Oxide protection layer for lithium metal negative electrode and preparation method and application thereof

Abstract

The invention belongs to the technical field of batteries, and relates to an oxide protection layer for a lithium metal negative electrode and a preparation method and application of the oxide protection layer, a prepared composite electrode comprises a lithium foil substrate and the oxide protection layer covering the surface of the lithium foil substrate, and the oxide protection layer is prepared through a sol-gel method. Comprising the following steps: sol preparation: dissolving metal alkoxide or metal salt in an aprotic polar solvent, adding a stabilizer and / or a catalyst, and stirring; and coating the sol on the surface of the lithium foil, drying and annealing. The oxide protective layer prepared by the method has a uniform and compact structure, can effectively inhibit the growth of lithium dendrites, improve interface contact and improve the cycle performance and coulombic efficiency of the lithium metal negative electrode, and is simple in preparation method, low in cost and easy to realize large-scale production. The invention also discloses an all-solid-state lithium metal battery comprising the composite electrode.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to an oxide protective layer for a lithium metal negative electrode and a preparation method and application thereof. Background Art

[0002] Lithium metal has a very high theoretical specific capacity (3860 mAh g -1 ) and the lowest electrochemical potential (-3.04 Vvs. standard hydrogen electrode), it is considered to be one of the most promising negative electrode materials for the next generation of high energy density batteries. Compared with the graphite negative electrode used in traditional lithium-ion batteries, the lithium metal negative electrode can significantly increase the energy density of the battery, thereby meeting the growing needs of portable electronic devices, electric vehicles and energy storage systems.

[0003] However, lithium metal anodes face many challenges in practical applications, which seriously restrict their commercialization process: Lithium dendrite growth: During the charge and discharge process, lithium ions are unevenly deposited on the negative electrode surface, which easily forms needle-shaped or branch-shaped lithium metal crystals, namely lithium dendrites. The growth of lithium dendrites can pierce the diaphragm, causing internal short circuits in the battery, leading to safety issues such as thermal runaway and even explosion. In addition, the formation of lithium dendrites will consume active lithium and reduce the coulombic efficiency and cycle life of the battery.

[0004] Volume expansion: Lithium metal undergoes dramatic volume changes during the charge and discharge process, resulting in unstable electrode structure, shedding of active substances, and intensified side reactions between the electrode and the electrolyte, further reducing battery performance.

[0005] Side reactions with electrolytes: Lithium metal has extremely high chemical activity and is prone to side reactions with traditional organic electrolytes to generate unstable solid electrolyte interface films (SEI). This unstable SEI film will continue to grow and rupture, consuming a large amount of lithium ions and electrolytes, resulting in battery capacity attenuation and shortened cycle life.

[0006] In order to solve the above problems, researchers have proposed a variety of strategies to improve the performance of lithium metal anodes, mainly including: Three-dimensional current collector design: By constructing a three-dimensional current collector with a special structure (such as porous copper foil, carbon nanotube array, etc.) to guide the uniform deposition of lithium ions and reduce the formation of lithium dendrites. However, this method has a high preparation cost and is difficult to achieve large-scale production.

[0007] Surface modification: Building a protective layer on the surface of lithium metal is a relatively effective and economical method. Currently reported protective layer materials include inorganic materials (such as oxides, nitrides, fluorides), organic materials (such as polymers, conjugated polymers) and composite materials. These protective layers are intended to improve the performance of lithium metal anodes through the following mechanisms: Guiding uniform deposition of lithium ions: Providing uniform nucleation sites to promote the uniform deposition of lithium ions.

[0008] However, there are still some deficiencies in the protective layer materials and preparation methods used in the prior art: The ionic conductivity of some protective layer materials is relatively low, which will increase the internal resistance of the battery and reduce the rate performance.

[0009] The interfacial bonding force between some protective layers and lithium metal is poor, and peeling is likely to occur, affecting the cycle stability of the battery.

[0010] Some preparation methods are relatively complex and costly, making it difficult to achieve large-scale production.

[0011] Therefore, developing a protective layer for lithium metal anodes with simple preparation, low cost, and excellent performance is still an important research direction at present.

[0012] In view of the problems existing in the prior art, the present invention proposes a new method for preparing an oxide protective layer on the surface of a lithium foil by the sol-gel method. The oxide protective layer prepared by this method has a uniform and dense structure, can effectively inhibit the growth of lithium dendrites, improve the interfacial contact, enhance the cycle performance and Coulomb efficiency of the lithium metal anode, and has a simple preparation method, low cost, and is easy to achieve large-scale production. Summary of the Invention

[0013] In view of this, the purpose of the present invention is to provide an oxide protective layer for a lithium metal anode, its preparation method, and application. This protective layer can effectively inhibit the growth of lithium dendrites, improve the interfacial contact, and enhance the cycle performance and Coulomb efficiency of the lithium metal anode. The oxide protective layer shows great potential in improving the interfacial contact of solid-state batteries. Specifically, the oxide protective layer effectively improves the interfacial contact of solid-state batteries through various mechanisms such as improving wettability, inhibiting side reactions, regulating ion transport, and enhancing mechanical strength. On the one hand, the oxide protective layer can act as a "buffer layer" to isolate lithium metal from the solid electrolyte, reducing or even inhibiting their direct contact, thereby reducing the occurrence of side reactions. On the other hand, some oxides have high mechanical strength and hardness, which can act as a physical barrier to block the penetration of lithium dendrites. Furthermore, some oxides have high ionic conductivity and can serve as a fast channel for lithium ions to promote the uniform transport of ions at the interface. In addition, the surface structure and crystal orientation of the oxide protective layer also affect the deposition behavior of lithium ions.

[0014] The oxide protective layer effectively improves the interfacial contact of solid-state batteries through various mechanisms such as improving wettability, inhibiting side reactions, regulating ion transport, and enhancing mechanical strength, thereby enhancing the performance and safety of the batteries. By selecting appropriate oxide materials and controlling their structure and thickness, the interfacial performance can be further optimized, promoting the development of solid-state batteries. This method provides an effective solution to the key problems in the development of all-solid-state batteries. The oxide protective layer of the present invention can maintain more than 90% of its capacity after 500 cycles when applied to all-solid-state lithium-metal batteries, and the cycle life is increased by approximately 2500%.

[0015] To achieve the above object, the technical solution adopted by the present invention is as follows: <First aspect> An oxide protective layer for a lithium-metal negative electrode is prepared by a method including the following steps: S1. Sol preparation: Dissolve a metal alkoxide or a metal salt in an aprotic polar solvent, add a stabilizer to obtain a sol precursor, and then dropwise add a catalyst and stir evenly to form a sol; S2. Preparation of the oxide protective layer: Coat the prepared sol on the surface of the lithium negative electrode, and perform annealing treatment after drying to obtain the oxide protective layer loaded on the surface of the lithium-metal negative electrode.

[0016] As an embodiment of the present invention, in step S1, the metal alkoxide includes one of tetrabutyl titanate, tri-sec-butyl aluminate, and tetraethyl orthosilicate.

[0017] As an embodiment of the present invention, in step S1, the metal salt includes one or more of metal nitrates and metal carboxylates. The metal nitrate is zirconium nitrate, and the metal carboxylate is zinc acetate.

[0018] As an embodiment of the present invention, in step S1, the volume ratio of the metal alkoxide to the aprotic polar solvent is 1:5 - 50, preferably 1:5 - 15; the dosage ratio of the metal salt to the aprotic polar solvent is 1 g:10 - 50 ml, preferably 1 g:20 - 30 ml.

[0019] The raw material corresponding to titanium dioxide (TiO 2 ), is tetrabutyl titanate, the raw material corresponding to aluminum oxide (Al 2 O 3 ), is tri-sec-butyl aluminate, the raw material corresponding to zinc oxide (ZnO) is zinc acetate, the raw material corresponding to zirconium oxide (ZrO 2 ), is zirconium nitrate, and the raw material corresponding to silicon oxide (SiO 2 ), is tetraethyl orthosilicate.

[0020] As an embodiment of the present invention, in step S1, the aprotic polar solvent includes one or more of N-methylpyrrolidone (NMP), acetonitrile (ACN), dimethylformamide (DMF), and anhydrous ether.

[0021] As an embodiment of the present invention, in step S1, the stabilizer is selected from one of acetylacetone and diethanolamine (DEA). The catalyst is one of sodium hydroxide, ammonia water, and deionized water.

[0022] As an embodiment of the present invention, in step S1, the stabilizer of the metal alkoxide is acetylacetone, and the catalyst is deionized water. The volume ratio of the metal alkoxide, stabilizer, and catalyst is 4-6:0.8-1.2:0.8-1.2.

[0023] As an embodiment of the present invention, in step S1, the stabilizer of the metal salt is diethanolamine (DEA), and the catalyst is one or more of sodium hydroxide solution and ammonia water; the concentration is 0.4-0.6 mol / L. Adding the catalyst makes the pH value of the sol precursor reach 7.5-8.5. The volume ratio of the metal salt to the stabilizer is 2-2.4:2.

[0024] As an embodiment of the present invention, in step S1, the stirring time is 1-3 h.

[0025] As an embodiment of the present invention, in step S2, the lithium negative electrode is a lithium metal negative electrode or a lithium alloy negative electrode. The lithium negative electrode is preferably a lithium foil.

[0026] As an embodiment of the present invention, in step S2, the lithium metal negative electrode is pretreated before coating; Pretreatment: Under an inert gas protection environment, the surface of the lithium metal negative electrode is mechanically scratched or cleaned to remove the surface oxide layer and impurities.

[0027] As an embodiment of the present invention, in step S2, the coating method is one of spin coating, dip coating, spraying, and blade coating.

[0028] As an embodiment of the present invention, in step S2, the drying temperature is 30-60 °C, and the time is 6-20 h. Drying is carried out under inert gas protection or in a vacuum environment.

[0029] As an embodiment of the present invention, in step S2, the annealing temperature is 80-120 °C, the annealing time is 1-10 h, preferably 1-4 h. The heating rate during annealing is 0.5-1 °C / min, preferably 0.5 °C / min As an embodiment of the present invention, in step S2, the oxides in the oxide protection layer include titanium dioxide (TiO 2 ), aluminum oxide (Al2 O 3 ), zinc oxide (ZnO), zirconia (ZrO 2 ), silica (SiO 2 ), or one or more thereof.

[0030] As an embodiment of the present invention, in step S2, the thickness of the obtained oxide protective layer is 30 - 100 nm.

[0031] As an embodiment of the present invention, in step S2, the oxide in the obtained oxide protective layer has a nanocrystalline structure. Different from conventional oxides, the oxides of the present invention are prepared by sol, and the annealed product has a nanocrystalline structure.

[0032] As an embodiment of the present invention, steps S1 and S2 are both carried out under an inert gas protection environment.

[0033] In the sol preparation stage of the present invention, the key lies in dissolving metal alkoxide or metal salt in an aprotic polar solvent, adding a stabilizer and a catalyst, and stirring evenly to form a stable sol.

[0034] Taking the preparation of titanium dioxide (TiO 2 ) sol as an example: The commonly used metal alkoxide precursor is tetrabutyl titanate (Ti(OC 4 H 9 ). Tetrabutyl titanate undergoes a hydrolysis reaction with water (catalyst) to generate titanium hydroxide (Ti(OH) 4 ) and butanol (C 4 H 4 H 9 OH). Titanium hydroxide then undergoes a condensation reaction to form Ti - O - Ti bonds, and finally TiO 2 sol is formed.

[0035] Hydrolysis reaction: Ti(OC 4 H 9 ) 4 + 4H 2 O → Ti(OH) 4 + 4C 4 H 9 OH; Condensation reaction: nTi(OH) 4 → (TiO 2 )n + 2nH 2 O.

[0036] Taking the preparation of zinc oxide (ZnO) sol as an example: The commonly used metal salt precursor is zinc acetate dihydrate (Zn(CH 3 COO) 2 ·2H 2(O). Zinc acetate dihydrate undergoes a complexation reaction with diethanolamine (DEA) to form a stable zinc complex. The zinc complex undergoes hydrolysis and polycondensation reactions under alkaline conditions to form a ZnO sol.

[0037] Complexation reaction: Zn(CH 3 COO) 2 ·2H 2 O + 2NH(CH 2 CH 2 OH) 2 → Zn(DEA) 2 (H 2 O) 2 ; Hydrolysis and condensation reaction: Zn(DEA) 2 (H 2 O) 2 + NaOH → ZnO + H 2 O + other products.

[0038] The main purpose of the annealing stage of the present invention is to remove the organic substances (such as solvents, stabilizers, etc.) in the sol and to form a nanocrystalline structure of the oxide, thereby improving its performance.

[0039] For example, for TiO 2 sol, the following reactions mainly occur during the annealing process: Decomposition of organic substances: The organic substances in the sol decompose at high temperature to generate gases such as carbon dioxide (CO 2 ), water vapor (H 2 O), etc.

[0040] Crystal growth: TiO 2 nanoparticles undergo crystal growth at high temperature to form a TiO 2 thin film with a certain crystal structure.

[0041] For ZnO sol, the following reactions mainly occur during the annealing process: Decomposition of organic substances: The organic substances (such as DEA) in the sol decompose at high temperature to generate gases such as carbon dioxide (CO 2 ), water vapor (H 2 O), ammonia (NH 3 ), etc.

[0042] Crystal growth: ZnO nanoparticles undergo crystal growth at high temperature to form a ZnO thin film with a certain crystal structure.

[0043] The preparation processes of other oxide thin films have similar chemical changes to the above oxide preparation processes.

[0044] Conventional methods, such as the calcination method, usually produce an oxide protective layer with a polycrystalline structure, but the grain size is relatively large, generally in the micrometer or submicrometer range, and it is difficult to reach the nanocrystalline level. Due to the high-temperature calcination process, the grains are prone to grow, and it is difficult to precisely control the grain size and morphology. In addition, the density of the thin film prepared by the calcination method may be relatively low, the porosity is relatively high, and the uniformity may also be poor.

[0045] In the prior art, the deposition method and the calcination method are used to prepare the oxide protective layer. Among them, the cost of the deposition equipment is relatively high, the process is more complex, and it cannot be mass-produced. In particular, only some materials can be deposited, and the material selection is greatly limited. The oxide protective layer prepared by the calcination method has fewer grain boundaries. The nanocrystalline oxide protective layer prepared by the sol-gel method of the present invention is superior to the oxide protective layer prepared by the calcination or deposition method in the prior art in terms of battery performance, especially in the application of all-solid-state lithium metal batteries.

[0046] It is mainly reflected in the following aspects: Higher ionic conductivity: The high-density grain boundaries brought about by the nanocrystalline structure can serve as channels for rapid lithium ion transport, reduce the interfacial ion transport impedance, and improve the rate performance of the battery. In the prior art, the oxide protective layer prepared by the calcination method has fewer grain boundaries, and the amorphous thin film prepared by the deposition method lacks grain boundaries, and the ionic conductivity may be limited.

[0047] Better interfacial contact: The nanocrystalline structure has higher surface activity and wettability, can form a closer interfacial contact with the solid electrolyte, reduce the interfacial resistance, and improve ion transport. In addition, the nanocrystalline structure may have a certain flexibility, can better adapt to the volume change of the lithium metal negative electrode during charge and discharge, and maintain the stability of the interfacial contact.

[0048] In the present invention, the oxide protective layer is prepared on the surface of the negative electrode. The oxide layer is in direct contact with the lithium metal and participates in the electrochemical reaction as a part of the negative electrode, which can improve the properties of the negative electrode surface, such as improving its chemical stability and mechanical strength, inhibiting the nucleation and growth of lithium dendrites, and reducing the direct contact with the sulfide electrolyte.

[0049] <Second aspect> A composite lithium metal negative electrode, comprising a lithium metal negative electrode and the oxide protective layer.

[0050] An application of the composite lithium metal negative electrode in an all-solid-state lithium alloy secondary battery. The application of the oxide protective layer for the lithium metal negative electrode in an all-solid-state lithium alloy secondary battery also belongs to the protection scope of the present invention.

[0051] The thickness of the oxide protective layer has a significant impact on the performance of all-solid-state lithium-metal batteries, especially in the all-solid-state lithium-metal battery system. If the oxide film is too thin, it may not effectively block the growth of lithium dendrites and may not sufficiently improve the interfacial contact. In addition, the integrity of the film may be poor and defects are likely to occur. If the oxide film is too thick, it will lead to an increase in ion transport resistance and reduce the rate performance of the battery. At the same time, the too-thick film may also cause a decrease in interfacial bonding force, affecting the cycle stability of the battery.

[0052] An all-solid-state lithium-metal secondary battery includes a solid electrolyte, a positive electrode, and the composite lithium-metal negative electrode.

[0053] The solid electrolyte includes one of a polymer electrolyte, an oxide electrolyte, and a sulfide electrolyte.

[0054] The positive electrode includes one of lithium iron phosphate, lithium cobaltate, lithium nickel cobalt manganate, lithium manganese phosphate, lithium titanate, lithium nickelate, and lithium manganate.

[0055] Compared with the all-solid-state lithium-metal batteries in the prior art, the beneficial effects of the present invention are as follows: (1) In traditional all-solid-state lithium-metal batteries, the poor interfacial contact between the lithium-metal negative electrode and the solid electrolyte is a long-standing bottleneck problem. Due to the large difference in plasticity between lithium metal and the rigidity of the solid electrolyte, it is difficult for the two to form a tight physical contact, resulting in a high interfacial resistance, which severely restricts the ion transport and performance of the battery. Some methods, such as applying external pressure, can temporarily improve the contact, but during cycling, due to the volume change of lithium, the interfacial contact will deteriorate again. The present invention effectively improves the interfacial contact between the lithium metal and the solid electrolyte by constructing an oxide protective layer on the surface of the lithium foil. Oxide materials usually have a high surface energy and can better wet the solid electrolyte, thereby increasing the actual contact area and reducing the interfacial resistance. In addition, the oxide film prepared by the sol-gel method usually has a nanostructure, which can further increase the contact area and provide more ion transport channels.

[0056] (2) In the prior art, lithium metal has extremely high chemical activity and is prone to side reactions with the solid electrolyte, forming unstable interfacial products, resulting in an increase in interfacial resistance and a decrease in battery performance. Especially when using some high-voltage positive electrode materials, the interfacial side reactions are more intense. The oxide protective layer in the present invention can act as a "buffer layer" to effectively isolate the lithium metal from the solid electrolyte, reduce or even inhibit the direct contact between the two, thereby reducing the occurrence of side reactions.

[0057] (3) The ionic conductivity of solid electrolytes is usually lower than that of liquid electrolytes, and the ionic transport behavior at the interface is crucial for the performance of lithium metal anodes. Uneven ionic transport can lead to the growth of lithium dendrites, posing a safety hazard. Some oxides of the present invention itself have certain ionic conductivity and can serve as fast channels for lithium ions to promote uniform ion transport at the interface. In addition, by controlling the preparation conditions of the sol-gel method, the microstructure of the oxide protective layer can be regulated, such as forming a vertically aligned nano-column or nanowire structure, providing more ion transport channels and uniform nucleation sites, thereby guiding uniform deposition of lithium ions and inhibiting the growth of lithium dendrites.

[0058] (4) Some methods for improving the interface of lithium metal anodes, such as atomic layer deposition (ALD), magnetron sputtering, etc., although high-quality thin films can be prepared, the equipment is expensive, the process is complex, and the production efficiency is low, making it difficult to achieve large-scale production. The sol-gel method adopted in the present invention is a mature thin film preparation technology, which has the advantages of simple operation, low cost, easy control, and large-area preparation, and is very suitable for industrial production.

[0059] (5) Some traditional lithium metal anode protection methods may not be applicable to all-solid-state battery systems, such as using certain liquid electrolyte additives. The oxide materials used in the present invention usually have high chemical stability and electrochemical stability, and have good compatibility with various solid electrolytes (including polymers, oxides, sulfides, etc.), and can be widely applied to various all-solid-state lithium metal battery systems. Description of the Drawings

[0060] By reading the detailed description of the non-limiting embodiments with reference to the following drawings, other features, objects, and advantages of the present invention will become more apparent: Figure 1 Cycling performance graph of the oxide protective layer applied to all-solid-state lithium metal for Example 1; Figure 2 Coulombic efficiency graph of the oxide protective layer applied to all-solid-state lithium metal battery for Example 1; Figure 3 Cycling performance comparison graph of all-solid-state lithium metal batteries between Example 1 and Comparative Example 1; Figure 4 Schematic structural diagram of the oxide protective layer of the present invention; Figure 5 TEM graph of the oxide for Example 1. Detailed Embodiments

[0061] The present invention will be described in detail below in conjunction with embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several adjustments and improvements can still be made. These all belong to the protection scope of the present invention.

[0062] The present invention relates to a composite electrode for a lithium metal anode and a preparation method thereof, aiming to solve problems such as dendrite growth, volume expansion, and side reactions with electrolytes existing in the lithium metal anode during the cycling process. The composite electrode includes a lithium foil substrate and an oxide protective layer covering its surface. The oxide protective layer is prepared by the sol-gel method, and the oxide is selected from one or more of titanium dioxide (TiO 2 ), aluminum oxide (Al 2 O 3 ), zinc oxide (ZnO), zirconium oxide (ZrO 2 ), silicon oxide (SiO 2 ), silver oxide (Ag 2 O) and their mixtures, and the thickness of the oxide protective layer is 30 nanometers to 100 nanometers. The preparation method includes: sol preparation (dissolving metal alkoxide or metal salt in an aprotic polar solvent under an inert gas protection environment, adding a stabilizer and / or a catalyst and stirring); lithium foil pretreatment (performing mechanical scratching or cleaning under an inert gas protection environment); coating (coating the sol on the surface of the lithium foil under an inert gas protection environment, using spin coating, dip coating, spraying or doctor blade coating); drying (under an inert gas protection environment); and annealing (under an inert gas protection environment). The oxide protective layer prepared by the present invention has a uniform and dense structure, can effectively inhibit the growth of lithium dendrites, improve the interfacial contact, and improve the cycling performance and Coulomb efficiency of the lithium metal anode. Moreover, the preparation method is simple, the cost is low, and it is easy to realize large-scale production. The present invention also discloses a all-solid-state lithium metal battery including the composite electrode.

[0063] Example 1 This example provides a lithium metal anode with a titanium dioxide protective film (50 nm), including the following steps: S1. Sol preparation: The following operations are carried out in a glove box filled with argon to avoid the reaction of tetrabutyl titanate with moisture in the air.

[0064] A certain amount of tetrabutyl titanate (5 mL) is slowly dropped into anhydrous ether (50 mL), and stirred evenly with a magnetic stirrer.

[0065] Acetylacetone (1 mL) is slowly dropped, and stirring is continued for 30 minutes to form a stable titanium precursor solution (sol precursor). Acetylacetone complexes with tetrabutyl titanate to slow down the hydrolysis rate.

[0066] Slowly add deionized water (1 mL) dropwise with a dropper to control the hydrolysis rate. Continue to stir vigorously for 2 hours to obtain a transparent TiO 2 sol. The reaction of water with tetrabutyl titanate produces TiO 2 sol.

[0067] S2. Lithium foil pretreatment: In a glove box filled with argon, cut the lithium foil into the required size.

[0068] Gently scrape the surface of the lithium foil with a spatula to remove the surface oxide layer and impurities.

[0069] S3. Coating: Fix the pretreated lithium foil on the platform of a spin coater.

[0070] Use a dropper to drop an appropriate amount of TiO 2 sol on the surface of the lithium foil.

[0071] Set the spin coating parameters: rotate at low speed (500 rpm) for 5 seconds, and then rotate at high speed (3000 rpm) for 30 seconds.

[0072] Repeat the coating steps 1 - 2 times to achieve the required film thickness. After each coating, perform a short drying.

[0073] S4. Drying: Put the coated lithium foil into a vacuum drying oven, set the temperature to 60 °C, the vacuum degree to -0.08 MPa, and the drying time to 12 hours to remove the solvent. This step needs to be carried out in a vacuum environment to prevent lithium from reacting with oxygen and water vapor in the air.

[0074] S5. Annealing: Put the dried lithium foil into a tube furnace, under argon protection, heat it to 120 °C at a certain heating rate (0.5 °C / min), and anneal it at this temperature for 2 hours. The TEM image of the obtained oxide is as shown in Figure 5 and has a nanocrystalline structure. The purpose of annealing is to decompose the organic matter in the sol and make TiO 2 form a crystalline structure to improve its performance.

[0075] Example 2 This example provides a lithium metal anode with a zinc oxide protective film (30 nm). This example includes the following steps: S1. Sol preparation: Perform the following operations in a glove box filled with argon to avoid the reaction of zinc salts with moisture in the air.

[0076] Dissolve a certain amount of zinc acetate dihydrate (2.2 g) in dimethylformamide (50 mL), and stir with a magnetic stirrer until completely dissolved.

[0077] Slowly add diethanolamine (DEA) (2 mL), and continue stirring for 1 hour to form a stable zinc precursor solution (sol precursor).

[0078] Under vigorous stirring, slowly add dropwise the sodium hydroxide dimethylformamide solution diluted with dimethylformamide (concentration: 0.5 mol / L), and monitor the pH value of the solution in real time with a pH meter until the pH value reaches 8. Continue stirring for 2 hours to obtain a transparent ZnO sol. The whole process is carried out in a glove box filled with argon.

[0079] S2. Lithium foil pretreatment: In a glove box filled with argon, cut the lithium foil into the required size.

[0080] Gently scrape the surface of the lithium foil with a spatula to remove the surface oxide layer and impurities.

[0081] S3. Coating: Fix the pretreated lithium foil on the platform of the spin coater.

[0082] Use a dropper to drop an appropriate amount of ZnO sol onto the surface of the lithium foil.

[0083] Set the spin coating parameters: rotate at low speed (500 rpm) for 5 seconds, and then rotate at high speed (3000 rpm) for 30 seconds.

[0084] S4. Drying: Put the coated lithium foil into a vacuum drying oven, set the temperature to 80 °C, the vacuum degree to -0.08 MPa, and the drying time to 12 hours to remove the solvent. This step needs to be carried out in a vacuum environment to prevent lithium from reacting with oxygen and water vapor in the air.

[0085] S5. Annealing: Put the dried lithium foil into a tube furnace, and under argon protection, heat it to 90 °C at a certain heating rate (0.5 °C / min), and anneal at this temperature for 2 hours. The purpose of annealing is to decompose the organic matter in the sol and make ZnO form a crystalline structure to improve its performance.

[0086] Example 3 This example provides a lithium metal negative electrode with a titanium oxide protective film (100 nm). The difference between this example and Example 1 is that in the S3 spin coating step, the high-speed rotation parameter is set to 2500 rpm, and other parameters remain unchanged. Finally, the oxide protective film is 100 nm.

[0087] Example 4 This embodiment provides a lithium metal anode with a titanium oxide protective film (30 nm). The difference between this embodiment and Embodiment 1 is that in the S3 spin-coating step, the high-speed rotation parameter is set to 5000 rpm, and other parameters remain unchanged. Finally, the oxide protective film is 30 nm.

[0088] Comparative Example 1 This comparative example provides a lithium metal without an oxide protective film; the steps are as follows: Prepare a lithium metal anode: Commercial lithium copper composite tape (Tianjin Zhongneng Lithium Industry Co., Ltd.); Comparative Example 2 This comparative example provides a lithium metal anode with a titanium oxide protective film (20 nm). The difference between this embodiment and Embodiment 1 is that in the S3 spin-coating step, the high-speed rotation parameter is set to 8000 rpm, and other parameters remain unchanged. Finally, the oxide protective film is 20 nm.

[0089] Comparative Example 3 This comparative example provides a lithium metal anode with a titanium oxide protective film (110 nm). The difference between this embodiment and Embodiment 1 is that in the S3 spin-coating step, the high-speed rotation parameter is set to 2000 rpm, and other parameters remain unchanged. Finally, the oxide protective film is 110 nm.

[0090] Comparative Example 4 This comparative example provides a lithium metal anode with a physically deposited titanium oxide protective film (50 nm). The preparation method of the titanium oxide protective film is as follows: S1. Target preparation: Select a target: Select a titanium dioxide target (Jiangxi Ketai New Materials Co., Ltd.).

[0091] Purity: Select a high-purity titanium dioxide ceramic target with a purity of 99.99% or higher to reduce impurity introduction.

[0092] Size: The diameter is 2 inches.

[0093] Installation: Firmly install the titanium dioxide ceramic target on the target gun of the magnetron sputtering equipment.

[0094] S2. Lithium foil pretreatment: Operating environment: Conduct the operation in a glove box filled with argon to avoid the reaction of lithium with oxygen and water vapor in the air.

[0095] Surface treatment: Gently scrape the surface of the lithium foil with a scraper to remove the surface oxide layer, passivation layer, and impurities. The scraping force should be moderate to avoid scratching the lithium foil body. After scraping, the surface of the lithium foil should show a metallic luster.

[0096] S3. Deposition (magnetron sputtering): Equipment preparation: Start the magnetron sputtering equipment to ensure that the equipment is in good operating condition.

[0097] Substrate Fixation: Fix the pre-treated lithium foil on the sample stage of the magnetron sputtering equipment. Ensure that the lithium foil is firmly and flatly fixed on the sample stage and as close to the target as possible.

[0098] Vacuum Pumping: Close the reaction vacuum chamber in the sputtering magnetron sputtering system, start the vacuum pump, and pump the reaction vacuum chamber to high vacuum. The background vacuum degree is better than 5 × 10 -4 Pa.

[0099] Argon Introduction: After the vacuum degree is stable, introduce high-purity argon (Ar) as the sputtering gas. Adjust the argon flow rate so that the working gas pressure in the reaction vacuum chamber is maintained within the range of 0.5 - 1 Pa.

[0100] Sputtering Parameter Setting: Sputtering Mode: Usually, the direct current sputtering mode is used to sputter the titanium dioxide ceramic target.

[0101] Sputtering Power: Set the sputtering power to 100W.

[0102] Deposition Time: Control the sputtering deposition time to about 1 minute and 40 seconds to obtain a titanium oxide film with a thickness of 50nm. Substrate Temperature: Do not perform intentional heating and adopt warm deposition. Room temperature deposition is a common process condition for preparing titanium oxide films by magnetron sputtering, avoiding adverse reactions of the lithium metal negative electrode at higher temperatures. Working Distance (Target-Substrate Distance): Adjust the distance between the target and the substrate to 5cm.

[0103] Start Sputtering: Turn on the sputtering power supply to start magnetron sputtering deposition. During the sputtering process, monitor parameters such as sputtering current, voltage, and working gas pressure in real time to ensure the stability of the sputtering process.

[0104] End of Sputtering: After reaching the preset deposition time, turn off the sputtering power supply to stop sputtering. Stop introducing argon and slowly release the vacuum in the reaction vacuum chamber to atmospheric pressure.

[0105] Take out the Sample: Carefully take out the lithium metal negative electrode loaded with the titanium oxide protective film prepared by magnetron sputtering.

[0106] Comparative Example 5 This comparative example provides a lithium metal negative electrode with a titanium oxide protective film (50nm). The difference from Example 1 is that annealing treatment is not carried out.

[0107] Performance Test Example Assemble the products obtained in the above examples and comparative examples: In an inert gas glove box with a water and oxygen content ≤ 0.01ppm, Li 6 PS 5Cl is used as the electrolyte layer, the positive electrode is a pole piece with a NCM811 loading of 10 mg·cm per unit area, and the lithium metal with an oxide protection layer is the negative electrode layer. A all-solid-state lithium metal battery is assembled, and the structure is as -2 shown. Finally, without external pressure, cyclic discharge is carried out. First, charge and discharge at 0.1C for two cycles, and then conduct long-term cyclic tests at 0.5C. Test its initial efficiency and the capacity retention rate after 500 cycles. The results are shown in Table 1 below: Figure 4 Table 1

[0108] It can be seen from Table 1 that when using lithium metal with a 50-nm oxide protection layer, higher initial efficiency and capacity retention can be achieved, improving the performance of the all-solid-state lithium metal battery life. Figure 1 The excellent cyclic ability and capacity retention rate of Example 1 can be seen; Figure 2 Its excellent Coulomb efficiency can be seen. The comparison of the cyclic performance of the all-solid-state lithium metal batteries of Example 1 and Comparative Example 1 is as Figure 3 shown.

[0109] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which do not affect the essence of the present invention.​

Claims

1. An oxide protective layer for a lithium metal negative electrode, characterized in that: Prepared by a method comprising the following steps: S1. Preparation of sol: Dissolving a metal alkoxide or a metal salt in a non-protonic polar solvent, adding a stabilizer to obtain a sol precursor, then dropping a catalyst, stirring evenly, and forming a sol; S2. Preparation of oxide protective layer: The prepared sol is coated on the surface of the lithium negative electrode, and then annealed after drying to obtain an oxide protective layer loaded on the surface of the lithium metal negative electrode.

2. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S1, the metal alkoxide includes one of tetrabutyl titanate, tri-sec-butyl aluminate, and tetraethyl orthosilicate; and / or, the metal salt comprises one or more of metal nitrates and metal carboxylates; and / or, the aprotic polar solvent comprises one or more of N-methylpyrrolidone, acetonitrile, dimethylformamide, and anhydrous ether; and / or, the stabilizer is one of acetylacetone and diethanolamine; And / or, the catalyst is one of sodium hydroxide, ammonia water, and deionized water.

3. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S1, and / or, the volume ratio of the metal alkoxide to the aprotic polar solvent is 1:5-50; and / or, the metal salt and the aprotic polar solvent are used in a ratio of 1 g: 10-50 ml; And / or, the volume ratio of metal alkoxide, stabilizer and catalyst is 4-6: 0.8-1.2：0.8-1.2； And / or, the volume ratio of metal salt to stabilizer is 2-2.4:

2.

4. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S2, the coating method is one of spin coating, dip coating, spray coating, and scraper coating.

5. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S2, the drying temperature is 30-60°C and the drying time is 6-20 h; And / or, the annealing temperature is 80-120° C., and the annealing time is 1-10 hours.

6. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S2, the thickness of the obtained oxide protective layer is 30-100 nm.

7. The oxide protective layer for lithium metal negative electrode according to claim 1, characterized in that: In step S2, the oxide in the obtained oxide protection layer has a nanocrystalline structure.

8. A composite lithium metal negative electrode, characterized in that It comprises a lithium metal negative electrode and the oxide protective layer as claimed in claim 1.

9. Use of the composite lithium metal negative electrode as claimed in claim 8 in an all-solid-state lithium alloy secondary battery.

10. An all-solid-state lithium metal secondary battery, characterized in that: It comprises a solid electrolyte, a positive electrode and the composite lithium metal negative electrode as claimed in claim 8.

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