A method for interfacial modification of a double-layer protective layer of metallic lithium and a metallic lithium negative electrode

Abstract

The present invention relates to a double-layer protective layer for metallic lithium and a method for modifying the interface of a metallic lithium negative electrode, including a bottom layer in contact with the surface of metallic lithium and a top layer as a protective layer; the bottom layer is a lithium-containing alloy, and the top layer includes lithium fluoride and a lithium salt of an oxyacid type. The present invention is prepared by simply immersing a lithium foil in a solution of a fluorine-containing metal salt, drying it, and then using it as a negative electrode to spontaneously form in situ through a charge / discharge process. The preparation method is simple. A double-layer interface containing lithium fluoride and a lithium-containing alloy is formed on the surface of metallic lithium. This double-layer interface can provide uniformly distributed lithiumophilic sites, promote the uniform nucleation and deposition of lithium, improve the stability of the metallic lithium negative electrode, promote the rapid diffusion of lithium ions, effectively inhibit the growth of lithium dendrites, and improve the safety performance and cycle stability of metallic lithium batteries.

Description

Technical Field

[0001] The present invention belongs to the field of lithium ion batteries, and particularly relates to a double-layer protective layer for metallic lithium and a method for modifying the interface of a metallic lithium negative electrode. Background Art

[0002] Developing clean energy, constructing a new energy system that is safe, efficient, clean, and sustainable, and reducing environmental pollution caused by the use of fossil energy have important and far-reaching impacts on sustainable development. In recent years, as a strategic emerging industry and a key field, new energy vehicles use secondary batteries as the power source. Their energy density and safety are crucial. Research and development of battery systems with high energy density and safety and reliability are the core of the development of new energy electric vehicles. Compared with the graphite negative electrode in traditional lithium ion batteries, the metallic lithium negative electrode has a high specific capacity (3860 mAh g -1 ) and the most negative reduction potential (-3.04 V vs. standard hydrogen electrode), and is considered an ideal negative electrode material. Batteries based on metallic lithium negative electrodes (such as Li-S, Li-air, etc.) are also considered the next-generation ideal battery systems. However, there are still problems in the application of lithium metal negative electrodes: on the one hand, during the deposition / dissolution process of lithium metal, due to the uneven distribution of lithium ion concentration in the liquid electrolyte, it will cause uneven deposition of lithium on the current collector, generating lithium dendrites and triggering safety problems. Moreover, during the lithium stripping process, it will first fall off from the root, resulting in a part of lithium losing electrical contact and becoming dead lithium, leading to the loss of active lithium. In addition, serious volume expansion and other problems will also occur during repeated charge and discharge processes, resulting in continuous reconstruction of the SEI, reducing the Coulomb efficiency and cycle life. Especially at high current densities, the slow migration of Li ions will lead to an increase in the concentration difference polarization of lithium ions, exacerbating the formation of lithium dendrites and the loss of active substances.

[0003] Due to the extremely high reactivity of metallic lithium, it will react with the electrolyte components during the reaction process to form an unstable SEI film, which cannot effectively inhibit the occurrence of side reactions and the growth of lithium dendrites. The cycle performance of metallic lithium mainly depends on whether a stable interface can be formed on its surface. For planar metallic lithium negative electrodes such as lithium foil or lithium copper composite tape, directly modifying the surface of metallic lithium is the most direct and simplest method to improve the metallic lithium negative electrode.

[0004] In recent years, extensive research has been carried out on improvement methods for metallic lithium negative electrodes, including modified electrolyte additives, artificial SEI films, solid electrolytes, alloyed negative electrodes, and three-dimensional current collectors. SEIs mainly composed of inorganic components, such as LiF, Li2S, Li3PO4, have good mechanical properties and can effectively inhibit lithium dendrites. Among them, LiF has been proven to be very effective in inhibiting the generation of lithium dendrites, but the low ionic conductivity of LiF leads to a large overpotential at high current densities.

[0005] In the prior art, a protective layer is formed in-situ on the surface of lithium by reacting a fluoride-containing salt with a lithium metal anode, preventing direct contact between the lithium surface and the electrolyte and suppressing lithium dendrites. For example, in Patent CN107093705A, a fluoride-containing additive is dissolved in an organic solvent and reacted with lithium metal, and a protective layer grows in-situ on the lithium foil. In Patent CN109950476A, metal fluoride powder and a binder are mixed, coated on a copper foil and vacuum dried to obtain a metal fluoride electrode, which is laminated and compacted with polished lithium metal and heat-treated to obtain a lithium metal anode material with a protective layer. The lithium fluoride generated by the reaction of the metal fluoride and lithium metal forms a protective layer on the lithium surface, which is uniform and dense, inhibits the generation of lithium dendrites, and improves the electrochemical performance of the lithium anode. Patent CN115036465A discloses that a metal fluoride is coated on the surface of a lithium metal matrix for a displacement reaction to generate lithium fluoride, and under reducing conditions, a lithium-philic metal ion is reduced to a lithium-philic metal element to form a composite protective layer.

[0006] The above patent methods all form a protective layer containing lithium fluoride on the surface of lithium metal by reacting with a fluoride-containing reagent, but the ionic conductivity of lithium fluoride is very low (LiF ionic conductivity = 10 -31 S cm -1 ), which is not conducive to the diffusion of lithium ions inside the SEI, resulting in uneven lithium deposition. The fluoride-containing additive may be unstable to the positive electrode. Coating a binder on the lithium surface, the ionic conductivity of the binder is poor, which will generate a large interfacial resistance, increase the lithium deposition overpotential, and easily form dendrites. Summary of the Invention

[0007] In order to modify the surface of the lithium metal anode and further improve the electrochemical performance of the modified anode, the present invention proposes a method for modifying the interface of a lithium metal anode and a double-layer protective layer of lithium metal obtained after interface modification. The double-layer protective layer is formed in-situ and spontaneously through a simple process of immersing a lithium foil in a solution of a fluoride-containing metal salt, drying it, and then using it as an anode through a charge / discharge process. The preparation method is simple. Through this very simple method, a double-layer interface containing lithium fluoride and a lithium-containing alloy is formed on the surface of lithium metal. This double-layer interface can provide uniformly distributed lithium-philic sites, promote the uniform nucleation and deposition of lithium, improve the stability of the lithium metal anode, and at the same time promote the rapid diffusion of lithium ions, effectively inhibit the growth of lithium dendrites, and improve the safety performance and cycle stability of lithium metal batteries. This method has the characteristics of simplicity and wide applicability, has little impact on the energy density of the battery, and lithium batteries based on this method exhibit good cycle stability.

[0008] To solve the above technical problems, the present invention provides the following technical solutions:

[0009] A double-layer protective layer for metallic lithium, comprising a bottom layer in contact with the surface of metallic lithium and a top layer as the protective layer; the bottom layer is a lithium-containing alloy, and the top layer comprises lithium fluoride and a lithium salt of an oxyacid.

[0010] The lithium-containing alloy is selected from at least one of lithium-tin alloy, lithium-zinc alloy, lithium-aluminum alloy, lithium-magnesium alloy, and lithium-silver alloy; the lithium salt of an oxyacid is selected from at least one of lithium borate and lithium phosphate.

[0011] In a preferred technical solution of the present invention, the lithium-containing alloy is a lithium-tin alloy, and the lithium salt of an oxyacid is lithium borate (LiBO x , including LiBO2, LiB2O4, Li3BO3, Li2B4O7, Li2B8O 13 etc.).

[0012] Furthermore, when the lithium-containing alloy is a lithium-tin alloy and the lithium salt of an oxyacid is lithium borate, the double-layer protective layer of metallic lithium has at least one of the following physical and chemical parameters:

[0013] a) The infrared spectrum includes the following characteristic peaks: 1049 ± 10 cm -1 , 1081 ± 10 cm -1 , 1166 ± 10 cm -1 and 1355 ± 10 cm -1 ;

[0014] b) Time-of-flight secondary ion mass spectrometry (TOF-Sims) includes the following mass-to-charge ratios (m / Z): 7.01 ± 0.1, 18.99 ± 0.1, 43.00 ± 0.1, 93.03 ± 0.1, and 112.0 ± 0.1.

[0015] c) The scanning electron microscope energy dispersive spectrum includes elements Sn, F, B, and O.

[0016] The present invention also provides a method for modifying the interface of a metallic lithium negative electrode, comprising the following steps: contacting a clean lithium surface with a solution of an organic solvent containing a modifier, drying to obtain a modified metallic lithium negative electrode, and forming the above double-layer protective layer on the surface of metallic lithium through charge-discharge cycles; the modifier is a salt, the cation of which is a metal cation capable of alloying with metallic lithium, and the anion is a fluorine-containing anion.

[0017] Furthermore, the metal cation capable of alloying with metallic lithium is selected from at least one of Sn, Zn, Al, Mg, Ag, and Cs; the fluorine-containing anion is selected from at least one of tetrafluoroborate (BF4 - ), hexafluorophosphate (PF6 - ).

[0018] Preferably, the modifier is tin tetrafluoroborate (Sn(BF4)2).

[0019] Furthermore, the organic solvent is selected from at least one of ethylene glycol dimethyl ether ((DME), dioxolane (DOL), dimethyl carbonate (DMC), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), and ethanol).

[0020] Furthermore, the concentration of the modifier in the solution is 0.01 - 1 M, preferably 0.1 - 0.5 M.

[0021] Furthermore, the thickness of the metallic lithium is 5 - 1000 μm, and the method for cleaning its surface is well-known in the art, such as cleaning with an organic solvent or polishing in an inert atmosphere.

[0022] After the metallic lithium is immersed in the solution of Sn(BF4)2 in an organic solvent, tin particles, LiF, and amorphous lithium borate (LiBO x ) will be formed on the surface of the metallic lithium, and the three substances are in a mixed state. The surface-modified metallic lithium is used as the negative electrode, and during the electrochemical cycling process of charging / discharging, lithium is electrochemically deposited. LiF and LiBO x have good electronic insulation, so lithium will not be deposited on their surfaces. Tin has a lithiophilic property and can induce the deposition of lithium ions on its surface, and they diffuse with each other to form a lithium-tin alloy layer as the bottom layer. The bottom layer of the lithium-tin alloy can reduce the lithium deposition overpotential and reduce the phenomenon of lithium dendrites. In the top layer, LiF has excellent mechanical strength and can effectively prevent dendrites; amorphous LiBO x has more defects and higher ionic conductivity, which is beneficial to the transport of lithium ions in the SEI and can support the rapid horizontal diffusion and vertical migration of lithium ions; at the same time, the interface between the amorphous LiBO x and LiF in the top layer causes the lattice expansion of LiF, resulting in a stronger adsorption energy of lithium ions at the interface and a faster lithium ion migration rate. It can be seen that the present invention forms a double-layer protective layer on the surface of the metallic lithium negative electrode through a very simple method (immersion in a fluorine-containing metal salt + electrochemical cycling), that is, the bottom layer of the lithium-tin alloy and the top layer of LiF / LiBO x . The bottom layer and the top layer each play their own advantages and cooperate with each other, which can effectively prevent the growth of lithium dendrites, inhibit the decomposition of the electrolyte, and guide the directional deposition of lithium ions, improving the electrochemical performance of the metallic lithium negative electrode. Under the condition of a high-rate current density, very excellent cycle stability is achieved.

[0023] Figure 1 Schematically illustrates the formation of LiF / LiBO on the surface of the metallic lithium in the present invention xProcess of the -Sn interface. For metallic lithium, the original passivation layer contains Li2O, Li2CO3, and LiOH. When metallic lithium is immersed in a solution of an organic solvent containing Sn(BF4)2, a substitution reaction occurs: Li + Sn(BF4)2 = Sn + LiBF4. The formed LiBF4 reacts with trace water in the solvent to form LiF and amorphous LiBO x .

[0024] The present invention also provides an interface modification method for a metallic lithium negative electrode in a current collector based on the above strategy, including the following steps: The current collector is immersed in a solution of an organic solvent containing a modifier for modification, and then a modified metallic lithium negative electrode is obtained by Method A or Method B:

[0025] Method A: Assemble the current collector and metallic lithium into a half-cell and perform electrochemical deposition to obtain a metallic lithium negative electrode;

[0026] Method B: Pour molten lithium into the current collector to obtain a metallic lithium negative electrode.

[0027] The principle of the present invention is that: The nucleation overpotential of lithium ions on the surface of a metal that can form an alloy with metallic lithium is relatively small, and lithium ions will preferentially deposit on its surface and form a stable lithium-containing alloy. In addition, SEI can passivate the surface of metallic lithium, inhibit the side reaction between metallic lithium and the electrolyte, and play an important role in the stability of the metallic lithium negative electrode. LiF has high chemical stability, low solubility, high shear modulus, and low lithium ion diffusion barrier, which is beneficial to the stability of the metallic lithium negative electrode; Amorphous LiBO x has relatively high ionic conductivity, which is beneficial to the migration of lithium ions. The combined action of multiple factors enables the obtained metallic lithium negative electrode with a double-layer protective layer to fully exert its electrochemical performance, and the lithium ion battery combined with the NCM ternary positive electrode has excellent cycle stability and rate performance.

[0028] The excellent effects of the present invention are as follows:

[0029] First, the method of the present invention is an interface modification method for a metallic lithium negative electrode that is widely applicable to various lithium metal negative electrodes and can inhibit the growth of lithium dendrites. It has simple operation, a wide application range, and little impact on the energy density of the battery. The lithium battery based on this method exhibits good cycle stability and is an interface modification method for metallic lithium with great industrial advantages.

[0030] Second, the double-layer protective layer near the metallic lithium provided by the present invention improves the negative electrode / electrolyte interface synergistically between the bottom layer and the upper layer, significantly enhancing the electrochemical performance of the metallic lithium negative electrode, especially the cycle stability at high current densities, and having good rate performance. Description of the Drawings

[0031] Figure 1It is a schematic diagram of the process of forming a double-layer interface on the surface of metallic lithium in the present invention;

[0032] Figure 2 It is the electron microscope photograph of Sn(BF4)2@Li obtained in Example 1;

[0033] Figure 3 It is the scanning electron microscope energy dispersive spectrum of Sn(BF4)2@Li obtained in Example 1;

[0034] Figure 4 It is the transmission electron microscope image of Sn(BF4)2@Li obtained in Example 1;

[0035] Figure 5 It is the transmission electron microscope energy dispersive spectrum of Sn(BF4)2@Li obtained in Example 1;

[0036] Figure 6 It is the Fourier transform infrared spectrum of Sn(BF4)2@Li obtained in Example 1;

[0037] Figure 7 It is the cross-sectional scanning electron microscope energy dispersive spectrum of Sn(BF4)@Li after electrochemical cycling in Example 1;

[0038] Figure 8 It is the time-of-flight secondary mass spectrum of Sn(BF4)2@Li obtained in Example 1 changing with time;

[0039] Figure 9 It is the cycle performance graph of the symmetric battery using Sn(BF4)2@Li in Example 1;

[0040] Figure 10 It is the electrochemical impedance spectrum of the surface-modified lithium negative electrode and the pure lithium negative electrode obtained in Example 1;

[0041] Figure 11 It is the SEM image of the modified metallic lithium negative electrode after the 50th cycle in Example 1;

[0042] Figure 12 It is the SEM image of pure lithium after the 50th cycle;

[0043] Figure 13 It is the SEM image of the modified carbon paper in Example 5;

[0044] Figure 14 It is the deposition overpotential of current collectors such as copper foil, carbon paper, and modified carbon paper in Example 5;

[0045] Figure 15 It is the cycle performance test graph of the full cell assembled with the negative electrode obtained in Example 1. Detailed implementation manners

[0046] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be described in detail below. The following embodiments facilitate a better understanding of the present invention but do not limit the present invention. The experimental methods in the following embodiments are conventional methods unless otherwise specified.

[0047] Example 1

[0048] Sn(BF4)2 and DME were prepared into a solution at a concentration of 0.1 M, and after magnetic stirring for 30 minutes, a Sn(BF4)2 / DME solution was obtained. A lithium foil with a clean surface (diameter 12.5 mm, thickness 550 μm) was immersed in the obtained Sn(BF4)2 / DME solution for 3 minutes and then taken out. After rinsing the surface 3 times with DME, it was dried under an argon atmosphere to obtain Sn(BF4)2-modified metallic lithium (Sn(BF4)2@Li) for standby.

[0049] Figure 2 is the SEM image of the obtained Sn(BF4)2@Li. It can be seen that a large number of Sn nanoparticles are covered by a continuous interfacial phase and are uniformly distributed on the Li surface

[0050] Figure 3 is the scanning electron microscope energy dispersive spectroscopy map (SEM EDS-Mapping) of the obtained Sn(BF4)2@Li, which confirms the uniform distribution of Sn, F, and O elements on the surface of metallic lithium.

[0051] Figure 4 is the transmission electron microscope image of the surface modification layer of the obtained Sn(BF4)2@Li. It can be seen that the Sn nanoparticles are coated with an amorphous substance.

[0052] Figure 5 is the transmission electron microscope energy dispersive spectroscopy (TEM EDS-Mapping) of the surface modification layer of the obtained Sn(BF4)2@Li, indicating the distribution of elements such as B, O, Sn, and F in the obtained modification layer, and proving that the nanoparticles are metallic Sn and the outer amorphous compound is LiBO x .

[0053] Figure 6 is the Fourier transform infrared spectroscopy (FT-IR) image of the obtained Sn(BF4)2@Li, where the peaks at 1049, 1081, 1166, and 1355 cm -1 can be attributed to the bending of B-O-B or the stretching vibration of O-B-O. It shows the existence of amorphous LiBO x .

[0054] Figure 7It is the cross-sectional SEM-EDS Mapping of Sn(BF4)2@Li after cycling, which proves that after cycling, Sn elements are distributed in the lower layer, and F and O elements are distributed in the upper layer, forming a bilayer structure.

[0055] Figure 8 It is the time-of-flight secondary ion mass spectrometry (TOF-Sims) graph of the obtained Sn(BF4)2@Li varying with depth. The signals of LiF and LiBO x reach the strongest at about 50 seconds of etching time and then gradually weaken, indicating that LiF and LiBO x are distributed at the upper interface. The signal intensity of Sn reaches the strongest at about 150 seconds and does not weaken significantly with the increase of etching time, indicating that it is distributed at the lower interface.

[0056] (S2) Using 1M LiTFSI, DOL:DME (1:1) as the electrolyte, Celgard 2400 as the separator, and Sn(BF4)2@Li for both the positive and negative electrodes, a symmetric battery is assembled. At a current density of 8 mA cm -2 and a cycling capacity of 8 mAh cm -2 under the condition, it cycles for more than 1000 hours. Figure 9 It is the cycling performance graph of the symmetric battery, and the polarization voltage is 50 mV. If pure lithium is used, a large polarization voltage (573 mV) is shown.

[0057] Figure 10 It is the electrochemical impedance spectrum of the surface-modified lithium negative electrode and the pure lithium negative electrode obtained in Example 1. The results show that the modified lithium negative electrode has a smaller interfacial impedance, proving that the bilayer protective layer significantly improves the interfacial kinetics of the negative electrode surface.

[0058] Figure 11 and 12 are the SEM images of the pure lithium and the modified metallic lithium negative electrode obtained in Example 1 under the condition of a current density of 2 mA cm -2 after 50 cycles. After 50 cycles, the morphology of the lithium deposited on the surface-modified negative electrode in Example 1 remains uniform, while a large amount of dendritic lithium appears on the bare lithium negative electrode.

[0059] Example 2

[0060] Other conditions are the same as those in Example 1, except that the concentration of Sn(BF4)2 in DME is 0.5 M.

[0061] Example 3

[0062] Other conditions are the same as those in Example 1, except that the solvent DME is replaced by DOL.

[0063] Example 4

[0064] Other conditions are the same as those in Example 1, except that Sn(BF4)2 is replaced by Sn(PF6)2.

[0065] Comparative Example 1

[0066] Other conditions are the same as those in Example 1. The difference is that an unmodified lithium foil is used to assemble a symmetric cell under the same conditions. At a current density of 8 mA cm -2 , and a cycling capacity of 8 mAh cm -2 Under the condition, the polarization voltage reaches greater than 500 mV after 200 hours of cycling.

[0067] The lithium anodes of Examples 1-4 and Comparative Example 1 were used to prepare symmetric cells according to the method described in step (S2) of Example 1 and tested under cycling conditions at a current density of 8 mA cm -2 . The results are shown in Table 1 below:

[0068] Table 1 Performance of symmetric cells with different lithium anodes

[0069] electrode cycle time polarization voltage Example 1 > 1000 hours 50 mV Example 2 > 1200 hours 63 mV Example 3 > 1000 hours 66 mV Example 4 > 600 hours 104 mV Comparative Example 1 ＜200h > 500 mV

[0070] By comparing the data in Table 1, it can be seen that the metal lithium anode with a double-layer protective layer modification provided by the present invention has a longer cycling time and a lower polarization voltage, indicating that the lithium-tin alloy layer induces uniform deposition of lithium, reduces the overpotential, and the LiF-rich SEI improves the stability of the charge-discharge cycle of metallic lithium, proving that the metal lithium with a double-layer interface modification has better cycling stability. A similar strategy was used to modify the surface of metallic lithium with Sn(PF6). Although the electrochemical performance was improved compared to pure lithium, it was not as good as the modification effect of Sn(BF4).

[0071] Example 5

[0072] Sn(BF4)2-modified carbon paper current collector

[0073] (S1) Prepare a solution of Sn(BF4)2 and DME at a concentration of 0.1 M. Immerse the pretreated carbon paper (CP) in the 0.1 M Sn(BF4)2 solution, take it out after ultrasonic treatment for 1 h, and dry it under vacuum at 0.01 MPa and 80 °C for 6 h. Figure 13 is the morphological characterization of Sn(BF4)2@CP, indicating that Sn(BF4)2 nanoparticles are uniformly distributed on the surface of CP.

[0074] (S2) Using a lithium foil as the negative electrode, 1 M LiPF6|EC:DMC:DEC (volume ratio 1:1:1) + 2 wt% VC as the electrolyte, Celgard 2400 as the separator, assemble Sn(BF4)2@CP and a lithium sheet into a half-cell, and at 2 mA cm-2 Electrodeposition was carried out at a current density, Figure 14 which are the deposition overpotential curves of Cu, CP, and Sn(BF4)2@CP. The overpotential of Sn(BF4)2@CP is 79 mV, which is significantly lower than that of Cu (156 mV) and CP (113 mV).

[0075] Battery performance test

[0076] Using the lithium with a double-layer protective layer obtained in Example 1 as the negative electrode, commercial NCM811 as the positive electrode, 1M LiPF6|EC:DMC:DEC (volume ratio 1:1:1) + 2 wt% VC as the electrolyte, and Celgard 2400 as the separator to assemble a full cell, and the cycle performance test was carried out under the condition of 2C, Figure 15 which is the cycle performance diagram of the full cell assembled with the lithium negative electrode obtained in Example 1. The discharge specific capacity in the first cycle is 195.5 mAh g -1 and the discharge specific capacity after 150 cycles is 180.3 mAh g -1 with a capacity retention rate of 84.8%.

[0077] Similarly, the negative electrodes obtained in Examples 2-4 and Comparative Example 1 were assembled into full cells according to the above method for battery performance testing. The results of Examples 1-4 and Comparative Example 1 are shown in Table 2 below:

[0078] Table 2 Full cell performance test

[0079]

[0080] It can be seen that the modified lithium metal negative electrode or the negative electrode current collector of the present invention exhibits a lower deposition overpotential, induces the deposition of metallic lithium, and the current collector after the obtained deposited lithium exhibits higher stability during the lithium deposition / stripping process, proving that the double-layer interface of the lithium fluoride-rich SEI and the lithium-tin alloy layer improves the stability of the negative electrode. There is a significant improvement in cycle stability.

Claims

1. A preparation method of a double-layer protective layer for a lithium metal anode, characterized in that, Comprising the following steps: Using a clean lithium surface to contact a solution of an organic solvent dissolved with a modifier, and drying to obtain a modified lithium metal anode. After charge-discharge cycling, the above double-layer protective layer is formed on the lithium metal surface; the modifier is tin tetrafluoroborate, and the concentration of the modifier in the solution is 0.1 - 0.5 M; The double-layer protective layer of the lithium metal anode includes a bottom layer in contact with the lithium metal surface and a top layer as the protective layer; the bottom layer is a lithium-tin alloy, and the top layer is lithium fluoride and amorphous lithium borate; the double-layer protective layer of the lithium metal anode has the following physicochemical parameters: the energy dispersive spectrogram of the scanning electron microscope includes elements Sn, F, B, and O, and Sn, F, and O are uniformly distributed on the lithium metal surface.

2. The preparation method according to claim 1, characterized in that, The lithium borate is selected from LiBO2, LiB2O4, Li3BO3, Li2B4O7 or Li2B8O 13 .

3. The preparation method according to claim 1, characterized in that, The double-layer protective layer of the lithium metal anode has at least one of the following physicochemical parameters: a) The infrared spectrum includes the following characteristic peaks: 1049 ± 10 cm -1 , 1081 ± 10 cm -1 , 1166 ± 10 cm -1 and 1355 ± 10 cm -1 ; b) Time-of-flight secondary ion mass spectrometry (TOF-Sims) includes the following mass-to-charge ratios (m / Z): 7.01 ± 0.1, 18.99 ± 0.1, 43.00 ± 0.1, 93.03 ± 0.1, and 112.0 ± 0.

1.

4. The preparation method according to claim 1, characterized in that, The organic solvent is selected from at least one of ethylene glycol dimethyl ether, dioxolane, dimethyl carbonate, tetrahydrofuran, dimethyl sulfoxide, N,N-dimethylformamide, and ethanol.

5. The preparation method according to claim 1, characterized in that, The thickness of the lithium metal is 5 - 1000 μm.

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