A lithium ribbon with high carbon content and a method of making the same

Abstract

This invention provides a high-carbon-content lithium strip and its preparation method. The high-carbon-content lithium strip comprises 20% to 80% by mass of a lithiophilic carbon material, an inner layer of a composite of the lithiophilic carbon material and metallic lithium, an outer layer of an oxidized lithium compound, and a gap layer supported by the lithiophilic carbon material between the inner and outer layers. The thickness of the gap layer between the inner and outer layers is 0.1 to 2 micrometers, and the thickness of the outer layer is 50 nm to 1 micrometer. This carbon-containing lithium strip outer layer (surface shell) can serve as an artificial solid electrolyte layer (SEI), effectively preventing oxidation of the carbon-containing lithium strip and avoiding the safety hazards associated with high-carbon-content lithium strips. The presence of the gap layer between the inner and outer layers provides space for lithium deposition, effectively inhibiting the formation and growth of dendrites on the lithium strip surface. Furthermore, the metallic lithium in the inner layer of the carbon-containing lithium strip can serve as a positive electrode capacity compensator, replenishing the active lithium lost from the positive electrode during cycling and extending the battery's cycle life.

Description

Technical Field

[0001] This invention belongs to the field of lithium metal strip processing, and particularly relates to a high-carbon-content lithium strip and its preparation method. Background Technology

[0002] Lithium metal, as the ultimate anode material, has always been a hot research topic in the field of anode materials. However, the inherent defects of lithium metal itself cause it to exhibit huge volume expansion when used as an anode. This characteristic leads to the fracture of lithium dendrites on the surface of lithium metal and the formation of a large number of "dead lithium". After the formation of dead lithium, the capacity and coulombic efficiency of lithium metal further decrease, and the surface impedance of lithium metal increases, which greatly shortens the cycle life of lithium metal anodes.

[0003] To address the issue of lithium metal volume expansion, carbon materials are generally used as host materials to mitigate this expansion. However, carbon materials have poor wettability with lithium metal, making it difficult to prepare lithium metal products with high carbon content. Some patents involve directly rolling carbon materials onto the surface of lithium metal strips to prepare carbon-containing lithium strips. However, the carbon material is only distributed on the surface of the lithium metal, offering little effect on mitigating the overall volume expansion of the lithium metal strip. Furthermore, after the lithium metal at the bottom of the carbon material is removed, structural collapse occurs on the carbon material surface, causing the solid electrolyte layer to rupture. Additionally, while processing carbon materials such as carbon cloth and then combining them with molten lithium to prepare carbon-containing lithium strips prevents structural collapse after lithium metal removal, when used as the negative electrode, positive electrode lithium metal directly deposits on its surface, offering negligible protection against lithium dendrite formation.

[0004] Although carbon materials, as host materials, can not only stabilize the structure but also provide space for lithium metal deposition, the low carbon content in lithium bands cannot achieve these effects.

[0005] In summary, it is indeed necessary to provide a high-carbon-content lithium strip with a stable structure that can effectively suppress the formation of lithium dendrites. Summary of the Invention

[0006] To address the above technical problems, the inventors provide a high-carbon-content lithium strip and its preparation method. The outer layer (surface shell) of the carbon-containing lithium strip can serve as an artificial solid electrolyte layer (SEI), effectively preventing the oxidation of the carbon-containing lithium strip and avoiding the safety hazards of high-carbon-content lithium strips. There is an interstitial layer between the inner and outer layers, which can reserve some space for lithium deposition and effectively inhibit the formation and growth of dendrites on the surface of the carbon-containing lithium strip. The metallic lithium in the inner layer can serve as a positive electrode capacity compensator, replenishing the active lithium lost by the positive electrode during cycling and extending the cycle life of the battery.

[0007] To achieve the above-mentioned objectives, one aspect of the present invention provides a high-carbon-content lithium strip comprising 20% ​​to 80% by mass of a lithium-loving carbon material, and said lithium strip having:

[0008] The inner layer is formed of a composite of the lithiophilic carbon material and metallic lithium;

[0009] The outer layer is a shell formed of lithium oxide compounds, with a thickness ranging from 50 nm to 1 micrometer; and

[0010] A gap layer supported by the lithiophilic carbon material is located between the inner and outer layers, and the gap layer thickness is 0.1 micrometers to 2 micrometers.

[0011] In the high-carbon lithium strip of this invention, the content of lithium-philic carbon material is 20-80%. When the carbon content is too low, there is less carbon material and the carbon skeleton plays a smaller role, so it does not significantly improve the performance of the pure lithium strip. When the carbon content is too high, there is less lithium content and the carbon material has a larger specific surface area, which requires a large amount of active lithium to form a solid electrolyte interface. If the lithium content in the lithium strip is too low, it is insufficient to compensate for the active lithium consumed in forming the solid electrolyte interface, which will consume the active lithium in the positive electrode, resulting in a low battery capacity.

[0012] In some embodiments, the compound of the oxidized lithium is at least one selected from lithium nitride, lithium carbonate, lithium oxide, lithium peroxide, and lithium halide.

[0013] In some embodiments, the lithiophilic carbon material includes at least one selected from graphite, carbon nanotubes, graphene, soft carbon, hard carbon, and mesophase carbon microspheres. Preferably, the lithiophilic carbon material is a nanomaterial. More preferably, the lithiophilic carbon material has undergone lithiophilic treatment.

[0014] In some implementations, the inner lithium metal is pure lithium metal or a lithium alloy.

[0015] In some embodiments, the interstitial layer has unfilled voids formed by the lithiophilic carbon material.

[0016] In some embodiments, the high-carbon-content lithium strip has a width of 0.5 cm to 100 cm and a thickness of 5 μm to 200 μm.

[0017] Another aspect of the present invention provides a method for preparing the above-mentioned high-carbon-content lithium strip, comprising the following steps:

[0018] Step 1: Mix the lithiophilic carbon material with molten lithium and stir.

[0019] Step 2: Use a container with micropores at the bottom to scoop up the lithium metal liquid containing the lithium-loving carbon material obtained in Step 1. The pore diameter of the micropores is 10 micrometers to 50 micrometers.

[0020] Step 3: Collect the residue in the container, cool it, and then extrude it into a strip under a mixed dry gas purging to obtain a carbon-containing lithium strip with a carbon mass content between 20% and 80%.

[0021] In the above methods, the pore diameter of the micropores in the container with micropores at the bottom ranges from 10 to 50 micrometers, which is mainly determined by the surface tension of the molten lithium (affected by the temperature of the molten lithium). At lower temperatures, the surface tension of molten lithium is higher, requiring a container with a larger pore size to filter the molten lithium, while at higher temperatures, the surface tension of molten lithium is lower, allowing the use of a container with a smaller pore size. The goal is to control the ratio of metallic lithium to carbon-containing materials to increase the carbon content or control the carbon content within a suitable range.

[0022] In some embodiments, the molten lithium temperature is 300°C to 700°C.

[0023] In some embodiments, the stirring time in step one is 10 min to 30 min.

[0024] In some embodiments, the container with micropores at the bottom is a colander.

[0025] In some embodiments, the mixing ratio of the lithiophilic carbon material and molten lithium in step one is 1:4 to 4:1 by mass.

[0026] In some embodiments, the water content of the mixed dry gas is less than 1000 ppm, and the mixed gas is a combination of at least two of nitrogen, oxygen, carbon dioxide, and halogen gases.

[0027] In some embodiments, the extrusion into strip is performed using a roller pressing method.

[0028] This invention has at least the following advantages:

[0029] 1. The surface shell of this carbon-containing lithium strip can serve as an artificial solid electrolyte layer (SEI), which can effectively prevent the oxidation of the carbon-containing lithium strip and avoid the safety hazards of high-carbon lithium strips.

[0030] 2. The presence of the interstitial layer between the inner and outer layers of the lithium strip can reserve some space for lithium deposition and effectively inhibit the formation and growth of dendrites on the surface of the carbon-containing lithium strip.

[0031] 3. The metallic lithium in the inner layer of the carbon-containing lithium strip can be used as a positive electrode capacity compensator to replenish the active lithium lost by the positive electrode during cycling and extend the cycle life of the battery.

[0032] 4. The inner layer of the lithium strip contains a high content of carbon material, so there is no problem of structural collapse after the metallic lithium is pulled out. Attached Figure Description

[0033] Figure 1 Photograph of the carbon-containing lithium strip prepared after rolling in Example 1.

[0034] Figure 2 The specific capacity test curve of the carbon-containing lithium belt prepared in Example 1.

[0035] Figure 3 The image shows a carbon-containing lithium strip prepared in Example 1 after being cut by an ion beam using an electron microscope.

[0036] Figure 4 The specific capacity test curve of the carbon-containing lithium belt prepared in Example 2.

[0037] Figure 5 The test curves for the lithium-ion battery assembled using Examples 1, 2 and Comparative Example 1 are shown. Detailed Implementation

[0038] To illustrate the invention in detail, the following specific embodiments will be used to describe the invention in detail.

[0039] Example 1

[0040] Lithophilic carbon nanotubes, which have undergone acid washing and high-temperature carbonization, are mixed with molten lithium at 600°C in a ratio of 12:13 and stirred for 15 minutes using a stainless steel stirrer. The carbon-containing lithium metal solution is then scooped up using a strainer with a pore diameter of 20 micrometers, and the residue in the strainer is collected in a stainless steel mold.

[0041] The collected residue was exposed to dry air and then thinned using a roller press to produce a carbon-containing lithium strip. The carbon-containing lithium strip was purged using a mixture of carbon dioxide and nitrogen during the rolling process.

[0042] Figure 1 To prepare a carbon-containing lithium strip after roll pressing, as shown in the figure, the thickness of the carbon-containing lithium strip obtained by roll pressing is 35 micrometers.

[0043] The prepared lithium metal strip was used as the working electrode, and the pure lithium metal sheet was used as the counter electrode. A coin cell was assembled using a polypropylene (PP) separator and a carbonate electrolyte (lithium salt LiPF6 concentration of 1 mol / L, solvent ratio of ethylene carbonate (EC): diethyl carbonate (DEC) = 1:1). Lithium extraction experiments were conducted to determine the carbon content in the carbon-containing lithium strip. Figure 2 The voltage curves for the lithium stripping process of the prepared carbon-containing lithium belt are shown. From... Figure 2 As can be seen, the specific capacity of the carbon-containing lithium tape is 2017 mAh / g, while the theoretical specific capacity of metallic lithium is 3860 mAh / g. The lower specific capacity of the carbon-containing lithium tape compared to the theoretical capacity of metallic lithium is due to the presence of carbon. Calculations based on the specific capacity of the carbon-containing lithium tape indicate that the carbon content in this material is 47.7%.

[0044] A corner of the lithium strip is cut using a focused ion beam (FIB). Figure 3 This is an electron microscope (EM) image of the cross-section of a lithium strip after ion beam cutting. The image clearly shows that the lithium metal strip has a shell layer approximately 2.5 micrometers thick. The interstitial layer between the shell and the inner layer varies considerably. This interstitial layer is mainly composed of rod-shaped carbon nanotubes, with no lithium metal filling between the carbon nanotubes. The thickness of this interstitial layer is approximately 2.5 micrometers. The white areas and dots in the inner layer represent carbon nanotubes, while the remaining areas are lithium metal. The image shows that carbon nanotubes are widely distributed in the inner layer, without any aggregation.

[0045] Example 2

[0046] Other conditions were the same as in Example 1, except that the diameter of the pores in the strainer was changed to 10 micrometers. The prepared carbon-containing lithium strip was used for lithium extraction experiments to determine the carbon content in the carbon-containing lithium strip. Figure 4 The voltage curves for the lithium stripping process of the prepared carbon-containing lithium belt are shown. From... Figure 4 As can be seen, the specific capacity of the carbon-containing lithium strip is 2720mAh / g. Based on this specific capacity, the carbon content in the carbon-containing lithium strip is 29.5%.

[0047] Comparative Example 1

[0048] Commercially available 35-micron pure metallic lithium strip.

[0049] Lithium iron phosphate was used as the positive electrode, and the carbon-containing lithium tape prepared in Example 1, the carbon-containing lithium tape prepared in Example 2, and the pure metallic lithium tape of Comparative Example 1 were used as the negative electrodes. A PP separator and an ester electrolyte (lithium salt LiPF6 concentration of 1 mol / L, solvent ratio EC:DEC = 1:1) were used to assemble the full battery. Charge-discharge cycle tests were performed on the assembled full battery. The test procedure was as follows: charging with a current of 2 mAh, with a cutoff voltage of 3.8 V, followed by discharging with a current of 2 mAh, with a discharge cutoff voltage of 2.2 V. The number of cycles and the discharge capacity were recorded. Figure 5 The figures show the cycle life of full cells assembled using the carbon-containing lithium strip prepared in Example 1, the carbon-containing lithium strip prepared in Example 2, and the pure metallic lithium strip of Comparative Example 1 as negative electrodes. As can be seen from the figures, the battery using the carbon-containing lithium strip prepared in Example 1 as the negative electrode has the longest cycle life, reaching approximately 175 cycles before its discharge capacity begins to rapidly decline, ending the battery's lifespan. The battery using the carbon-containing lithium strip prepared in Example 2 as the negative electrode has the second longest cycle life, with its discharge capacity rapidly declining after approximately 110 cycles, ending the battery's lifespan. The battery using the pure metallic lithium strip as the negative electrode has the shortest cycle life, with its discharge capacity starting to fluctuate significantly after only about 40 cycles, and its discharge capacity rapidly declining after 75 cycles, ending the battery's lifespan.

[0050] While specific embodiments of the present invention have been described in detail, they are not intended to limit the invention. Any modifications, substitutions, and improvements made to those details within the spirit and principles of the invention should be included within the scope of protection of the invention. The full scope of the invention is given by the appended claims and any equivalents.

Claims

1. A lithium strip with high carbon content, characterized in that, Containing 20% ​​to 80% by mass of lithium-loving carbon material, and the lithium strip having: The inner layer is formed of a composite of the lithiophilic carbon material and metallic lithium; The outer layer is a shell formed of lithium oxide compounds, with a thickness of 50 nm to 1 micrometer; and A gap layer supported by the lithiophilic carbon material is located between the inner and outer layers. The gap layer has a thickness of 0.1 micrometers to 2 micrometers and has unfilled voids formed by the lithiophilic carbon material.

2. The high-carbon-content lithium strip according to claim 1, characterized in that, The compound of the oxidized lithium is at least one of lithium nitride, lithium carbonate, lithium oxide, lithium peroxide, and lithium halide.

3. The high-carbon-content lithium strip according to claim 1, characterized in that, The lithiophilic carbon material includes at least one of graphite, carbon nanotubes, graphene, soft carbon, hard carbon, and mesophase carbon microspheres.

4. The high-carbon-content lithium strip according to claim 1, characterized in that, The lithium metal in the inner layer is pure lithium metal or a lithium alloy.

5. The high-carbon-content lithium strip according to claim 1, characterized in that, The high-carbon-content lithium strip has a width of 0.5 cm to 100 cm and a thickness of 5 μm to 200 μm.

6. A method for preparing a high-carbon-content lithium strip as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step 1: Mix the lithiophilic carbon material with molten lithium and stir. Step 2: Use a container with micropores at the bottom to scoop up the lithium metal liquid containing the lithium-loving carbon material obtained in Step 1. The pore diameter of the micropores is 10 micrometers to 50 micrometers. Step 3: Collect the residue in the container, cool it, and then extrude it into a strip under mixed dry gas purging to obtain a carbon-containing lithium strip with a carbon mass content between 20% and 80%.

7. The preparation method according to claim 6, characterized in that, The molten lithium temperature is between 300°C and 700°C.

8. The preparation method according to claim 6, characterized in that, The stirring time in step one is 10 to 30 minutes.

9. The preparation method according to claim 6, characterized in that, The water content of the mixed dry gas is less than 1000 ppm, and the mixed gas is a combination of at least two of nitrogen, oxygen, carbon dioxide, and halogen gases.

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