Preparation method of ultrathin lithium foil containing surface protection layer, product thereof and use method of ultrathin lithium foil

Abstract

The invention belongs to the field of ultra-thin lithium foils, and particularly discloses a preparation method of an ultra-thin lithium foil containing a surface protection layer, a product of the ultra-thin lithium foil and a use method of the ultra-thin lithium foil containing the surface protection layer. The surface protection layer comprises a chemical inert layer growing into the skin of the ultrathin lithium foil substrate and a physical protection layer covering the surface of the chemical inert layer, and the chemical inert layer grows into a part of the ultrathin lithium foil and is not separated from the ultrathin lithium foil substrate from beginning to end after being formed; the chemical inert layer is obtained by reacting one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate and polydimethylsiloxane with the surface of the freshly prepared ultrathin lithium foil substrate. The ultrathin lithium foil can be directly used as a negative electrode of a lithium ion battery, a diaphragm, an electrolyte and a positive electrode are added to form the battery, and the stability of the ultrathin lithium foil in air can be improved by the method.

Description

Technical Field

[0001] The present invention belongs to the field of ultra-thin lithium foil, and more specifically, relates to a preparation method of an ultra-thin lithium foil containing a surface protective layer, a product thereof and a use method thereof. Background Art

[0002] With the popularization of smart devices and new energy vehicles and the construction of large-scale energy storage facilities, improving the energy density of lithium-ion batteries and reducing their costs are urgent needs of the current market. Metallic lithium is regarded as the most important negative electrode material in the future due to its low redox potential (–3.04V vs. standard hydrogen electrode) and high theoretical specific capacity (3860 mAh g-1, more than ten times that of existing graphite negative electrodes).

[0003] When ultra-thin lithium foil is used as a lithium negative electrode, it can achieve the matching of positive and negative electrode surface capacity and improve the energy density of lithium metal batteries. The patent application with publication number CN 114583312 A discloses a processing and recycling method and product of ultra-thin lithium foil. The invention uses a liquid phase thinning agent to contact and react with lithium metal to prepare an ultra-thin lithium foil with a thickness of less than 20 μm, a flat surface, and an adjustable thickness with good uniformity. It does not disclose how such ultra-thin lithium is protected on the surface.

[0004] As a typical alkali metal, metallic lithium has extremely active chemical properties. In the actual preparation process, it has strict environmental and safety requirements during processing and storage. The chemically active metallic lithium will react with a variety of substances in the air to form a surface passivation layer, resulting in a large loss of active substances, and destroying the morphology of the lithium metal surface, thereby affecting the subsequent electrochemical performance of the battery, greatly limiting the practical application of lithium metal anodes. On the other hand, the lithium anode is prone to form irregular lithium dendrites during the deposition process. The irreversible reaction of the lithium anode with the organic electrolyte will also cause irreversible capacity loss, causing the cycle performance to decline rapidly. The generated lithium dendrites are easy to fall off to form dead lithium, which not only reduces the coulomb efficiency of the battery but also aggravates the occurrence of side reactions. In addition, the formed lithium dendrites can easily pierce the diaphragm and cause internal short circuits, and even cause safety accidents such as fire or explosion.

[0005] In view of the above problems, it is very important to protect the surface of ultra-thin lithium foil, among which, constructing a surface protection layer is considered to be the most effective solution. The ideal surface protection layer needs to have good chemical and electrochemical stability, be able to protect the metal lithium foil from direct contact with the air, and also need to reduce the side reactions between the metal lithium and the electrolyte, ensure the rapid migration of lithium ions and the appropriate lithium deposition site, so as to ensure that it can effectively protect the metal lithium surface during long cycles.

[0006] However, at present, the construction technology of ultra-thin lithium metal foil surface protection layer still has shortcomings and challenges. This is mainly because lithium metal has extremely high chemical reactivity and easily reacts with most atmospheres or reagents, releasing a large amount of heat or explosive gases such as hydrogen, which brings great insecurity and complexity to the operation and storage of lithium metal. In addition, many material systems can be used as lithium negative electrode surface protection materials according to their own composition structure, but due to the incompatibility of the construction atmosphere and reagents with lithium, they cannot be applied to existing surface protection technologies.

[0007] Therefore, it is necessary to develop a surface protective layer with excellent chemical and electrochemical stability, which can protect the surface of ultra-thin lithium foil and improve the stability of ultra-thin lithium foil in the air. This is of great significance for applying it to the negative electrode of high energy density batteries, so that it has excellent electrochemical performance and safety and meets the needs of practical applications. Summary of the invention

[0008] In view of the defects of the prior art, the purpose of the present invention is to provide a preparation method of an ultra-thin lithium foil containing a surface protective layer, a product thereof and a use method thereof, in order to prevent the problem that the effective thickness of the ultra-thin lithium foil is further reduced after being oxidized.

[0009] To achieve the above-mentioned object, the first aspect of the present invention provides an ultra-thin lithium foil with a surface protective layer, which comprises an ultra-thin lithium foil substrate and a surface protective layer, wherein the surface protective layer comprises a chemically inert layer grown as the skin of the ultra-thin lithium foil substrate, wherein the chemically inert layer grows as a part of the ultra-thin lithium foil and is not separated from the ultra-thin lithium foil substrate from the beginning to the end after its formation. The chemically inert layer is obtained by reacting one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, polydimethylsiloxane, polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene with the surface of a freshly prepared ultrathin lithium foil substrate.

[0010] In the above invention concept, the chemically inert layer grows on the surface of the ultra-thin lithium foil substrate, similar to skin, and is dense itself, and gas molecules cannot penetrate. The chemically inert layer grows continuously and tightly on the surface of the ultra-thin lithium foil substrate without any gaps, thereby preventing any possible oxidizing gas molecules from penetrating the chemically inert layer and reacting with the lithium foil.

[0011] Furthermore, the chemically inert layer has a lithium product in its composition, including Li-X, wherein X is selected from lithium fluoride, lithium oxide, lithium peroxide, lithium nitride, lithium carbonate, lithium phosphate, ethylene dicarbonate dilithium, alkyl lithium carbonate and other lithium-containing inorganic or organic substances, and the chemically inert layer is used as a SEI film and has Li-conducting inertness. In fact, the raw materials for preparing the chemically inert layer can react with fresh ultra-thin lithium to form an integrated inner protective layer, which is inseparable, to obtain the lithium product Li-X, which is combined with fresh Li to form a skin layer without any gaps, continuous and tight, and grown on the surface of Li, which can achieve chemical protection.

[0012] Furthermore, the thickness of the chemically inert layer is 0.1μm~10μm. The chemically inert layer should not be too thick, as it will increase the internal resistance of the battery and reduce the transmission rate of lithium ions. In the process of assembling the battery, the protection time provided by the chemically inert layer does not need to be too long, about half an hour is enough, and half an hour can fully complete the assembly of the battery. When the ultra-thin lithium foil is assembled as part of the battery, there is no need to consider whether it is oxidized, because after being assembled, it is naturally isolated from the air. The chemically inert layer does not need to be too thick, and is mainly used to protect the ultra-thin lithium foil from possible oxidation caused by exposure during use. It only requires a short period of protection, rather than a lifelong anti-oxidation.

[0013] Furthermore, the thickness of the ultra-thin lithium foil substrate does not exceed 30μm, the surface is flat and smooth, the surface impurity element content is less than 5%, and the roughness is less than 1μm. In actual engineering practice, the thickness of the chemically inert layer itself is relatively thin, and the raw materials for preparing the chemically inert layer are relatively small. The surface of the ultra-thin lithium foil substrate is required to be absolutely fresh and not oxidized, and there are few surface impurity elements to achieve the generation of Li-X lithiation products, and to ensure that the chemically inert layer grows densely and continuously on the surface of the ultra-thin lithium foil substrate like skin, with tight bonding and no gaps. If the surface roughness of the ultra-thin lithium foil substrate is large, greater than 1μm, and the impurity element content is high, the generated chemically inert layer is easily damaged or not dense, or not connected, and it is difficult to be tightly bonded to the substrate, and gas penetration may occur.

[0014] Furthermore, it also includes a physical protective layer covering the surface of the chemically inert layer, and the physical protective layer is a film made of one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, polyoxymethylene, polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane, and its thickness is less than 200μm. The physical protective layer serves as an outer layer and mainly provides physical protection. The physical protective layer can be thick or thin. When the physical protective layer is thick, the physical protective layer film can be torn off first, and then the ultra-thin lithium foil can be used as the lithium negative electrode to assemble the lithium battery. When the physical protective layer is thin, it can be torn off, or it can be prepared with a material that can be dissolved by the electrolyte, and the physical protective layer is retained without removal until the battery is assembled and the electrolyte dissolves it.

[0015] The physical protection layer is continuously and densely attached to the chemically inert layer, which can not only play a physical protection role, but also play a role in isolating the oxidizing atmosphere, and has at least a dual role. According to the actual engineering needs and the storage time requirements of the ultra-thin lithium foil, it can be prepared thicker or thinner. If it is prepared thicker, the thicker physical protection layer is matched with the appropriate chemically inert layer, so that the ultra-thin lithium foil can be stored for a long time without being oxidized, for example, it can be stored for up to three years. Conversely, if it is prepared thinner, the thinner physical protection layer is matched with the thinner chemically inert layer, so that the ultra-thin lithium foil can be stored for a short time without being oxidized, for example, it can be stored for half a year or a year. In short, the simultaneous preparation of a chemically inert layer and a physical protection layer is a relatively flexible, efficient and low-cost surface protection method.

[0016] According to a second aspect of the present invention, there is also provided a method for preparing an ultra-thin lithium foil as described above, wherein an ultra-thin lithium foil substrate is prepared under a protective atmosphere, and a chemically inert layer is prepared on one or both sides of the ultra-thin lithium foil substrate within 30 minutes of the ultra-thin lithium foil substrate being prepared, and the chemically inert layer is prepared by a solution coating method, a melt cooling film forming method, or a finished polymer film co-rolling method.

[0017] In the above invention concept, a chemically inert layer with good insulation performance must be prepared on the surface of the freshly prepared ultra-thin lithium foil before it is oxidized. In actual engineering practice, after the preparation of the fresh lithium foil is completed, the chemically inert layer and the physical protective layer are immediately prepared on the surface, and the interval between the two processes is preferably less than 30 minutes, so that the surface of the freshly prepared ultra-thin lithium foil can be protected before it is oxidized.

[0018] The solution coating method is a simple and low-cost method for preparing thin films, which is widely used in the fields of solar cells, sensors, displays, etc. This method can prepare high-quality thin film materials by accurately controlling the material preparation, solution preparation, coating operation and heat treatment. Furthermore, in the solution coating method, the solution includes a polymer and an organic solvent, the polymer is dissolved in the organic solvent, the polymer is selected from one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane, and the organic solvent is selected from one or more of dimethylformamide, N-methyl-2-pyrrolidone, dimethylacetamide, tetramethylurea, dimethyl sulfoxide, triethyl phosphate, ethylene carbonate, propylene carbonate, acetonitrile, toluene, xylene, dichloromethane, butyl acetate, ethyl acetate, n-hexane, and cyclohexane. The ratio of polymer to organic solvent is generally 5wt% ~ 30wt%, and the concentration after the ratio is convenient for coating, and there is no special requirement.

[0019] The melt cooling film forming method is a method for preparing a film through a melting and cooling process. Specifically, this method includes melting and plasticizing the plastic raw material, extruding it into a sheet film blank through a die head, and then stretching it to the desired size using a casting roller, and cooling and shaping it through a cooling roller to form a film. In the melt cooling film forming method, the molten material is selected from one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene.

[0020] The finished polymer film co-rolling method is a method of stacking different materials together through roll forming technology. First, the molten polymer material is transported to the preforming mold through the outlet mold for preliminary forming. The preforming mold preforms the molten polymer material, and then cools and shapes it through a cooling device. The polymer product after cooling and shaping enters the traction device, and the traction device sends it to the roll forming machine. The upper and lower forming rollers roll and press to roll the hot polymer product. This method is suitable for batch production, can efficiently stack different materials together, and is suitable for the preparation of polymer films that require a multi-layer structure. In addition, the roll forming method is a solid-state processing method that deforms and fuses particles through high pressure, and is suitable for materials such as ultra-high molecular weight polyethylene. In the finished polymer film co-rolling method, the polymer film is selected from one or more of polypropylene film, polyethylene film, polyvinyl chloride film, polyethylene terephthalate film, polyoxymethylene film, polyethylene oxide film, polyvinylidene fluoride film, polymethyl methacrylate film, and polydimethylsiloxane film.

[0021] According to the second aspect of the present invention, the application of the ultra-thin lithium foil with a surface protective layer as described above, when being assembled to form a battery and in the subsequent battery operation process, the chemically inert layer remains on the surface of the ultra-thin lithium foil substrate until it is dissolved by the electrolyte. When being assembled to form a battery and in the subsequent battery operation process, the physical protective layer is torn off in advance or retained until it is dissolved by the electrolyte. In actual engineering practice, the chemically inert layer as the inner protective layer is retained, and the physical protective layer as the outer protective layer can be torn off, and the chemically inert layer or the physical protective layer can protect the lithium foil on one side or both sides. In the present invention, the chemically inert layer or the physical protective layer are both thin films with good air tightness and extremely low air permeability. The ultra-thin lithium foil with a surface protective layer of the present invention can be directly used as the negative electrode of a lithium-ion battery, and a separator, an electrolyte and a positive electrode are added to form a battery.

[0022] In general, the method and product of the present invention have extremely high engineering application value. Compared with the prior art, the above technical solution conceived by the present invention mainly has the following advantages: (1) In the present invention, a dense and complete polymer surface protection layer is coated on the surface of the ultra-thin metal lithium foil to protect the metal lithium foil from direct contact with the air, thereby preventing the fresh metal lithium from reacting with the air environment and preventing the formation of an oxidized impurity layer on the surface of the metal lithium foil, thereby improving the stability of the ultra-thin lithium foil in the air.

[0023] (2) In the present invention, the raw material of the chemically inert layer can react with fresh ultra-thin lithium to form an integrated inner protective layer, which is inseparable, and the lithium product Li-X is obtained. Li-X is combined with fresh Li to form a skin layer without any gaps, which is continuous and tight. In fact, the chemically inert layer forms a natural SEI film (Solid Electrolyte Interface, full name solid electrolyte interface film), which is used as an SEI film and has Li conductive inertness and can achieve chemical protection. The polymer surface protective layer has good chemical and electrochemical stability, can reduce the side reactions between metal lithium and electrolyte, ensure the rapid migration of lithium ions and suitable lithium deposition sites, so as to ensure that it can effectively protect the metal lithium surface during long cycles. The ultra-thin lithium foil with a surface protective layer can be directly used as the negative electrode of a lithium ion battery. When used in a high energy density battery, it has excellent electrochemical performance and safety. The ultra-thin lithium foil with a surface protective layer of the present invention has the effect of killing two birds with one stone due to the polymer protective film set on the surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 is a schematic diagram of an ultra-thin lithium foil material with a surface protection layer provided by one embodiment of the present invention; Figure 2 Schematic diagram of an ultra-thin lithium-copper composite foil with a surface protective layer provided by one embodiment of the present invention; Figure 3 Schematic diagram of an ultra-thin lithium foil material with double-sided surface protection layers according to an embodiment of the present invention.

[0025] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein: 1 is a chemically inert layer, 2 is a physical protective layer, 3 is an ultra-thin lithium-copper composite foil, 4 is a metallic lithium layer, 5 is a metallic copper base layer, and 6 is an ultra-thin lithium foil substrate. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0027] In the description of the present invention, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of the present invention, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0028] In addition, throughout this specification, "one embodiment", "one example" or similar language indicates that a particular feature, structure or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present invention. Therefore, the phrase "in one embodiment" and similar language may, but do not necessarily, refer to the same embodiment.

[0029] Figure 1 It is a schematic diagram of an ultrathin lithium foil with a surface protective layer provided by an embodiment of the present invention. As can be seen from the figure, the present invention provides an ultrathin lithium foil with a surface protective layer, which includes an ultrathin lithium foil substrate 6 and a surface protective layer, the surface protective layer includes a chemically inert layer 1 (the chemically inert layer is an inner protective layer) grown into the skin of the ultrathin lithium foil substrate and a physical protective layer 2 (the physical protective layer is an outer protective layer) covering the surface of the chemically inert layer. The chemically inert layer grows to become a part of the ultrathin lithium foil, and is not separated from the ultrathin lithium foil substrate from the beginning to the end after its formation, similar to skin, and is dense in itself, gas molecules cannot penetrate, and the chemically inert layer grows continuously and tightly on the surface of the ultrathin lithium foil substrate without any gaps, thereby preventing any possible oxidizing gas molecules from penetrating the chemically inert layer and reacting with the lithium foil.

[0030] In the present invention, the inner protective layer in the surface protective layer provides chemical protection, and the inner protective layer is selected from one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane. The specific reason is that when the above materials are used as inner materials to protect the metal lithium foil, they have rich polar functional groups as organic matter, which can maintain the electrochemical stability of the interface between the electrode and the electrolyte and achieve uniform lithium deposition. The outer protective layer provides physical protection, and the outer protective layer is selected from one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene. The specific reason is that when the above materials are used as outer protective layer materials to protect the metal lithium foil, they can isolate the influence of air on the lithium foil, and the above organic protective layer has excellent mechanical properties, which can avoid the phenomenon of wrinkles and the like in the lithium foil during the preparation process.

[0031] In one embodiment of the present invention, the chemically inert layer contains a lithium product, including Li-X, where X is selected from lithium fluoride, lithium oxide, lithium peroxide, lithium nitride, lithium carbonate, lithium phosphate, ethylene dicarbonate dilithium, alkyl lithium carbonate and other lithium-containing inorganic or organic substances. The raw materials for preparing the chemically inert layer can react with fresh ultra-thin lithium to form an integrated inner protective layer that cannot be separated to obtain a lithium product Li-X. Li-X is combined with fresh Li to form a skin layer without any gaps, continuous and tight, and grown on the surface of Li to achieve chemical protection.

[0032] In one embodiment of the present invention, the ultra-thin lithium foil is fresh and bright, with a flat surface, a roughness of less than 1 μm, no crack defects, and surface impurity elements less than 5%. The metal lithium foil with a flat and smooth surface can ensure that it fits tightly with the inner protective layer. The metal lithium foil has no crack defects on the surface and low impurity element content, which can help ensure that the lithium foil has good surface morphology and chemical purity when used as an electrode, thereby improving the cycle performance and service life of the electrode and improving the electrochemical performance of the electrode.

[0033] In one embodiment of the present invention, the surface protection layer is in close contact with the ultra-thin lithium foil without any gaps. The specific reason is that the close fit between the surface protection layer and the ultra-thin lithium foil can ensure that the protection layer isolates the external air and the metal lithium foil, thereby better playing a protective role. The organic protection layer has excellent mechanical properties, so it can avoid the lithium foil from wrinkling during the preparation process.

[0034] The above-mentioned surface protection layer can be prepared by solution coating method, melt cooling film forming method, and finished polymer film co-rolling method. The specific reason is that the surface layer prepared by the method can fit tightly with the ultra-thin lithium foil material and play a surface protection role. Specifically, the solution coating method is to coat the surface of the metal lithium foil with a polymer solution, and after drying, a metal lithium foil with a surface protection layer is obtained, wherein the coating method includes but is not limited to scraping, spraying, spin coating or sputtering. The melt cooling film forming method is to cast a molten polymer material onto the surface of the metal lithium foil, and form a surface protection layer after cooling. The finished polymer film co-rolling method is to obtain an ultra-thin lithium foil with a surface protection layer by putting the metal lithium foil and the polymer film into a pressure roller together and then rolling them.

[0035] In one embodiment of the present invention, in the solution coating method, the solution includes a polymer and an organic solvent, the polymer is dissolved in the organic solvent, and the polymer is selected from one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane. The specific reason is that the organic material can be closely attached to the metal lithium foil after film formation, and its dense structure ensures the protection of the metal lithium foil. In the organic material, the inner protective layer can have rich organic polar functional groups, which can interact with the ions in the electrolyte when the metal lithium foil is used as the negative electrode of the battery. Taking polyvinylidene fluoride as an example, its highly polar CF group has a certain antioxidant ability, and can regulate the distribution of lithium ions at the interface of the electrode and the electrolyte, prevent the growth of lithium dendrites, and play a role in chemical and electrochemical protection.

[0036] The organic solvent is selected from one or more of dimethylformamide, N-methyl-2-pyrrolidone, dimethylacetamide, tetramethylurea, dimethyl sulfoxide, triethyl phosphate, ethylene carbonate, propylene carbonate, acetonitrile, toluene, xylene, dichloromethane, butyl acetate, ethyl acetate, n-hexane, and cyclohexane; in the melt cooling film forming method, the molten material is selected from one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene. The specific reason is that the organic material has a low melting point, has the characteristics of self-leveling, and can be well spread on the surface of the metal lithium foil to form a dense protective layer.

[0037] In the finished polymer film co-rolling method, the polymer film is selected from one or more of polypropylene film, polyethylene film, polyvinyl chloride film, polyethylene terephthalate film, polyoxymethylene film, polyethylene oxide film, polyvinylidene fluoride film, polymethyl methacrylate film, and polydimethylsiloxane film. The specific reason is that the organic material has good film-forming properties and mechanical properties, and can be laminated with the metal lithium foil by rolling without gaps.

[0038] In one embodiment of the present invention, a product of an ultra-thin lithium foil material containing a surface protective layer and its integrated preparation process, the ultra-thin lithium foil material has a thickness of less than 30 μm, the thickness of the outer protective layer is 0.1 μm ~ 200 μm, the thickness of the inner protective layer is 0.1 μm ~ 10 μm, the inner protective layer is always retained, the outer protective layer can be torn off, and the outer protective layer can also be made into a form that can be dissolved without being torn off, mainly due to different material selection. The specific reason is that the inner protective layer has a certain viscosity, fits tightly with the metal lithium foil, and can be used as an artificial solid electrolyte membrane to improve the lithium deposition and dissolution behavior. Therefore, in actual application, the inner protective layer can be used together with the metal lithium foil as a negative electrode material, and the outer protective layer has good mechanical properties and film-forming properties, which prevents the metal lithium foil from directly contacting the air, and can be torn off when used. On the other hand, the protective layer can protect the lithium foil on one side or both sides. The specific reason is that for pure metal lithium foil, since both sides of it are in contact with the air, the lithium foil can be protected on both sides during surface protection; for lithium composite foil, such as lithium-copper composite foil, one side is pure lithium and the other side is copper, so when protecting it, only the pure lithium side can be protected on the surface. Figure 2 This is a schematic diagram of an ultra-thin lithium-copper composite strip foil with a surface protection layer provided by an embodiment of the present invention. As can be seen from the figure, 1 is a chemically inert layer, 2 is a physical protection layer, 3 is an ultra-thin lithium-copper composite strip foil, 4 is a metallic lithium layer, and 5 is a metallic copper base layer. The metallic lithium layer 4 in the ultra-thin lithium-copper composite strip foil 3 is coated on the metallic copper base layer 5, the chemically inert layer 1 is tightly attached to the metallic lithium layer 4, and the physical protection layer 2 is arranged on the chemically inert layer 1.

[0039] Figure 3 It is a schematic diagram of an ultra-thin lithium foil material containing a double-sided surface protective layer according to an embodiment of the present invention. As can be seen from the figure, both sides of the ultra-thin lithium foil substrate 6 have protective layers, and the protective layer structure on each side is similar, both of which include a chemically inert layer 1 bonded to the ultra-thin lithium foil substrate 6 and a physical protective layer 2 bonded to the chemically inert layer 1.

[0040] In the process of preparing the surface protection layer, the previous process is the preparation of ultra-thin metal lithium foil. Therefore, after the preparation of fresh lithium foil is completed, the protective layer is immediately covered on its surface, and the interval between the two processes is preferably less than 30 minutes. The specific reason is that after the preparation of ultra-thin metal lithium foil, due to the extremely active chemical properties of metal lithium, the fresh metal lithium foil is very easy to react with the air environment, and then an oxidized impurity layer is generated on the surface. Therefore, after the preparation of fresh lithium foil is completed, the protective layer needs to be immediately covered on its surface. The interval between the two processes is preferably less than 30 minutes, so as to ensure that no passivation layer is formed on the surface of the metal lithium foil and to maximize the surface protection effect.

[0041] The ultra-thin lithium foil material with a surface protective layer as described above can be directly used as the negative electrode of a lithium-ion battery, and a separator, an electrolyte and a positive electrode are added to form a battery for the next application.

[0042] It is worth noting that the method of the present invention is particularly suitable for ultra-thin lithium foil. For ultra-thick lithium, the surface protection method of the present invention may not be used, because, for ultra-thick lithium, after its surface is oxidized, it will further lose a certain thickness on the basis of the original "ultra-thin" thickness, which may cause the ultra-thin lithium foil to be unusable. Therefore, the ultra-thin lithium foil "cannot withstand" the thickness lost by oxidation. When the ultra-thin lithium foil is continuously produced, after the fresh lithium foil is prepared, one or more dense and complete polymer surface protection layers are immediately covered on the surface of the ultra-thin lithium foil. The surface protection layer prepared by the present invention can improve the stability of the ultra-thin lithium foil in the air, can meet the needs of actual applications, and has broad application prospects.

[0043] In order to explain the method of the present invention in more detail, it is further explained in detail below in conjunction with specific embodiments.

[0044] Example 1 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 1 μm is completed, polyvinylidene fluoride is dispersed in N-methyl-2-pyrrolidone to prepare a polymer solution, and the polymer solution is sprayed onto the surface of the fresh ultra-thin lithium foil by a solution coating method. The thickness of the protective layer is adjusted to 0.1 μm, and after drying, an ultra-thin lithium foil containing a chemically inert layer of polyvinylidene fluoride is obtained.

[0045] Example 2 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 2 μm is completed, polyethylene oxide is dispersed in dimethyl sulfoxide to prepare a polymer solution, and the polymer solution is spin-coated onto the surface of the ultra-thin lithium foil using a solution coating method. The thickness of the protective layer is adjusted to 0.2 μm, and after drying, an ultra-thin lithium foil containing a chemically inert layer of polyethylene oxide is obtained.

[0046] Example 3 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 3 μm is completed, polymethyl methacrylate is dispersed in dimethylformamide to prepare a polymer solution, and the polymer solution is scraped onto the surface of the ultra-thin lithium foil by a solution coating method. The thickness of the protective layer is adjusted to 0.3 μm, and after drying, an ultra-thin lithium foil containing a chemically inert layer of polymethyl methacrylate is obtained.

[0047] Example 4 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 4 μm is completed, polydimethylsiloxane is dispersed in toluene to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by a solution coating method. The thickness of the protective layer is adjusted to 0.4 μm, and after drying, an ultra-thin lithium foil containing a chemically inert layer of polydimethylsiloxane is obtained.

[0048] Example 5 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 5 μm is completed, polypropylene is heated to a molten state, and the molten polypropylene is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method, the thickness of the protective layer is adjusted to 5 μm, and after drying, an ultra-thin lithium foil containing a polypropylene surface protective layer is obtained.

[0049] Example 6 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 6 μm is completed, polyethylene is heated into a molten state, and the molten polyethylene is cast onto the surface of the ultra-thin lithium foil using a melt cooling film-forming method. The thickness of the protective layer is adjusted to 10 μm, and after drying, an ultra-thin lithium foil containing a polyethylene surface protective layer is obtained.

[0050] Example 7 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 7 μm is completed, polyvinyl chloride is heated into a molten state, and the molten polyvinyl chloride is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method, and the thickness of the protective layer is adjusted to 20 μm. After drying, an ultra-thin lithium foil containing a polyvinyl chloride surface protective layer is obtained.

[0051] Example 8 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 8 μm, polyethylene oxide is first dispersed in acetonitrile to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by a solution coating method, and the thickness of the protective layer is adjusted to 0.5 μm. Secondly, polyethylene terephthalate is heated to a molten state, and the molten polyethylene terephthalate is cast onto the surface of the ultra-thin lithium foil by a melt cooling film forming method, and the thickness of the protective layer is adjusted to 10 μm. After drying, an ultra-thin lithium foil with an inner layer of polyethylene oxide protective layer and an outer layer of polyethylene terephthalate protective layer is obtained.

[0052] Example 9 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 9 μm, polyvinylidene fluoride is first dispersed in tetramethyl urea to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by solution coating method, and the thickness of the protective layer is adjusted to 1 μm. Secondly, polyethylene terephthalate is heated to a molten state, and the molten polyoxymethylene is cast onto the surface of the ultra-thin lithium foil by melt cooling film forming method, and the thickness of the protective layer is adjusted to 20 μm. After drying, an ultra-thin lithium foil with a chemically inert layer of polyvinylidene fluoride protective layer and a physical protective layer of polyoxymethylene protective layer is obtained.

[0053] Example 10 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 10 μm, polyvinylidene fluoride is first dispersed in triethyl phosphate to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by solution coating method, and the thickness of the protective layer is adjusted to 1 μm. Secondly, polypropylene is heated to a molten state, and the molten polypropylene is cast onto the surface of the ultra-thin lithium foil by melt cooling film forming method, and the thickness of the protective layer is adjusted to 30 μm. After drying, an ultra-thin lithium foil with an inner layer of polyvinylidene fluoride protective layer and an outer layer of polypropylene protective layer is obtained.

[0054] Embodiment 11 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 11 μm, polyvinylidene fluoride is first dispersed in ethylene carbonate to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by solution coating method, and the thickness of the protective layer is adjusted to 2 μm. Secondly, polyethylene is heated to a molten state, and the molten polyethylene is cast onto the surface of the ultra-thin lithium foil by melt cooling film forming method, and the thickness of the protective layer is adjusted to 50 μm. After drying, an ultra-thin lithium foil with an inner layer of polyvinylidene fluoride protective layer and an outer layer of polyethylene protective layer is obtained.

[0055] Example 12 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 12 μm, polyvinylidene fluoride is first dispersed in propylene carbonate to prepare a polymer solution, and the polymer solution is scraped onto the surface of the ultra-thin lithium foil by a solution coating method, and the thickness of the protective layer is adjusted to 3 μm. Secondly, the polyvinyl chloride is heated to a molten state, and the molten polyvinyl chloride is cast onto the surface of the ultra-thin lithium foil by a melt cooling film forming method, and the thickness of the protective layer is adjusted to 80 μm. After drying, an ultra-thin lithium foil with an inner layer of polyvinylidene fluoride protective layer and an outer layer of polyvinyl chloride protective layer is obtained.

[0056] Example 13 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 13 μm, polydimethylsiloxane is first dispersed in toluene to prepare a polymer solution, and the polymer solution is scraped onto the surface of the ultra-thin lithium foil using a solution coating method to adjust the thickness of the protective layer to 5 μm. Secondly, polypropylene is heated to a molten state, and the molten polypropylene is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method to adjust the thickness of the protective layer to 100 μm. After drying, an ultra-thin lithium foil with an inner layer of polydimethylsiloxane protective layer and an outer layer of polypropylene protective layer is obtained.

[0057] Embodiment 14 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 14 μm, polyethylene oxide is first dispersed in dichloromethane to prepare a polymer solution, and the polymer solution is scraped onto the surface of the ultra-thin lithium foil using a solution coating method to adjust the thickness of the protective layer to 5 μm. Secondly, polypropylene is heated to a molten state, and the molten polypropylene is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method to adjust the thickness of the protective layer to 120 μm. After drying, an ultra-thin lithium foil with a polyethylene oxide protective layer as the inner layer and a polypropylene protective layer as the outer layer is obtained.

[0058] Embodiment 15 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 15 μm, polyethylene oxide is first dispersed in butyl acetate to prepare a polymer solution, and the polymer solution is scraped onto the surface of the ultra-thin lithium foil using a solution coating method to adjust the thickness of the protective layer to 6 μm. Secondly, polypropylene is heated to a molten state, and the molten polypropylene is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method to adjust the thickness of the protective layer to 150 μm. After drying, an ultra-thin lithium foil with a polyethylene oxide protective layer as the inner layer and a polypropylene protective layer as the outer layer is obtained.

[0059] Example 16 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 16 μm, polyethylene oxide is first dispersed in ethyl acetate to prepare a polymer solution, and the polymer solution is sprayed onto the surface of the ultra-thin lithium foil by a solution coating method, and the thickness of the protective layer is adjusted to 8 μm. Secondly, polyvinyl chloride is heated to a molten state, and the molten polyvinyl chloride is cast onto the surface of the ultra-thin lithium foil by a melt cooling film forming method, and the thickness of the protective layer is adjusted to 200 μm. After drying, an ultra-thin lithium foil with a polyethylene oxide protective layer as the inner layer and a polyvinyl chloride protective layer as the outer layer is obtained.

[0060] Embodiment 17 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 17 μm, polyethylene oxide is first dispersed in n-hexane to prepare a polymer solution, and the polymer solution is sprayed onto the surface of the ultra-thin lithium foil by a solution coating method, and the thickness of the protective layer is adjusted to 8 μm. Secondly, polyvinyl chloride is heated to a molten state, and the molten polyvinyl chloride is cast onto the surface of the ultra-thin lithium foil by a melt cooling film forming method, and the thickness of the protective layer is adjusted to 20 μm. After drying, an ultra-thin lithium foil with a polyethylene oxide protective layer as the inner layer and a polyvinyl chloride protective layer as the outer layer is obtained.

[0061] Embodiment 18 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 18 μm, polyethylene oxide is first dispersed in cyclohexane to prepare a polymer solution, and the polymer solution is spin-coated onto the surface of the ultra-thin lithium foil using a solution coating method, and the thickness of the protective layer is adjusted to 10 μm. Secondly, polyvinyl chloride is heated to a molten state, and the molten polyvinyl chloride is cast onto the surface of the ultra-thin lithium foil using a melt cooling film forming method, and the thickness of the protective layer is adjusted to 10 μm. After drying, an ultra-thin lithium foil with a polyethylene oxide protective layer as the inner layer and a polyvinyl chloride protective layer as the outer layer is obtained.

[0062] Embodiment 19 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 19 μm, polymethyl methacrylate is first dispersed in dimethylformamide to prepare a polymer solution, and the polymer solution is spin-coated onto the surface of the ultra-thin lithium foil using a solution coating method to adjust the thickness of the protective layer to 1 μm. Secondly, a polyoxymethylene film with a thickness of 10 μm is co-rolled with an ultra-thin lithium foil with a polymethyl methacrylate protective layer to obtain an ultra-thin lithium foil with a polymethyl methacrylate protective layer as the inner layer and a polyoxymethylene protective layer as the outer layer.

[0063] Embodiment 20 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 20 μm, polymethyl methacrylate is first dispersed in dimethylformamide to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil by solution coating method to adjust the thickness of the protective layer to 0.1 μm. Secondly, a polypropylene film with a thickness of 10 μm is co-rolled with an ultra-thin lithium foil with a polymethyl methacrylate protective layer to obtain an ultra-thin lithium foil with a polymethyl methacrylate protective layer as the inner layer and a polypropylene protective layer as the outer layer.

[0064] Embodiment 21 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 1 μm, polydimethylsiloxane is first dispersed in toluene to prepare a polymer solution, and the polymer solution is sputtered onto the surface of the ultra-thin lithium foil using a solution coating method to adjust the thickness of the protective layer to 0.2 μm. Secondly, a polypropylene film with a thickness of 20 μm is co-rolled with an ultra-thin lithium foil with a polydimethylsiloxane protective layer to obtain an ultra-thin lithium foil with a polydimethylsiloxane protective layer as the inner layer and a polypropylene protective layer as the outer layer.

[0065] Embodiment 22 In the continuous production of ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 5 μm is completed, a polydimethylsiloxane film with a thickness of 0.5 μm is first rolled together with the ultra-thin lithium foil, and then a polypropylene film with a thickness of 10 μm is rolled together with the ultra-thin lithium foil with a polydimethylsiloxane protective layer to obtain an ultra-thin lithium foil with a polydimethylsiloxane protective layer as an inner layer and a polypropylene protective layer as an outer layer.

[0066] Embodiment 23 During the continuous production of ultra-thin lithium foil, after a fresh lithium foil with a thickness of 10 μm is prepared, a polydimethylsiloxane film with a thickness of 1 μm and a polypropylene film with a thickness of 30 μm are co-rolled with the ultra-thin lithium foil to obtain an ultra-thin lithium foil with a polydimethylsiloxane protective layer as an inner layer and a polypropylene protective layer as an outer layer.

[0067] Embodiment 24 When continuously producing ultra-thin lithium foil, after preparing fresh lithium foil with a thickness of 10 μm, a polydimethylsiloxane film with a thickness of 1 μm and a polypropylene film with a thickness of 30 μm are co-rolled with the ultra-thin lithium foil to obtain an ultra-thin lithium foil with an inner layer of polydimethylsiloxane protective layer and an outer layer of polypropylene protective layer. The same operation is performed on the other side of the lithium foil to perform double-sided protection on the lithium foil, so that both sides have a surface protective layer with an inner layer of polydimethylsiloxane and an outer layer of polypropylene.

[0068] Embodiment 25 When continuously producing ultra-thin lithium foil, after the preparation of fresh lithium foil with a thickness of 15 μm, first disperse polyvinylidene fluoride in ethylene carbonate to prepare a polymer solution, and use a solution coating method to sputter the polymer solution onto the surface of the ultra-thin lithium foil, and adjust the thickness of the protective layer to 2 μm. Secondly, heat the polyethylene to a molten state, and use a melt cooling film forming method to cast the molten polyethylene onto the surface of the ultra-thin lithium foil, and adjust the thickness of the protective layer to 50 μm. After drying, an ultra-thin lithium foil with an inner layer of polyvinylidene fluoride protective layer and an outer layer of polyethylene protective layer is obtained. The same operation is performed on the other side of the lithium foil to protect the lithium foil on both sides, so that both sides have a surface protective layer with an inner layer of polyvinylidene fluoride and an outer layer of polyethylene.

[0069] Embodiment 26 During the continuous production of ultra-thin lithium-copper composite strip foil, after the preparation of a fresh ultra-thin lithium-copper composite strip foil with a metal lithium layer thickness of 10 μm is completed, a polydimethylsiloxane film with a thickness of 1 μm and a polypropylene film with a thickness of 30 μm are co-rolled with the metal lithium side of the ultra-thin lithium-copper composite strip foil to obtain an ultra-thin lithium-copper composite strip foil with an inner layer of a polydimethylsiloxane protective layer and an outer layer of a polypropylene protective layer.

[0070] Embodiment 27 When continuously producing ultra-thin lithium-copper composite foil, after the preparation of fresh ultra-thin lithium-copper composite foil with a metal lithium layer thickness of 15 μm, polyvinylidene fluoride is first dispersed in ethylene carbonate to prepare a polymer solution, and the polymer solution is sputtered onto the metal lithium side of the ultra-thin lithium-copper composite foil by a solution coating method, and the thickness of the protective layer is adjusted to 2 μm. Secondly, the polyethylene is heated to a molten state, and the molten polyethylene is cast onto the metal lithium side of the ultra-thin lithium-copper composite foil by a melt cooling film forming method, and the thickness of the protective layer is adjusted to 50 μm. After drying, an ultra-thin lithium-copper composite foil with a polyvinylidene fluoride protective layer as the inner layer and a polyethylene protective layer as the outer layer is obtained.

[0071] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. An ultra-thin lithium foil material with a surface protective layer, characterized in that: The invention comprises an ultra-thin lithium foil substrate and a surface protective layer, wherein the surface protective layer comprises a chemically inert layer grown as the skin of the ultra-thin lithium foil substrate, wherein the chemically inert layer grows as a part of the ultra-thin lithium foil and is not separated from the ultra-thin lithium foil substrate from the beginning to the end after being formed. The chemically inert layer is obtained by reacting one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, polydimethylsiloxane, polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene with the surface of a freshly prepared ultrathin lithium foil substrate.

2. The ultra-thin lithium foil material according to claim 1, characterized in that: The chemically inert layer contains a lithium product, including a lithium-containing inorganic or organic Li-X, wherein X is selected from lithium fluoride, lithium oxide, lithium peroxide, lithium nitride, lithium carbonate, lithium phosphate, lithium ethylene dicarbonate, and alkyl lithium carbonate, and has Li-conducting inertness.

3. The ultra-thin lithium foil material according to claim 2, characterized in that: The thickness of the chemically inert layer is 0.1μm~10μm.

4. The ultra-thin lithium foil material according to claim 3, characterized in that: The thickness of the ultra-thin lithium foil matrix does not exceed 30μm, the surface is flat and smooth, the surface impurity element content is less than 5%, and the roughness is less than 1μm.

5. The ultra-thin lithium foil material according to claim 4, characterized in that: It also includes a physical protective layer covering the surface of the chemically inert layer, the physical protective layer is a film made of one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, polyoxymethylene, polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane, and its thickness is less than 200μm.

6. A method for preparing an ultra-thin lithium foil as claimed in any one of claims 1 to 5, characterized in that: An ultra-thin lithium foil substrate is prepared under a protective atmosphere, and a chemically inert layer is prepared on one or both sides of the ultra-thin lithium foil substrate within 30 minutes of the preparation of the ultra-thin lithium foil substrate. The chemically inert layer is prepared by a solution coating method, a melt cooling film forming method, and a finished polymer film co-rolling method.

7. The method according to claim 6, characterized in that In the solution coating method, the solution includes a polymer and an organic solvent, the polymer is dissolved in the organic solvent, and the polymer is selected from one or more of polyethylene oxide, polyvinylidene fluoride, polymethyl methacrylate, and polydimethylsiloxane. The organic solvent is selected from one or more of dimethylformamide, N-methyl-2-pyrrolidone, dimethylacetamide, tetramethylurea, dimethyl sulfoxide, triethyl phosphate, ethylene carbonate, propylene carbonate, acetonitrile, toluene, xylene, dichloromethane, butyl acetate, ethyl acetate, n-hexane, and cyclohexane.

8. The method according to claim 7, characterized in that In the melt cooling film forming method, the molten material is selected from one or more of polypropylene, polyethylene, polyvinyl chloride, polyethylene terephthalate, and polyoxymethylene. In the finished polymer film co-rolling method, the polymer film is selected from one or more of polypropylene film, polyethylene film, polyvinyl chloride film, polyethylene terephthalate film, polyoxymethylene film, polyethylene oxide film, polyvinylidene fluoride film, polymethyl methacrylate film, and polydimethylsiloxane film.

9. The method for using the ultra-thin lithium foil with a surface protection layer as claimed in any one of claims 1 to 5, characterized in that: During the assembly to form a battery and the subsequent battery operation process, the chemically inert layer remains on the surface of the ultra-thin lithium foil matrix until it is dissolved by the electrolyte.

10. The method of use according to claim 9, characterized in that: During the assembly of the battery and the subsequent battery operation process, the physical protective layer is torn off in advance or retained until it is dissolved by the electrolyte.

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