A lithium metal anode containing an artificial SEI film protective layer and a preparation method therefor, and a lithium metal battery

Abstract

The application provides a lithium metal negative electrode containing an artificial SEI film protective layer and a preparation method thereof 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 mixture on the surface of metal lithium, and performing a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protective 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 oxygens, a six-membered heterocyclic compound containing one oxygen, a six-membered heterocyclic compound containing two oxygens, an epoxy propane derivative, maleic anhydride and a derivative thereof; and the inorganic monomer comprises one or more of hexafluorocyclotriphosphazene and a derivative thereof. The preparation method provided by the application forms an artificial SEI film with good flexibility, can relieve the volume expansion of the lithium metal negative electrode, and further inhibit the growth of lithium dendrites.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium metal negative electrodes, and particularly relates to a lithium metal negative electrode containing an artificial SEI film protective layer and a preparation method thereof and a lithium metal battery. BACKGROUND

[0002] Metal lithium has an ultra-high theoretical specific capacity (3860 mAh g -1 ) and an extremely low redox potential (-3.040 V vs. standard hydrogen electrode), and is the best material for a high-energy-density metal lithium battery (>500 Wh Kg -1 ) negative electrode. Existing batteries use lithium metal as the negative electrode, and the prepared lithium metal battery causes the volume of the lithium metal negative electrode to increase during the battery charging process, in which lithium ions migrate from the positive electrode to the negative electrode and are deposited into metal lithium; during the battery discharging process, the metal 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 peeling 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, which causes the unevenness of the lithium metal surface, the uneven deposition of lithium and the generation of lithium dendrites, and the uneven composition and surface structure of the subsequently generated SEI film. Therefore, it is urgent to develop an artificial SEI film with uniform structure, good toughness and surface electronic insulation on the surface of the lithium metal negative electrode to inhibit the generation of lithium dendrites on the surface of the lithium metal negative electrode. SUMMARY

[0003] In view of the technical problem that the volume expansion of the existing lithium metal negative electrode causes the unevenness of the lithium metal surface, and easily causes the uneven deposition of lithium and the generation of lithium dendrites, the application provides a lithium metal negative electrode containing an artificial SEI film protective layer and a preparation method thereof and a lithium metal battery.

[0004] In a first aspect, the application provides a preparation method of a lithium metal negative electrode containing an artificial SEI film protective layer, comprising the following steps:

[0005] obtaining a precursor solution, mixing the precursor solution and an initiator, coating the mixture on the surface of metal lithium, and performing a heating reaction to obtain the lithium metal negative electrode containing the artificial SEI film protective layer;

[0006] 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.

[0007] The cyclic organic monomer comprises one or more of an epoxy compound containing at least one oxygen and a derivative thereof, and maleic anhydride and a derivative thereof.

[0008] The inorganic monomer comprises one or more of hexafluorocyclotriphosphazene and a derivative thereof.

[0009] Preferably, the precursor solution is obtained by the following steps:

[0010] mixing the monomer, the lithium salt and the solvent to obtain the precursor solution;

[0011] The molar ratio of the monomer to the lithium salt is (2-18):1.

[0012] Preferably, the heating temperature in the heating reaction is 50-200°C, and the heating time is 30 min-12 h.

[0013] Preferably, the at least one oxygen-containing 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.

[0014] Preferably, the derivative of tetrahydrofuran includes a compound shown in structural formula 1, the derivative of 1,3-dioxolane includes a compound shown in structural formula 2, the derivative of tetrahydropyran includes a compound shown in structural formula 3, the derivative of 1,3-dioxane includes a compound shown in structural formula 4, the derivative of maleic anhydride includes a compound shown in structural formula 5, the propylene oxide derivative includes a compound shown in structural formula 6, and the hexafluorotriphosphazene derivative includes a compound shown in structural formula 7.

[0015]

[0016] wherein R1, R2, R3, R4, R5, R6 are each 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 substituted or unsubstituted by halogen, C1-C5 alkoxy substituted or unsubstituted by halogen.

[0017]

[0018] 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, C1-C5 alkenyloxy substituted or unsubstituted by halogen.

[0019] Preferably, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide, lithium bis-trifluorosulfonylimide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethylsulfonate.

[0020] In a second aspect, the present application provides a lithium metal negative electrode with an artificial SEI film protective layer, which is prepared by the preparation method of the lithium metal negative electrode with an artificial SEI film protective layer.

[0021] 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.

[0022] Preferably, the thickness of the artificial SEI film protective layer is 5-15 μm.

[0023] In a third aspect, the present application provides a lithium metal battery, which comprises a positive electrode, a negative electrode and an electrolyte, and the negative electrode is the lithium metal negative electrode with an artificial SEI film protective layer described above.

[0024] The preparation method of the lithium metal negative electrode with an artificial SEI film protective layer provided by the present application contains monomers and lithium salts in the precursor solution, wherein the cyclic organic monomers can be in-situ polymerized on the surface of lithium metal under the conditions of an initiator and heating to form an artificial SEI film with good flexibility, thereby relieving the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be more uniformly 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. The lithium salts can generate in-situ SEI films containing lithium ions, such as lithium fluoride, lithium nitride, lithium oxide and lithium sulfate, on the surface of lithium metal, which can improve the ionic conductivity of the artificial SEI film on one hand, and form an organic-inorganic composite film with the polymer generated by the cyclic organic monomers 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 cycle performance of the battery. The inorganic monomers contained in the precursor solution are polymerized under heating conditions to form an inorganic polymer film on the surface of lithium metal, i.e. an inorganic artificial SEI film, which can improve the flexibility of the artificial SEI film, relieve the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allow the metal lithium to be more uniformly 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; at the same time, the inorganic polymer also has a flame-retardant property, which improves the flame-retardant effect of the lithium metal negative electrode. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and should not be used to limit the present application.

[0026] In order to illustrate the technical solutions of the present application, the following specific embodiments are used for illustration.

[0027] In a first aspect, the present application provides a method for preparing a lithium metal anode with an artificial SEI film protective layer, comprising the following steps:

[0028] obtaining a precursor solution, coating the precursor solution on the surface of the metal lithium after mixing with an initiator, and performing a heating reaction to obtain the lithium metal anode with the artificial SEI film protective layer;

[0029] The precursor solution comprises a lithium salt and a monomer, and the monomer comprises one or more of a cyclic organic monomer, an inorganic monomer;

[0030] The cyclic organic monomer comprises one or more of an epoxide compound containing at least one oxygen and derivatives thereof, maleic anhydride and derivatives thereof;

[0031] The inorganic monomer comprises one or more of hexafluorotriphosphazene and derivatives thereof.

[0032] Specifically, the precursor solution is directly coated on the surface of the metal lithium, and both the cyclic organic monomer and the inorganic monomer can undergo polymerization under the conditions of the initiator and heating, corresponding to the generation of an organic artificial SEI film or an inorganic artificial SEI film on the surface of the lithium metal. The generated artificial SEI film is coated on the surface of the metal lithium and has flexibility, which can to some extent alleviate the volume expansion of the lithium metal anode during the charge and discharge cycle process, avoid the non-uniformity of the surface of the metal lithium caused by the volume expansion of the lithium metal anode, so that the metal lithium can be more uniformly deposited on the surface of the lithium metal anode, avoid the initiation of non-uniform deposition of lithium, and further inhibit the growth of lithium dendrites.

[0033] The epoxide compound containing at least one oxygen is a cyclic ether with a -C-C- structure, such as oxirane, oxetane, 1,2-epoxybutane, 1,4-epoxybutane (tetrahydrofuran), etc.

[0034] The application provides a preparation method of a lithium metal negative electrode containing an artificial SEI film protective layer. A precursor solution contains monomers and lithium salts. The cyclic organic monomers can be in-situ polymerized on the surface of lithium metal under the action of an initiator and heating to form an artificial SEI film with good flexibility, thereby relieving the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be more uniformly 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. The lithium salts can generate in-situ SEI films containing lithium ion inorganic components such as lithium fluoride, lithium nitride, lithium oxide and lithium sulfate on the surface of lithium metal. On the one hand, the lithium salts can improve the ionic conductivity of the artificial SEI film, and on the other hand, the lithium salts can form an organic-inorganic composite film with the polymer generated by the 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, and improving the cycle performance of the battery. The inorganic monomers contained in the precursor solution are polymerized under heating to generate an inorganic polymer film on the surface of lithium metal, that is, an inorganic artificial SEI film, which can improve the flexibility of the artificial SEI film, relieve the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allow the metal lithium to be more uniformly 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. At the same time, the inorganic polymer also has a flame retardant property, which improves the flame retardant effect of the lithium metal negative electrode.

[0035] In some embodiments, obtaining the precursor solution comprises the following steps:

[0036] The monomers, the lithium salts and the solvent are uniformly mixed to obtain the precursor solution.

[0037] The molar ratio of the monomers to the lithium salts is (2-18):1.

[0038] Specifically, the solvent dissolves the monomers and the lithium salts, and the monomers and the lithium salts are added into the solvent and uniformly mixed to obtain the precursor solution. In the precursor solution, the molar ratio of the monomers to the lithium salts is (2-18):1, which is beneficial to control the thickness of the artificial SEI film formed in the range of 1-20 μm, has good flexibility, can effectively relieve 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 cycle performance of the battery.

[0039] If the molar ratio of the monomer and the lithium salt is less than 2:1, the monomer content is low, the lithium salt content is high, the lithium salt is easy to precipitate, the lithium salt forms a thick inorganic component SEI film containing lithium ions on the surface of the lithium metal, although the generation of lithium dendrites is inhibited, the cycle capacity retention rate of the battery is too low, which affects the normal use of the battery. If the molar ratio of the monomer and the lithium salt is higher than 18:1, the monomer content is too high, the lithium salt content is low, 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, the volume expansion of the lithium metal negative electrode during the charge and discharge cycle process cannot be effectively alleviated, the lithium precipitation is serious, and the battery impedance increases.

[0040] Specifically, the molar ratio of the monomer and the lithium salt can be in the following range: 2:1~5:1, 5:1~8:1, 8:1~12:1 or 12:1~18:1.

[0041] In some preferred embodiments, the molar ratio of the monomer and the lithium salt is (5~10):1.

[0042] The molar ratio of the monomer and the lithium salt is in the above preferred range, the flexibility of the artificial SEI film generated after the reaction is better, which can better alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle process, is conducive to the more uniform deposition of the metal lithium on the surface of the lithium metal negative electrode, better inhibits the growth of lithium dendrites, and improves the cycle performance of the battery.

[0043] In some embodiments, the solvent includes one or more of a carbonate solvent, an alcohol ether solvent, a carboxylic acid ester solvent, and an ether solvent.

[0044] In some embodiments, the carbonate solvent includes one or more of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, fluoroethylene carbonate, and vinylene carbonate.

[0045] In some embodiments, the alcohol ether solvent includes one or more of ethylene glycol dimethyl ether and ethylene glycol diethyl ether.

[0046] In some embodiments, the carboxylic acid ester solvent includes one or more of a fluoro carboxylic acid ester and a carboxylic acid ester.

[0047] In some embodiments, the ether solvent includes a fluoro ether.

[0048] In some embodiments, the heating temperature in the heating reaction is 50~200℃, and the heating time is 30min~12h.

[0049] Specifically, the heating temperature of the heating reaction is in the range of 50-200℃, which can provide reaction conditions for the polymerization of the cyclic organic monomer and the inorganic monomer, and is also conducive to the reaction of the lithium salt to generate the inorganic component containing lithium ion artificial SEI film. The heating time is controlled to be 30min-12h, which is conducive to the formation of the artificial SEI film with a thickness of 1-20μm on the surface of the metal lithium.

[0050] The heating temperature of the heating reaction can be 50℃, 70℃, 80℃, 100℃, 120℃, 140℃, 160℃, 180℃, 200℃, etc., as long as the heating temperature is in the range of 50-200℃. The heating time can be 30min, 1.5h, 1.0h, 2.0h, 3.0h, 4.0h, 5.0h, 6.0h, 9.0h, 10h, 12h, etc., as long as the heating time is in the range of 30min-12h.

[0051] It should be noted that the specific selection of the heating temperature needs to be adjusted appropriately according to the physical properties of the selected monomer.

[0052] In some embodiments, the at least one oxygen-containing 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, and propylene oxide and its derivatives.

[0053] The cyclic organic monomer includes the above-mentioned compounds, which is conducive to the in-situ polymerization of the lithium metal surface under heating conditions to form an artificial SEI film with good flexibility, which can further alleviate the volume expansion of the lithium metal anode during the charge and discharge cycle, enable the metal lithium to be more uniformly deposited on the surface of the lithium metal anode, and further inhibit the growth of lithium dendrites.

[0054] In some embodiments, the derivative of tetrahydrofuran includes a compound shown in structural formula 1, the derivative of 1,3-dioxolane includes a compound shown in structural formula 2, the derivative of tetrahydropyran includes a compound shown in structural formula 3, the derivative of 1,3-dioxane includes a compound shown in structural formula 4, the derivative of maleic anhydride includes a compound shown in structural formula 5, and the derivative of propylene oxide includes a compound shown in structural formula 6,

[0055]

[0056] wherein R1, R2, R3, R4, R5, and R6 are each 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 substituted or unsubstituted by halogen, and C1-C5 alkoxy substituted or unsubstituted by halogen.

[0057] Specifically, the compound shown in structural formula 1 contains four carbon atoms, and R1may be connected to any one of the carbon atoms; similarly, the compound in structural formula 2 contains three carbon atoms, and R2may be connected to any one of the carbon atoms; the compound in structural formula 3 contains five carbon atoms, and R3may be connected to any one of the carbon atoms; the compound in structural formula 4 contains four carbon atoms, and R4may be connected to any one of the carbon atoms; the compound in structural formula 5 contains two carbon atoms, and R5may be connected to any one of the carbon atoms.

[0058] C1-C5 alkyl substituted or unsubstituted by halogen includes alkyl having 1-5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or alkyl having 1-5 carbon atoms in which hydrogen atom is not substituted by halogen. Alkyl having 1-5 carbon atoms includes at least one of straight chain alkyl having 1-5 carbon atoms, and branched chain alkyl having 1-5 carbon atoms, such as -CH2CH2CH3, -CH(CH3)2, methyl, ethyl, butyl, isobutyl, etc. Halogen includes F, Cl, Br, I, etc.

[0059] C1-C5 alkenyl includes at least one of straight chain alkenyl having 1-5 carbon atoms, and branched chain alkenyl having 1-5 carbon atoms, such as -CH=CHCH3, -CH=C(CH3)CH(CH3)-, ethenyl, butenyl, pentenyl, etc.

[0060] C1-C5 alkoxy substituted or unsubstituted by halogen includes alkoxy having 1-5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or alkoxy having 1-5 carbon atoms in which hydrogen atom is not substituted by halogen. Alkoxy having 1-5 carbon atoms includes at least one of straight chain alkoxy having 1-5 carbon atoms, and branched chain alkoxy having 1-5 carbon atoms, such as methoxy, ethoxy, propoxy, butoxy, pentoxy, etc.

[0061] C1-C5 ester substituted or unsubstituted by halogen includes ester having 1-5 carbon atoms in which at least one hydrogen atom is substituted by halogen, or ester having 1-5 carbon atoms in which hydrogen atom is not substituted by halogen. Ester having 1-5 carbon atoms includes at least one of straight chain ester having 1-5 carbon atoms, and branched chain ester having 1-5 carbon atoms, such as butyl formate, isobutyl formate, propyl acetate, etc.

[0062] In some embodiments, the lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bisfluorosulfonylimide, lithium bis-trifluorosulfonylimide, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium trifluoromethylsulfonate.

[0063] Specifically, the lithium salt is selected from the above-mentioned types, which is helpful for the lithium salt to generate in situ on the surface of the lithium metal an SEI film containing inorganic components containing lithium ions such as lithium fluoride, lithium nitride, lithium oxide, lithium sulfate, etc. 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 the polymer generated by the cyclic organic monomer, 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.

[0064] In some embodiments, the heating reaction comprises the following steps: performing the heating reaction in an environment containing a protective gas.

[0065] 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).

[0066] In some embodiments, the derivative of hexafluorocyclotriphosphazene includes a compound shown in structural formula 7,

[0067]

[0068] 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.

[0069] The C1-C5 alkenyloxy includes a straight-chain alkenyloxy with a carbon atom number of 1-5, and a branched-chain alkenyloxy with a carbon atom number of 1-5, such as propenyloxy, ethenyloxy, butenyloxy, etc.

[0070] Specifically, the derivative of hexafluorocyclotriphosphazene selected from the above-mentioned types is helpful for the polymerization reaction under heating conditions to generate an inorganic polymer, i.e., an inorganic artificial SEI film, which can improve the flexibility of the artificial SEI film, relieve the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, enable the metal lithium to be more uniformly deposited on the surface of the lithium metal negative electrode, and thereby inhibit the growth of lithium dendrites; at the same time, the inorganic polymer also has a flame retardant property, which improves the flame retardant effect of the lithium metal negative electrode.

[0071] In some embodiments, the initiator includes one or more of aluminum triflate (Al(OTf)3), azobisisobutyronitrile (AIBN), 2-hydroxy-2-methylpropyl phenone (HMPP), and benzoyl peroxide (BPO).

[0072] In some embodiments, the amount of initiator is 0.1-1% by mass of the precursor solution.

[0073] The amount of initiator is 0.1-1% by mass of the precursor solution, which enables the ring-shaped organic monomer and inorganic monomer to initiate polymerization.

[0074] In a second aspect, the application provides a lithium metal negative electrode containing an artificial SEI film protective layer, which is prepared by the method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer described above.

[0075] The lithium metal negative electrode containing an artificial SEI film protective layer provided by the application has good flexibility, which can alleviate the volume expansion of the lithium metal negative electrode during the charge-discharge cycle, enable the metal lithium to be more uniformly deposited on the surface of the lithium metal negative electrode, and thus inhibit the growth of lithium dendrites and improve the cycle performance of the battery. 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 cycle performance of the battery. The inorganic polymer formed by the polymerization of the inorganic monomer has flame retardant properties, which improves the flame retardant effect of the battery.

[0076] In some embodiments, 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.

[0077] Specifically, the thickness of the artificial SEI film protective layer is in the range of 1-20 μm, the thickness of the artificial SEI film protective layer is moderate, the structure is complete, and the flexibility is good, which can alleviate the volume expansion of the lithium metal negative electrode during the charge-discharge cycle, enable the metal lithium to be more uniformly deposited on the surface of the lithium metal negative electrode, and thus inhibit the growth of lithium dendrites and improve the cycle performance of the battery.

[0078] In some preferred embodiments, the thickness of the artificial SEI film protective layer is 5-15 μm.

[0079] Specifically, the thickness of the artificial SEI film protective layer is in the range of 5-15 μm, the flexibility of the artificial SEI film protective layer is better, which can better weld the volume expansion of the lithium metal negative electrode, effectively inhibit the growth of lithium dendrites, improve the cycle performance of the battery, and at the same time, does not increase the thickness of the lithium metal negative electrode too much, and does not affect the energy density of the battery.

[0080] In a third aspect, the application provides a lithium metal battery, which comprises a positive electrode, a negative electrode, and an electrolyte, and the negative electrode is the lithium metal negative electrode containing an artificial SEI film protective layer described above.

[0081] The lithium metal battery provided in the application contains the lithium metal negative electrode with the artificial SEI film protective layer, reduces the reaction of the lithium metal negative electrode with the electrolyte, inhibits the generation of lithium dendrites, improves the cycle performance of the battery, and the contained lithium ions can improve the ionic conductivity of the artificial SEI film protective layer, reduce the electrode impedance and improve the safety performance. The inorganic polymer formed by polymerization of the inorganic monomer also has a flame retardant effect, and improves the flame retardant performance of the battery.

[0082] In some embodiments, the positive electrode comprises a positive electrode active material, which is a commonly used material in the prior art, such as a positive electrode active material comprising lithium cobaltate, lithium manganate, lithium iron phosphate, a ternary material or other new positive electrode material, etc. The electrolyte is also a prior art, which will not be described here.

[0083] The application is further described below through examples.

[0084] Example 1

[0085] A preparation method of a lithium metal negative electrode with an artificial SEI film protective layer, comprising the following steps:

[0086] The monomer is a cyclic organic monomer tetrahydrofuran, the lithium salt is lithium hexafluorophosphate, the solvent is EC:DEC with a volume ratio of 3:7. The initiator is aluminum trifluoromethanesulfonate Al(OTf)3.

[0087] S1: uniformly mixing tetrahydrofuran, lithium hexafluorophosphate and the solvent to obtain a precursor solution, wherein the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 10:1.

[0088] S2: taking 100 mL of the precursor solution, adding the initiator into the precursor solution, uniformly coating the surface of the metal lithium after uniform mixing, and then transferring to an oven containing argon inert gas protection, heating at a heating temperature of 50℃, and heating for 6h to obtain a lithium metal negative electrode with an artificial SEI film protective layer; wherein the thickness of the formed artificial SEI film protective layer is 10μm.

[0089] The mass of the added initiator is 0.2% of the mass of the precursor solution.

[0090] Example 2

[0091] Most of the steps in this example are the same as those in Example 1, except that the monomers are inorganic monomers hexafluorocyclotriphosphazene and ethoxy(pentafluoro)cyclotriphosphazene, the heating temperature in the S2 step is 80℃, and the rest is the same as in Example 1; wherein the thickness of the formed artificial SEI film protective layer is 10μm.

[0092] Example 3

[0093] The present embodiment is the same as most of the steps of Example 1, except that the monomer is selected as the cyclic organic monomer tetrahydrofuran, the inorganic monomer is hexafluorotriphosphazene and ethoxy (pentafluoro) triphosphazene, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 10 μm.

[0094] Example 4

[0095] The present embodiment is the same as most of the steps of Example 1, except that in the S1 step, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 5:1, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 14 μm.

[0096] Example 5

[0097] The present embodiment is the same as most of the steps of Example 1, except that in the S1 step, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 18:1, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 5 μm.

[0098] Example 6

[0099] The present embodiment is the same as most of the steps of Example 1, except that in the S1 step, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 1.5:1, and the rest is the same as Example 1.

[0100] Example 7

[0101] The present embodiment is the same as most of the steps of Example 1, except that in the S1 step, the molar ratio of tetrahydrofuran to lithium hexafluorophosphate is 20:1, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 2 μm.

[0102] Example 8

[0103] The present embodiment is the same as most of the steps of Example 1, except that the cyclic organic monomer is 1-methyl-dioxolane, and in the S2 step, heating is carried out at a temperature of 80°C, and the heating time is 4 h, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 12 μm.

[0104] Example 9

[0105] The present embodiment is the same as most of the steps of Example 1, except that the inorganic monomer is propyleneoxytriphosphazene, and in the S2 step, heating is carried out at a temperature of 150°C, and the heating time is 1 min, and the rest is the same as Example 1; wherein the thickness of the artificial SEI film protective layer formed is 8 μm.

[0106] Example 10

[0107] Most of the steps in this embodiment are the same as those in 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 in embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 13 μm.

[0108] Example 11

[0109] Most of the steps in this embodiment are the same as those in embodiment 1, except that the cyclic organic monomers selected as tetrahydropyran and 2-fluorotetrahydrofuran are the same as those in embodiment 1. The thickness of the artificial SEI protective layer is 12 μm.

[0110] Example 12

[0111] Most of the steps in this embodiment are the same as those in 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 in embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 13 μm.

[0112] Example 13

[0113] Most of the steps in this embodiment are the same as those in 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 in embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 12 μm.

[0114] Example 14

[0115] Most of the steps in this embodiment are the same as those in 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 in embodiment 1; wherein the thickness of the formed artificial SEI film protective layer is 9 μm.

[0116] Comparative Example 1

[0117] 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 lithium metal surface.

[0118] 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.

[0119] Lithium metal battery performance test:

[0120] (1) Normal temperature cycle performance test method: at normal temperature 25℃±2, charge to 4.4V at 1C constant current, then charge to 0.05C cut-off at 4.4V constant voltage, then discharge to 3.0V at 1C constant current, so cycle 300 times, test the capacity retention rate of the battery, the specific test results are shown in Table 1.

[0121] Lithium dendrite test

[0122] The lithium metal battery in full charge state after cycling for 300 times according to the method in (1) is disassembled, and whether lithium dendrites are deposited on the surface of the lithium metal negative electrode sheet is observed. If lithium dendrites are generated, the proportion of the generated area of the lithium dendrites to the area of the entire lithium metal negative electrode sheet is observed.

[0123] The specific test results are shown in Table 1.

[0124] Table 1

[0125]

[0126] From the test results in Table 1, it can be seen that the cycle capacity retention rate of Example 1 is high and the proportion of lithium dendrite area is low compared with Comparative Example 1, which shows that coating the precursor solution on the surface of lithium metal can form a flexible artificial SEI film under the action of the initiator, which can alleviate the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, enable the metal lithium to be more uniformly deposited on the surface of the lithium metal negative electrode, and further inhibit the growth of lithium dendrites, thereby improving the cycle performance of the battery.

[0127] Comparing Examples 1, 2 and 3, the artificial SEI film formed on the surface of the lithium metal has the same thickness, the monomer in the precursor solution is a cyclic organic monomer, an inorganic monomer or a cyclic organic monomer + an inorganic monomer, and the lithium metal negative electrode prepared therefrom all has similar effects, all of which can inhibit the growth of lithium dendrites and improve the cycle performance of the battery. Comparing Example 1, 4-5 and Example 6, 7, when the molar ratio of monomer to lithium salt is less than 2:1, the content of lithium salt is too high, and lithium salt precipitation occurs. The lithium salt will form a SEI film with thick inorganic components containing lithium ions on the surface of the lithium metal, and the cycle performance of the battery is poor. If the molar ratio of monomer to lithium salt is higher than 18:1, the content of lithium salt is low, the conductivity of the formed SEI film is reduced, the thickness is reduced, the cycle capacity retention rate of the battery is reduced, the volume expansion of the lithium metal negative electrode during the charge and discharge cycle cannot be effectively alleviated, and lithium precipitation is serious. When the molar ratio of monomer to lithium salt is in the range of (2-18):1, the generation of lithium dendrites can be effectively inhibited. Further preferably, the molar ratio of monomer to lithium salt is in the range of (5-10):1, the cycle capacity retention rate of the battery is high, and the proportion of lithium dendrite area is small.

[0128] The lithium metal surface is polymerized to form a flexible artificial SEI film by changing the type of cyclic organic monomer as long as it meets the compounds shown in structural formulas 1-7, relieving the volume expansion of the lithium metal negative electrode during the charge and discharge cycle, allowing the metal lithium to be more uniformly deposited on the surface of the lithium metal negative electrode, thereby inhibiting the growth of lithium dendrites and improving the cycle capacity retention rate of the battery.

[0129] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it. Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents. These modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

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 a 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 a cyclic organic monomer and an inorganic monomer; The cyclic organic monomers include tetrahydrofuran and its derivatives; The inorganic monomer includes one or more of hexafluorocyclotriphosphazene and its derivatives; Obtaining the precursor solution comprises the following steps: uniformly mixing the monomer, lithium salt and solvent to obtain the precursor solution; The molar ratio of the monomer to the lithium salt is (5-10):1; The heating temperature in the heating reaction is 50-200°C, and the heating time is 30 minutes to 12 hours; The thickness of the artificial SEI film protective layer is 5-15 μm.

2. The method for preparing a lithium metal negative electrode containing an artificial SEI film protective layer according to claim 1, characterized in that: The tetrahydrofuran and its derivatives include the compound shown in Structural Formula 1, and the hexafluorocyclotriphosphazene and its derivatives include the compound shown in Structural Formula 7; wherein R1 is 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 groups substituted or unsubstituted by halogen, C1~C5 alkenyl groups substituted or unsubstituted by halogen, halogen, C1~C5 ester groups substituted or unsubstituted by halogen, C1~C5 alkoxy groups substituted or unsubstituted by halogen, and C1~C5 alkenyloxy groups substituted or unsubstituted by halogen.

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 lithium salt includes one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(fluorosulfonylimide), lithium bis(trifluorosulfonylimide), lithium bis(oxalatoborate), lithium difluorooxalatoborate, and lithium trifluoromethanesulfonate.

4. A lithium metal negative electrode containing an artificial SEI film protective layer, characterized in that: The lithium metal negative electrode is prepared by the preparation method of any one of claims 1 to 3 containing an artificial SEI film protective layer.

5. A lithium metal battery, characterized in that The invention comprises 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 claimed in claim 4.

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