluoro)cyclotriphosphazene, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 10 μm.

[0070] Example 4 Most of the steps of this embodiment are the same as those of Embodiment 1, except that in step S1, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 5:1, and the rest is the same as that of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 14 μm.

[0071] Example 5 Most of the steps of this embodiment are the same as those of Embodiment 1, except that in step S1, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 18:1, and the rest is the same as that of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 5 μm.

[0072] Example 6 Most of the steps of this embodiment are the same as those of Embodiment 1, except that in step S1, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 1.5:1, and the rest is the same as that of Embodiment 1.

[0073] Example 7 Most of the steps of this embodiment are the same as those of Embodiment 1, except that in step S1, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 20:1, and the rest is the same as that of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 2 μm.

[0074] Example 8 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomer is 1-methyl-dioxolane, and in step S2, heating is performed at a heating temperature of 80° C. for 4 h, and the rest is the same as in Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 12 μm.

[0075] Example 9 Most of the steps of this embodiment are the same as those of Embodiment 1, except that: the inorganic monomer is propyleneoxycyclotriphosphazene, and in step S2, heating is performed at a heating temperature of 150° C. for a heating time of 1 min, and the rest is the same as in Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 8 μm.

[0076] Example 10 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomers selected as monomers are 1,3-dioxolane and 1-methyl-dioxolane, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 13 μm.

[0077] Embodiment 11 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomers selected as monomers are tetrahydropyran and 2-fluorotetrahydrofuran, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 12 μm.

[0078] Example 12 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomers selected as monomers are 1,3-dioxane and 1-methyl-1,3-dioxane, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 13 μm.

[0079] Example 13 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomers selected as monomers are maleic anhydride and dimethylmaleic anhydride, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 12 μm.

[0080] Embodiment 14 Most of the steps of this embodiment are the same as those of Embodiment 1, except that the cyclic organic monomers selected as monomers are propylene oxide and ethylpropylene oxide, and the rest are the same as those of Embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 9 μm.

[0081] Comparative Example 1 The negative electrode of this comparative example is an existing commercially available lithium metal negative electrode, and no artificial SEI film protective layer is provided on the surface of the lithium metal.

[0082] NCM811 was used as the positive electrode, the lithium metal negative electrode prepared in the above embodiments and comparative examples was used as the negative electrode, the separator was a PP separator, the battery used a commercially available electrolyte containing lithium hexafluorophosphate, and the lithium metal battery was prepared using existing technology.

[0083] Lithium metal battery performance test: (1) Room temperature cycle performance test method: At room temperature 25℃±2, charge at 1C constant current to 4.4V, then charge at 4.4V constant voltage to 0.05C cutoff, then discharge at 1C constant current to 3.0V, and cycle for 300 cycles to test the battery capacity retention rate. The specific test results are shown in Table 1.

[0084] Lithium dendrite test After the 300th cycle according to the method in (1), the fully charged lithium metal battery is disassembled to observe whether there are deposited lithium dendrites on the surface of the lithium metal negative electrode sheet. If lithium dendrites are generated, the ratio of the generated area of ​​the lithium dendrites to the area of ​​the entire lithium metal negative electrode sheet is observed.

[0085] The specific test results are shown in Table 1.

[0086] Table 1 From the test results in Table 1, it can be seen that compared with Example 1 and Comparative Example 1, Example 1 has a high cycle capacity retention rate and a low lithium dendrite area ratio, indicating that coating the precursor solution on the surface of lithium metal can polymerize under the action of an initiator to form an artificial SEI film with good flexibility, which can alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the cycle performance of the battery.

[0087] Comparing Examples 1, 2, and 3, the thickness of the artificial SEI film formed on the surface of lithium metal is the same, the monomers in the precursor solution are cyclic organic monomers, inorganic monomers, or cyclic organic monomers + inorganic monomers, and the prepared lithium metal negative electrodes have similar effects, and can inhibit the growth of lithium dendrites and improve the cycle performance of the battery. Compared with Examples 1, 4-5 and Examples 6 and 7, the molar ratio of the monomer to the lithium salt is lower than 2:1, the lithium salt content is too high, lithium salt precipitation occurs, and the lithium salt forms a thick SEI film containing inorganic components of lithium ions on the surface of the lithium metal, and the battery cycle performance is poor; if the molar ratio of the monomer to the lithium salt is higher than 18:1, the lithium salt component contained is small, the conductivity of the formed SEI film is reduced, the thickness is reduced, the battery cycle capacity retention rate is reduced, and the volume expansion of the lithium metal negative electrode during the charge and discharge cycle cannot be effectively alleviated, and lithium precipitation is serious; the molar ratio of the monomer to the lithium salt is in the range of (2~18):1, which can effectively inhibit the formation of lithium dendrites; further preferably, the molar ratio of the monomer to the lithium salt is in the range of (5~10):1, the battery cycle capacity retention rate is high, and the lithium dendrite area accounts for a small proportion.

[0088] By comparing Examples 1 and 10-14, by changing the type of cyclic organic monomers, as long as the compounds shown in structural formula 1-7 are satisfied, they can polymerize on the surface of lithium metal to form an artificial SEI film with good flexibility, thereby alleviating the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing metallic lithium to be more evenly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the battery's cycle capacity retention rate.

[0089] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them. Although the present invention has been described in detail with reference to the above embodiments, it should be understood by those skilled in the art that the technical solutions described in the above embodiments can still be modified, or some of the technical features can be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention, and should be included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer, characterized in that: The following steps are involved: Obtaining a precursor solution, mixing the precursor solution with an initiator and coating the mixture on the surface of metallic lithium, and performing a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protective layer; The precursor solution includes a lithium salt and a monomer, wherein the monomer includes one or more of a cyclic organic monomer and an inorganic monomer; The cyclic organic monomer includes one or more of epoxy compounds containing at least one oxygen and their derivatives, maleic anhydride and their derivatives; The inorganic monomer includes one or more of hexafluorocyclotriphosphazene and its derivatives.

2. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 1, characterized in that: Obtaining the precursor solution comprises the following steps: Mixing the monomer, lithium salt and solvent uniformly to obtain the precursor solution; The molar ratio of the monomer to the lithium salt is (2-18):

1.

3. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 1, characterized in that: The heating temperature in the heating reaction is 50-200° C., and the heating time is 30 min-12 h.

4. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 1, characterized in that: The epoxy compound containing at least one oxygen includes one or more of tetrahydrofuran and its derivatives, 1,3-dioxolane and its derivatives, 1,3-dioxane and its derivatives, tetrahydropyran and its derivatives, propylene oxide and its derivatives.

5. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 4, characterized in that: The tetrahydrofuran derivatives include the compound shown in structural formula 1, the 1,3-dioxolane derivatives include the compound shown in structural formula 2, the tetrahydropyran derivatives include the compound shown in structural formula 3, the 1,3-dioxolane derivatives include the compound shown in structural formula 4, the maleic anhydride derivatives include the compound shown in structural formula 5, the propylene oxide derivatives include the compound shown in structural formula 6, and the hexafluorocyclotriphosphazene derivatives include the compound shown in structural formula 7; Wherein, R1, R2, R3, R4, R5, and R6 are independently selected from one or more of H, C1-C5 alkyl substituted or unsubstituted by halogen, C1-C5 alkenyl substituted or unsubstituted by halogen, halogen, C1-C5 ester group substituted or unsubstituted by halogen, and C1-C5 alkoxy group substituted or unsubstituted by halogen; Wherein, R7 is selected from one or more of C1~C5 alkyl substituted or unsubstituted by halogen, C1~C5 alkenyl substituted or unsubstituted by halogen, halogen, C1~C5 ester substituted or unsubstituted by halogen, C1~C5 alkoxy substituted or unsubstituted by halogen, and C1~C5 alkenyloxy substituted or unsubstituted by halogen.

6. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 1, characterized in that: The lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonyl imide), lithium bis(trifluorosulfonyl imide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, and lithium trifluoromethanesulfonate.

7. A lithium metal negative electrode containing an artificial SEI film protective layer, characterized in that: The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to any one of claims 1 to 6 is used.

8. The lithium metal negative electrode containing an artificial SEI film protective layer according to claim 7, characterized in that: The lithium metal negative electrode comprises lithium metal and an artificial SEI film protective layer, wherein the artificial SEI film protective layer is arranged on the surface of the lithium metal, and the thickness of the artificial SEI film protective layer is 1-20 μm.

9. The lithium metal negative electrode containing an artificial SEI film protective layer according to claim 8, characterized in that: The thickness of the artificial SEI film protective layer is 5-15 μm.

10. A lithium metal battery, characterized in that: The invention comprises a positive electrode, a negative electrode and an electrolyte, wherein the negative electrode is a lithium metal negative electrode containing an artificial SEI film protective layer as claimed in any one of claims 7 to 9.

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