Ultra-thin composite foil and its preparation method and application

Abstract

The present invention provides an ultrathin composite foil and its preparation method and application, belonging to the field of new energy materials technology. The preparation method comprises the following steps: in an argon environment, red phosphorus powder is evenly applied to the surface of a first metal lithium foil, and then covered with a second metal lithium foil to form a Li|P|Li composite foil; wherein the red phosphorus powder accounts for 8-20wt% of the total mass of the reactants, and the total mass of the reactants is the sum of the masses of the red phosphorus powder, the first metal lithium foil, and the second metal lithium foil; the Li|P|Li composite foil is mechanically rolled and then folded in half, and the mechanical rolling and folding are repeated until a Li / Li3P composite foil with a thickness of 9 to 30μm is obtained. The present invention can produce an ultrathin lithium metal composite foil with a thickness of less than 30μm, which is mainly used in the preparation of lithium metal batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy materials, and in particular relates to an ultra-thin composite foil and a preparation method and application thereof. Background Art

[0002] The rapid development of electric vehicles has made lithium metal batteries a potential choice for next-generation energy storage devices. The key to the successful application of lithium metal batteries lies in the lithium metal anode. Due to the poor mechanical properties of lithium metal and the high cost of thin lithium foil, research on lithium metal anodes has mainly been based on thicker lithium foil, focusing on suppressing lithium dendrites and improving coulombic efficiency. Currently, research on lithium dendrite and coulombic efficiency performance has reached maturity.

[0003] Researchers have found that if we want to further improve other properties of lithium metal batteries, such as energy density, it is imperative to develop thinner lithium foils with higher application value. In existing technologies, a variety of technologies are used to reduce the thickness of lithium foil. For example, thermal evaporation or electrodeposition technology, but this method has harsh preparation conditions and is difficult to mass produce. For another example, using graphene oxide film to absorb molten metallic lithium, this method can prepare large-area ultra-thin graphite oxide films, which is technically difficult. For another example, silver powder and metallic lithium are melted and mixed, and then mechanically rolled after cooling, but silver powder is expensive and hot melting consumes a lot of energy. Summary of the Invention

[0004] The present invention provides an ultra-thin composite foil and a preparation method and application thereof, which can prepare an ultra-thin lithium-based composite foil with a thickness of less than 30 μm.

[0005] The present invention provides a method for preparing an ultra-thin composite foil, comprising the following steps:

[0006] In an argon environment, red phosphorus powder is evenly coated on the surface of a first metal lithium foil, and then covered with a second metal lithium foil to form a Li|P|Li composite foil; wherein the red phosphorus powder accounts for 8-20wt% of the total mass of the reactants, and the total mass of the reactants is the sum of the masses of the red phosphorus powder, the first metal lithium foil, and the second metal lithium foil;

[0007] The Li|P|Li composite foil is mechanically rolled and then folded in half, and the mechanical rolling and folding are repeated until a Li3P / Li (LLP) composite foil with a thickness of 9 to 30 μm is obtained.

[0008] Furthermore, the first metal lithium foil and the second metal lithium foil have the same shape.

[0009] Furthermore, the red phosphorus powder accounts for 20 wt% of the total mass of the reactants.

[0010] Furthermore, the red phosphorus powder is particles with a size below micron level.

[0011] Furthermore, mechanical rolling and folding were repeated until an LLP composite foil with a thickness of 15 μm was obtained.

[0012] The present invention also proposes the use of the ultra-thin composite foil prepared by any of the above-mentioned preparation methods in high-energy-density lithium metal batteries.

[0013] Furthermore, the positive electrode of the high-energy-density lithium metal battery is lithium transition metal oxide or sulfur.

[0014] The present invention has the following advantages:

[0015] The method for preparing an ultra-thin composite foil proposed in the present invention comprises the following steps: at room temperature, an appropriate amount of red phosphorus is coated between two layers of metallic lithium foil to form a composite foil having a Li|P|Li sandwich structure; a mechanical rolling method is used to cause the red phosphorus and metallic lithium to react at the interface; the generated Li3P nano-enhanced particles are uniformly introduced into the metallic lithium, thereby improving the machinability of the metallic lithium and preparing an ultra-thin lithium-metal composite foil having a thickness of less than 30 μm; and the introduced particles have high lithium ion conductivity and lithiophilicity, which can reduce the initial nucleation overpotential of lithium deposition and improve lithium ion diffusion kinetics, thereby improving the electrochemical performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The accompanying drawings, which constitute part of the present invention, are provided to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are provided to explain the present invention and do not constitute an undue limitation of the present invention. In the accompanying drawings:

[0017] Figure 1 The following are SEM images and electron photos of LLP composite foils of Examples 1 and 3 of the present invention;

[0018] Figure 2 This is the XRD pattern of the LLP composite foil of Example 1 of the present invention;

[0019] Figure 3 This is a SEM image of the remaining Li3P particles after the LLP composite foil is charged to completely remove lithium in Example 1 of the present invention;

[0020] Figure 4 This is an electronic photograph of the LLP composite foil of Example 5 of the present invention;

[0021] Figure 5 This is an electronic photograph of the composite foil obtained in Comparative Example 1 of the present invention;

[0022] Figure 6 This is an electronic photograph of the composite foil obtained in Comparative Example 2 of the present invention;

[0023] Figure 7 This is the time-voltage curve of the symmetrical battery assembled with the LLP composite foil electrode in Experimental Example 1 of the present invention;

[0024] Figure 8 This is the surface exchange current of the LLP composite foil of Test Example 1 of the present invention;

[0025] Figure 9 This is the graph of the experimental example 2 LiCoO2||LLP full battery of the present invention showing good electrochemical performance. DETAILED DESCRIPTION

[0026] It should be noted that, in the absence of conflict, the embodiments of the present invention and the features in the embodiments may be combined with each other.

[0027] Normally, the areal capacity of the positive electrode / negative electrode in a full battery should be less than 2 (preferably close to 1). The areal capacity of the commonly used positive electrode is generally about 3 mAh cm –2 The surface capacity of 5μm thick pure lithium foil is about 1mAh. Therefore, the thickness of lithium foil with high practical value should be less than 30μm.

[0028] Through research, the inventors of this application discovered that during the preparation of metal composite foil, the composite foil obtained by mixing lithium foil with tin foil contained large particles. This easily cracked when rolled to a thickness below 50 μm, making it difficult to form a complete ultra-thin composite foil. Furthermore, insufficient red phosphorus inclusion not only failed to effectively improve the machinability of the lithium metal, but also failed to improve the electrochemical performance.

[0029] An embodiment of the present invention provides a method for preparing an ultra-thin composite foil, comprising the following steps:

[0030] In an argon environment, red phosphorus powder is evenly coated on the surface of a first metal lithium foil, and then covered with a second metal lithium foil to form a Li|P|Li composite foil; wherein the red phosphorus powder accounts for 8-20wt% of the total mass of the reactants; and the total mass of the reactants is the sum of the masses of the red phosphorus powder, the first metal lithium foil, and the second metal lithium foil;

[0031] The Li|P|Li composite foil is mechanically rolled and then folded in half, and the mechanical rolling and folding are repeated until a Li3P / Li (LLP) composite foil with a thickness of 9 to 30 μm is obtained.

[0032] In this embodiment of the present invention, a composite foil is prepared based on lithium metal. An appropriate amount of red phosphorus is added. Leveraging its low hardness and chain-like structure, red phosphorus reacts with the lithium metal at the interface during rolling, generating nanometer-sized lithium phosphide (Li3P) particles. Rolling causes the Li3P to fall off the phosphorus powder surface and embed into the lithium metal bulk. Repeated rolling allows the red phosphorus to fully react and evenly disperse within the lithium metal, enhancing the machinability of the lithium metal. This results in the production of an ultra-thin lithium-metal composite foil, effectively improving electrochemical performance.

[0033] In the embodiment of the present invention, metallic lithium itself is relatively soft, has poor mechanical strength, and has a certain viscosity. However, the present invention coats a certain amount of red phosphorus between two layers of metallic lithium foil to form a composite foil with a Li|P|Li sandwich structure.

[0034] Preferably, the red phosphorus powder accounts for 8-20% by weight of the total mass of all reactants. More preferably, it accounts for 20% by weight. In the embodiments of the present invention, red phosphorus reacts with lithium to form Li3P, and the amount of phosphorus powder added directly determines the Li3P content in the composite foil. A relatively low amount of red phosphorus added (e.g., <5%) directly affects the Li3P particle content in the resulting composite foil. This makes it difficult to effectively improve the machinability of metallic lithium, preventing the production of an ultra-thin composite foil with a thickness of 30 μm. Furthermore, the improvement in electrochemical performance is relatively limited. Excessive red phosphorus will consume active lithium, thereby reducing the capacity of the composite foil.

[0035] More preferably, the red phosphorus powder is micron-sized particles.

[0036] In the embodiment of the present invention, repeated mechanical rolling and folding can obtain LLP composite foils with thicknesses of 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30 μm.

[0037] In a preferred embodiment of the present invention, the first metal lithium foil and the second metal lithium foil have the same shape, which is more conducive to rolling the mixed red phosphorus powder into the composite foil with a Li|P|Li sandwich structure.

[0038] An embodiment of the present invention further proposes the application of the ultra-thin composite foil in a high-energy lithium metal battery.

[0039] Furthermore, the positive electrode of the lithium metal battery is lithium transition metal oxide or sulfur.

[0040] The present invention will be described in detail below with reference to the embodiments and accompanying drawings.

[0041] Example 1 The method for preparing an ultra-thin composite foil comprises the following steps:

[0042] All of the above operations were performed in an argon glove box. A first metal lithium foil and a second metal lithium were cut into regular rectangular shapes. The ground micron-sized red phosphorus powder was evenly coated on the surface of the first metal lithium foil, with a lithium (total of the first and second metal lithium foils) to red phosphorus mass ratio of 80:20. The second metal lithium foil was then placed on top, covering the red phosphorus with lithium, forming a Li|P|Li composite foil.

[0043] The Li|P|Li composite foil is placed between a pair of rollers and rolled. The roller spacing is adjusted and the foil is rolled. The roller spacing is continuously reduced until the composite foil reaches a certain thickness, then it is folded in half. The folded red phosphorus-coated lithium foil is then rolled multiple times. This process is repeated until the red phosphorus is evenly distributed within the lithium metal matrix, resulting in a 15μm thick LLP composite electrode (composite foil).

[0044] To further investigate the morphology of the composite foil, the inventors characterized the composite foil using conventional X-ray diffraction (XRD), scanning electron microscopy (SEM), and photographs. The results are as follows.

[0045] Figure 1 (a) is the SEM image of LLP composite foil. Figure 1 (c) is an electronic photograph of the LLP composite foil.

[0046] Figure 2 This is the XRD pattern of LLP composite foil.

[0047] Figure 3 SEM image of the remaining Li3P particles after the LLP composite foil is charged to 0.5V to completely extract lithium.

[0048] As shown in the figure above, the composite foil's phase composition is Li and Li₃P. The red phosphorus is completely reacted with lithium to form Li₃P, while trace amounts of Li₃PO₄ originate from the oxide layer on the red phosphorus surface. The resulting sample is stable, with its phase composition remaining unchanged after a week.

[0049] Example 2 The method for preparing an ultra-thin composite foil comprises the following steps:

[0050] The same as Example 1, except that an LLP composite electrode with a thickness of 30 μm is obtained.

[0051] Example 3 The method for preparing an ultra-thin composite foil comprises the following steps:

[0052] The same as Example 1, except that an LLP composite electrode with a thickness of 9 μm is obtained.

[0053] The obtained composite foil was characterized by conventional scanning electron microscopy (SEM), and the results are as follows.

[0054] Figure 1 (b) is the SEM image of the LLP composite foil.

[0055] Example 4 The method for preparing an ultra-thin composite foil comprises the following steps:

[0056] The same as Example 1, except that the mass ratio of lithium to red phosphorus is 90:10, until an LLP composite electrode with a thickness of 30 μm is obtained.

[0057] Example 5 The method for preparing an ultra-thin composite foil comprises the following steps:

[0058] The same method as Example 1 was used, except that the mass ratio of lithium to red phosphorus was 92:8, until a LLP composite electrode with a thickness of 30 μm was obtained. The obtained composite foil was characterized using conventional electron photography, and the results are as follows.

[0059] Figure 4 This is an electron photo of a 30μm LLP composite foil prepared with 8% red phosphorus. When the composite foil was rolled to 30μm, obvious wrinkles and cracks appeared on the edges, indicating that adding 8% red phosphorus can produce a composite foil with a thickness of 30μm, but the effect is relatively unsatisfactory.

[0060] Comparative Example 1 The preparation method of the composite foil comprises the following steps:

[0061] The same as Example 1, except that tin foil was used instead of red phosphorus, wherein the mass ratio of lithium foil to tin foil was 80:20. The obtained composite foil was characterized by conventional electron photography, and the results are as follows.

[0062] Figure 5 This is an electronic photograph of the composite foil of Comparative Example 1. Since the formed lithium tin particles are relatively large, they break when the rolling thickness reaches 50 μm, making it difficult to form a complete ultra-thin composite foil.

[0063] Comparative Example 2 The preparation method of the composite foil comprises the following steps:

[0064] The same as Example 1, except that the mass ratio of lithium to red phosphorus in the ground micron-sized red phosphorus powder is 95:5.

[0065] Figure 6 This is an electron photo of a composite sheet rolled to 30μm with 5% red phosphorus. As can be seen, even with a low red phosphorus addition of 5%, an ultra-thin composite sheet cannot be formed. The nanoparticles in the composite sheet must reach a certain volume fraction to effectively improve the machinability of the metal substrate through the diffusion enhancement effect.

[0066] Test Example 1 Electrochemical performance test

[0067] 10% and 20% red phosphorus were added to prepare LLP composite foil electrodes according to Example 1 and Example 4, respectively, to assemble into symmetrical batteries. The time-voltage curve performance of the obtained symmetrical batteries was then tested, and the results are as follows.

[0068] Figure 7 The time-voltage curves of symmetrical cells assembled with LLP composite foil electrodes prepared by adding 10% and 20% red phosphorus to pure lithium foil. The horizontal axis is time and the vertical axis is voltage.

[0069] Depend on Figure 7 It can be seen that with the increase of phosphorus content, the initial nucleation overpotential and overvoltage of the composite foil decrease, indicating that Li3P can effectively reduce the nucleation potential of lithium deposition, increase the lithium ion diffusion rate of the electrode, and thus improve the electrochemical performance of the battery.

[0070] Figure 8 The surface exchange current of pure lithium and 20% LLP composite foil obtained based on linear sweep voltammetry test, where the abscissa represents voltage and the ordinate represents current.

[0071] Depend on Figure 8 It can be seen that the higher surface exchange current of LLP indicates that the presence of Li3P helps to improve the lithium ion diffusion kinetics on the electrode surface.

[0072] Test Example 2 Safety battery performance test

[0073] The 15 μm composite foil obtained in Example 1 was used to assemble a battery with a LiCoO2 positive electrode and perform relevant electrochemical performance tests. Figure 9 As shown, the horizontal axis is the number of cycles and the vertical axis is the capacity. Figure 9 It can be seen that the LiCoO2||LLP full battery exhibits good electrochemical performance.

[0074] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing an ultra-thin composite foil, characterized in that: The steps include: In an argon environment, red phosphorus powder is evenly coated on the surface of a first metal lithium foil, and then covered with a second metal lithium foil to form a Li|P|Li composite foil; wherein the red phosphorus powder accounts for 8-20wt% of the total mass of the reactants, and the total mass of the reactants is the sum of the masses of the red phosphorus powder, the first metal lithium foil, and the second metal lithium foil; The Li|P|Li composite foil is mechanically rolled and then folded in half, and the mechanical rolling and folding are repeated until a Li / Li3P composite foil with a thickness of 9 to 30 μm is obtained.

2. The preparation method according to claim 1, characterized in that The first metal lithium foil and the second metal lithium foil have the same shape.

3. The preparation method according to claim 1, characterized in that The red phosphorus powder accounts for 20 wt% of the total mass of the reactants.

4. The preparation method according to claim 1, characterized in that The red phosphorus powder is particles with a size below micron level.

5. The preparation method according to claim 1, characterized in that The mechanical rolling and folding were repeated until an LLP composite foil with a thickness of 15 μm was obtained.

6. Use of the ultra-thin composite foil prepared by the preparation method according to any one of claims 1 to 5 in high-energy-density lithium metal batteries.

7. The use according to claim 6, characterized in that The positive electrode of the high-energy-density lithium metal battery is a lithium transition metal oxide or sulfur.

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