Ultrathin lithium foil and method of making the same

Abstract

Provided are an ultrathin lithium foil product and a method for preparing the same. The ultrathin lithium foil is a continuous, self-supporting, ribbon-shaped foil with a lithium content of 99.90-99.99%. The thickness of the ultrathin lithium foil is 10-20 um, and the width is 10-500 mm. The surface of the ultrathin lithium foil is bright and silver-white in color; the lithium foil surface is flat and has no visible holes or damage; and the edges of the lithium foil are neat and have no defects.

Description

Technical Field

[0001] The present invention relates to the field of energy storage technology, and in particular to an ultra-thin lithium foil material that can be used for secondary batteries and a preparation method thereof. Background Art

[0002] Lithium batteries, due to their high energy density, long cycle life, and wide operating temperature range, are widely used in aerospace, computers, mobile communications, robotics, and electric vehicles. With the development of society and advancements in technology, the demand for lithium battery energy density is increasing. However, current lithium-ion batteries using graphite as the negative electrode are unable to meet this expectation, necessitating the development of new positive and negative electrode materials with higher specific capacities. Among negative electrode materials, lithium metal offers a high specific capacity (3860 mAh / g, 10 times that of graphite anodes) and the lowest redox potential (-3.04 V vs. standard hydrogen potential). Replacing existing graphite anodes with metallic lithium can increase the battery's operating voltage while reducing the amount of negative electrode required, significantly improving the energy density of lithium-ion batteries. Furthermore, since the negative electrode contains metallic lithium, serving as the battery's lithium source, a wider range of positive electrode materials is available, enabling the development of higher-energy electrochemical energy systems, such as lithium-sulfur batteries and lithium-air batteries. Currently, internationally, countries are striving to achieve an energy density of 500 Wh / kg or higher for lithium batteries.

[0003] Although lithium metal has such advantages, its usage in the battery, including the thickness of the lithium metal negative electrode, must be precisely controlled. Because the positive electrode materials currently used in existing lithium-ion batteries are all lithium-containing materials (such as lithium cobalt oxide, lithium iron phosphate, ternary materials, etc.), the lithium contained in the positive electrode is already able to meet the charging and discharging requirements of the lithium-ion battery, and the negative electrode lithium foil only needs to provide a very small amount of lithium to make up for the lithium loss during the cycle. In addition, metallic lithium itself also has very good conductivity and can be used as a current collector for the negative electrode. Moreover, the density of metallic lithium is 1 / 16 of the density of metallic copper. Directly using ultra-thin lithium metal foil with precisely controlled thickness can greatly increase the energy density of the battery. For example, researchers have reported that when the positive electrode uses a lithium nickel manganese cobalt oxide system and the negative electrode uses a lithium metal composite with a double-sided thickness of 100 microns on an 8-micron copper foil current collector, the battery energy just reaches 300wh / kg (Joule 3, 1–12, March 20, 2019). As mentioned earlier, the positive electrode contains lithium ions. If the negative electrode can be thinner, for example 20 microns, and does not contain copper foil, the battery energy density can be as high as 500wh / kg.

[0004] The ultimate tensile strength of lithium metal is 11.8 kg / mm 2Ultra-thin lithium foil production is prone to breakage. Lithium metal easily adheres to the roll due to its strong self-welding properties. Current technology makes it difficult to produce uniform, defect-free, rolled lithium foil 20 microns thick. Therefore, developing a roll of ultra-thin lithium foil with a thickness of less than 20 microns is crucial to achieving high energy density in batteries. Summary of the Invention

[0005] The present invention aims to provide an ultra-thin lithium foil material with a thickness of less than 20 microns that can be used in practice, and an industrial production method for preparing the ultra-thin lithium foil material.

[0006] In one aspect, the present invention provides an ultrathin lithium foil, characterized in that the lithium foil is a self-supporting continuous strip foil with a lithium content of 99.90-99.99%, a uniform thickness of less than 20 μm, and a thickness tolerance within ±1.5 μm.

[0007] Optionally, the thickness of the lithium foil is in the range of 10 to 20 μm; the width is 10 to 500 mm; and the length of the lithium foil exceeds 0.1 m.

[0008] Optionally, the surface of the lithium foil is smooth, without visually observable holes, damages and wrinkles.

[0009] Optionally, the surface of the lithium foil is bright and metallic silver-white; the surface lithium content is 99.90% to 99.95%.

[0010] Optionally, the lithium foil has neat edges and no crack defects.

[0011] Optionally, the lithium foil is a rolled strip.

[0012] In another aspect, the present invention provides a method for preparing the above-mentioned ultra-thin lithium foil, which is a roll-to-roll production method. A metallic lithium strip with a thickness of 100 to 300 μm is used as a raw material, and the ultra-thin lithium foil is obtained by multiple rolling, with the rolling compression ratio of each rolling being controlled to be 2 to 15.

[0013] Optionally, rolling is performed using rollers, and a laser thickness gauge is provided at the rolling exit to monitor the thickness change of the lithium foil in real time, and the roller gap spacing is adjusted according to the detection data.

[0014] Optionally, the surface of the roller has an anti-sticking material, and the anti-sticking material includes: polyethylene, polyoxymethylene, silicone polymer, and ceramic.

[0015] Optionally, a roller with a maximum tension range of 0.1 to 10 N is used for winding, wherein the supporting roller is itself powered.

[0016] By selecting extremely high-purity lithium metal strip, applying an anti-sticking material to the roller surface, and further precisely controlling the gap between the rollers and the tension during winding, the present invention solves the problem of lithium metal strips easily sticking to the rollers, breaking, wrinkling, or damaging. Through multiple rolling processes, an ultra-thin (less than 20 μm) lithium foil with a uniform thickness (with a thickness tolerance within ±1.5 μm) is obtained. The ultra-thin lithium foil of the present invention can be produced using a roll-to-roll process, making it suitable for industrial continuous production. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The schematic diagram of the process equipment for producing ultra-thin lithium foil of the present invention is shown.

[0018] Figure 2 The 20-micron-thick lithium foil product prepared in Example 1 of the present application is shown.

[0019] Figure 3 The 20-micron-thick rolled lithium foil product prepared in Example 1 of the present application is shown.

[0020] Figure 4 The 16-micron-thick lithium foil product prepared in Example 2 of the present application is shown.

[0021] Figure 5 The 16-micron thick rolled lithium foil product prepared in Example 2 of the present application is shown.

[0022] Figure 6 A 20 μm thick lithium foil prepared in Comparative Example 1 is shown.

[0023] Figure 7 A plan view of a 20-μm-thick lithium foil prepared in Comparative Example 1 is shown.

[0024] Figure 8 A 50 micron thick lithium foil prepared in Comparative Example 2 is shown.

[0025] Figure 9 An 80 micron thick lithium foil prepared in Comparative Example 3 is shown. DETAILED DESCRIPTION

[0026] In order to make the objectives, technical solutions, and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0027] According to certain embodiments, the present invention provides an ultrathin lithium foil material. The ultrathin lithium foil material is a continuous, self-supporting strip material that can be prepared and wound using a roll-to-roll process without the need for a support, such as a metal (copper) foil or a plastic carrier. When in use, the ultrathin lithium foil material can be directly used by unwinding. Of course, the ultrathin lithium foil material can also be laminated with a protective film before final winding to protect the ultrathin lithium foil material during storage and transportation.

[0028] The ultra-thin lithium foil has a thickness of less than 20 μm and is uniform with a thickness tolerance within ±1.5 μm, preferably within ±1.0 μm. The thickness of the lithium foil ranges from 10 to 20 microns, for example, 20 μm, 18 μm, 16 μm, 12 μm, or 10 μm.

[0029] The width of the ultra-thin lithium foil can be 10 to 500 mm, such as 50 mm, 100 mm, 200 mm, etc. The width tolerance can be within ±0.4 mm. The length of the lithium foil can exceed 0.1 m, such as 0.1-1000 m.

[0030] The surface of the metal lithium foil is smooth, without visible holes, damage or wrinkles; the surface is bright and metallic silver-white. The edges of the lithium foil are neat and free of cracks.

[0031] The lithium content of the ultra-thin lithium foil is 99.90-99.99%. The inventors found that when the lithium content of the lithium strip is below 99.9%, the lithium strip is prone to breakage and wrinkling during the preparation process of the ultra-thin lithium foil, especially during rolling. When the lithium content is increased to an extremely high purity of more than 99.90%, even after multiple rolling, the surface of the lithium foil rarely shows breakage and wrinkling. Although the mechanism is still not very clear, it is speculated that it may be because the impurities contained in the lithium strip cause local uneven force, which is prone to breakage and wrinkling. During rolling, since a considerable force is applied to the lithium strip, if there is uneven force, breakage and wrinkling are more likely to occur. It should be pointed out that the thinner the thickness of the lithium foil, the more likely it is to breakage and wrinkling. At this time, the lithium content of the ultra-thin lithium foil should be controlled. The content of other elements in the lithium strip besides lithium can be determined by inductively coupled plasma (ICP) technology.

[0032] Typically, lithium foil has a passivation film on its surface, resulting in a lower surface lithium content than the bulk of the foil. However, the ultra-thin lithium foil of the present invention can achieve a surface lithium content of 99.90% to 99.95% (this surface lithium content can be measured using X-ray photoelectron spectroscopy, with a thickness of a few nanometers), which is comparable to the lithium content of the bulk of the foil. This is because the passivation film on the surface of the ultra-thin lithium foil of the present invention is very thin.

[0033] According to other embodiments, the present invention provides a method for preparing the aforementioned ultra-thin lithium foil. The method is a roll-to-roll production process, wherein a raw metal lithium ribbon is unwound, rolled, and then rewound to obtain a thinned metal lithium foil. The rolling process is repeated several times, for example 2-10 times, preferably 3-4 times, to ultimately obtain an ultra-thin lithium foil having a thickness of less than 20 microns.

[0034] Lithium metal ribbon with a thickness of 100 to 300 microns (with a thickness tolerance of ±4 μm) can be used as raw material. After unwinding, it is rolled using a rolling mill. The rolling mill's rollers are coated with an anti-stick coating, which effectively prevents the lithium ribbon from sticking to the rollers, allowing the lithium ribbon to be rolled thinner. The anti-stick coating can be made of one or more materials selected from polyethylene, polyoxymethylene, silicone polymers, and ceramics. Polyethylene, polyoxymethylene, and silicone polymers are preferred because these organic coatings have a certain degree of flexibility, which helps reduce or eliminate damage or wrinkling on the surface of the rolled lithium ribbon that may be caused by uneven force. The thickness of the anti-stick coating can range from 0.005 mm to 2 mm. A thickness measuring device is installed at the mill outlet to measure the thickness of the rolled lithium ribbon online. If the thickness deviation exceeds ±1 μm of the set value, the mill can fine-tune the roller gap based on the measured thickness to ensure a uniform thickness of the lithium ribbon at the mill outlet. The thickness measuring device can be a laser thickness gauge, an X-ray thickness gauge, or the like. After rolling, the lithium ribbon is reeled up using a winding device comprising at least a support roller, a tension control roller, and a reeling roller. The maximum tension of the roller ranges from 0.1 to 10N. The support roller is preferably powered, which can pull the lithium ribbon forward with a small pulling force. The tension control roller can move up and down or swing, controlling both the tension of the lithium ribbon and the winding speed based on the height or swing angle of the tension control roller. Repeating the rolling process, for example 2-5 times, with a rolling reduction ratio of 2 to 15 (preferably 2 to 6) each time, can produce a uniform lithium foil of any thickness of 10 microns or more using this process.

[0035] Figure 1 The schematic diagram of the process equipment for producing ultra-thin lithium foil of the present invention is shown. Figure 1As shown, a metallic lithium strip is used as a raw material and is unwound through an unwinding device 10, wherein the unwinding device 10 comprises at least an unwinding roller 11 and an unwinding support roller 12; the raw lithium strip enters a rolling mill 20 after passing through the unwinding roller 11 and the unwinding support roller 12; the rolling mill 20 comprises at least a rolling roller 21 and an anti-sticking coating 22 on the rolling roller 21, and the rolling pressure of the rolling mill 20 and the roller gap between the rolling rollers 21 can be fine-tuned; the material of the anti-sticking coating 22 on the rolling roller 21 can be selected from one or more of polyethylene, polyoxymethylene, silicone polymer, ceramic, etc.; a thickness measuring device 30 is provided on the outlet side of the rolling mill 20, which can measure the thickness of the lithium strip after rolling. Online thickness measurement is carried out, and the rolling mill 20 can fine-tune the roll gap according to the measured thickness, so as to ensure that the thickness of the lithium strip at the outlet side of the rolling mill is uniform. The thickness measuring device 30 can adopt a laser thickness gauge, an X-ray thickness gauge, etc.; after rolling, the lithium strip is wound by a winding device 40, and the winding device 40 comprises at least a support roller 41, a tension control roller 42 and a winding roller 43; wherein the support roller 41 is powered and can use a small pulling force to pull the lithium strip forward; the tension control roller 42 can move up and down or swing, which can control the tension of the lithium strip and can be used to control the winding speed of the winding roller 43 according to the height or swing angle of the tension control roller 42.

[0036] The present invention is further described below through examples using the aforementioned process equipment. The various product structural parameters, various reaction participants, and process conditions used in the following examples are typical examples. However, through extensive experiments by the inventors, it has been verified that other structural parameters, other types of reaction participants, and other process conditions listed above are also applicable and can achieve the technical effects claimed in the present invention.

[0037] Example 1

[0038] A lithium metal strip with a lithium content of 99.95% and a thickness of 160 microns is used, and an auxiliary unwinding and winding device is used to thin it through an organic silicon compound coated roller. In the first rolling, the rolling compression ratio is controlled to be 2, and a lithium metal strip with a thickness of 80 microns is obtained; after the second rolling, the rolling compression ratio is controlled to be 2, and a lithium metal strip with a thickness of 40 microns is prepared; after the third rolling, the rolling compression ratio is controlled to be 2, and a 20-micron ultra-thin lithium foil with a thickness tolerance of ±1 micron is obtained. The prepared 20-micron thick lithium foil product and the rolled 20-micron thick lithium foil product are as follows: Figure 2 、 3 shown.

[0039] Example 2

[0040] A lithium metal strip with a lithium content of 99.93% and a thickness of 130 microns is used, and an auxiliary unwinding and winding device is used to thin it through a polyethylene-coated roller. The first rolling is controlled at a rolling compression ratio of 2 to obtain a lithium strip with a thickness of 65 microns; the second rolling is controlled at a rolling compression ratio of 2 to obtain a lithium metal strip with a thickness of 32 microns; and the third rolling is controlled at a rolling compression ratio of 2 to obtain an ultra-thin lithium foil with a thickness of 16 microns, with a thickness tolerance of ±1 micron. The prepared 16-micron thick lithium foil product and the rolled 16-micron thick lithium foil product are as follows: Figure 4 、 5 shown.

[0041] Comparative Example 1

[0042] A lithium metal strip with a lithium content of 99.95% and a thickness of 160 microns was used, and an auxiliary unwinding and winding device was used to thin it through a roller coated with an organic silicon compound. After a single rolling process, the rolling compression ratio was controlled to be 8. Due to the large deformation in a single rolling process, the ultra-thin lithium foil had cracks on the edges, and it was impossible to prepare a 20-micron thick metal lithium foil in a roll. Figure 6 As shown; a section of lithium foil with a thickness of 20 microns was cut and laid flat, and there were irregular edges and cracks; the surface of the lithium foil had wrinkles, and the plane view was as shown Figure 7 shown.

[0043] Comparative Example 2

[0044] A metal lithium strip with a lithium content of 98.30% and a thickness of 100 microns is used, and an auxiliary unwinding and winding device is used to thin it through a roller coated with an organic silicon compound. The first rolling is performed with a rolling compression ratio of 2 to obtain a lithium foil with a thickness of 50 microns. The 50-micron-thick lithium foil has cracks and wrinkles and cannot be rolled again. It is impossible to obtain an ultra-thin lithium foil with a thickness of 20 microns or less in a roll. A 50-micron-thick lithium foil is obtained by rolling once. Figure 8 shown.

[0045] Comparative Example 3

[0046] A lithium metal strip with a lithium content of 99.95% and a thickness of 160 microns was used, and an auxiliary unwinding and winding device was used to thin the strip through a roller without an anti-stick coating. The first rolling was carried out with a controlled rolling compression ratio of 2 to obtain a lithium strip with a thickness of 80 microns. The 80-micron thick lithium foil had cracks and holes on the surface and could not be rolled again. It was impossible to obtain a roll of ultra-thin lithium foil with a thickness of 20 microns or less. Figure 9 shown.

[0047] It should be understood 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 scope of protection of the present invention.

Claims

1. A method for preparing an ultrathin lithium foil, characterized in that: The method is a roll-to-roll production method, which uses a metallic lithium strip with a thickness of 100-300 μm and a lithium content of 99.90-99.99% as a raw material, and uses a roller with an anti-sticking material on the surface for rolling. The ultra-thin lithium foil is obtained by multiple rolling processes with a compression ratio of 2-6 each time. The prepared ultra-thin lithium foil is a self-supporting continuous strip foil with a uniform thickness of less than 20 μm, a thickness tolerance within ±1.5 μm, a width of 10 to 500 mm, a smooth surface without visually observable holes, damage, or wrinkles, and a neat edge without crack defects. The lithium foil has a bright surface and is metallic silver-white in color; the surface lithium content is 99.90% to 99.95%.

2. The method according to claim 1, wherein: A laser thickness gauge is set at the rolling exit to monitor the thickness changes of the lithium foil in real time, and the roller gap spacing is adjusted according to the detection data.

3. The method according to claim 1, wherein: The anti-sticking material is polyethylene, polyoxymethylene or silicone polymer.

4. The method according to claim 1, wherein: Rollers with a maximum tension range of 0.1~10N are used for winding, and the support rollers themselves are powered.

5. An ultra-thin lithium foil, characterized in that: The lithium foil is obtained by the method according to any one of claims 1 to 4, and is a self-supporting continuous strip foil having a uniform thickness of less than 20 μm, a thickness tolerance within ±1.5 μm, and a width of 10 to 500 mm. The surface of the lithium foil is flat, without visually observable holes, damage, or wrinkles, and the edges of the lithium foil are neat without crack defects. The surface of the lithium foil is bright and metallic silver-white; and the surface lithium content is 99.90% to 99.95%.

6. The ultra-thin lithium foil according to claim 5, characterized in that The thickness of the lithium foil is in the range of 10-20 μm; the length of the lithium foil exceeds 0.1 m.

7. The ultra-thin lithium foil according to claim 5, characterized in that The width of the lithium foil is 100-500 mm.

8. The ultra-thin lithium foil according to claim 5, characterized in that The lithium foil is in the form of a coiled strip.

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