Composite negative electrode, method for preparing the same, and battery

Abstract

This invention relates to a composite negative electrode, its preparation method, and a battery. The composite negative electrode includes a substrate and a negative electrode coating disposed on the substrate. The negative electrode coating includes a first material and a second material, wherein the first material is a lithium-containing material. Along the direction away from the substrate, the content of the lithium-containing material in at least a portion of the composite negative electrode gradually increases. The composite negative electrode of this invention can solve the problems of lithium dendrite formation and dead lithium formation during charge-discharge cycles in lithium metal batteries.

Description

Technical Field

[0001] This invention relates to the field of lithium metal batteries, and more specifically, to a composite anode, a method for preparing the same, and a battery thereof. More specifically, this invention relates to a composite anode for preparing a lithium metal battery, a method for preparing the composite anode, and a lithium metal battery comprising the composite anode. Background Technology

[0002] Lithium metal anodes are a promising high-energy-density battery anode material due to their large theoretical specific capacity and low electrochemical potential. However, lithium metal batteries can develop lithium dendrites during charge-discharge cycles, leading to short circuits.

[0003] To address this issue, lithium alloys are typically used to prevent lithium dendrite formation. However, currently used lithium alloy anode materials are prone to dead lithium formation, leading to interfacial reactions and consequently reducing battery capacity. How to solve the problems of lithium dendrite formation and dead lithium formation in lithium metal batteries during charge-discharge cycles, while ensuring cell capacity and cycle performance, has become a pressing issue in this field. Summary of the Invention

[0004] The main objective of this invention is to provide a composite negative electrode and a method for preparing the composite negative electrode, so as to solve the problems of lithium dendrites and dead lithium in lithium metal batteries during charge-discharge cycles in the prior art.

[0005] To achieve the above objectives, the present invention provides a composite negative electrode, which includes a substrate and a negative electrode coating disposed on the substrate. The negative electrode coating includes a first material and a second material, wherein the first material is a lithium-containing material, and the content of the lithium-containing material in at least a portion of the composite negative electrode gradually increases along the direction away from the substrate.

[0006] Furthermore, along the direction away from the substrate of the composite negative electrode, the content of the second material in at least a portion of the composite negative electrode gradually decreases.

[0007] Furthermore, in some composite anodes, the mass ratio of lithium-containing materials in the anode coating varies with the coating thickness as follows: (1-5 wt%) / μm.

[0008] Furthermore, the thickness of the negative electrode coating is d. In the thickness direction of the negative electrode coating, within the range of 2%d-25%d from the substrate, the mass of the lithium-containing material is 10%-20% of the total mass of the lithium-containing material in the negative electrode coating; within the range of 25%d-40%d from the substrate, the mass of the lithium-containing material is 20%-50% of the total mass of the lithium-containing material in the negative electrode coating; and within the range of 40%-100%d from the substrate, the mass of the lithium-containing material is 40%-70% of the total mass of the lithium-containing material in the negative electrode coating.

[0009] Furthermore, the lithium-containing material includes one or more of lithium metal and lithium alloys, and the lithium alloys include one or more of lithium silicon alloys and lithium carbon alloys.

[0010] Furthermore, the second material includes carbon materials and / or silicon materials, wherein the carbon materials include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, carbon black, Ketten carbon, acetylene black, Super P, graphene, and single-walled carbon nanotubes, and wherein the silicon materials include one or more of monocrystalline silicon, polycrystalline silicon, and amorphous silicon.

[0011] Furthermore, the negative electrode coating further includes an adhesive.

[0012] To achieve the above objectives, another aspect of the present invention provides a method for preparing the aforementioned composite negative electrode, the method comprising the following steps: step S1, providing a first material and a second material, wherein the first material is a lithium-containing material, and disposing the second material on a substrate to form a second material layer; step S2, disposing the first material on the second material layer to form a first lithium-containing material layer; and step S3, hot-pressing the first lithium-containing material layer and the second material layer to obtain a composite negative electrode in which the content of lithium-containing material gradually increases in at least a portion of the composite negative electrode along the direction away from the substrate.

[0013] Furthermore, the substrate is selected from copper foil and composite copper foil.

[0014] Furthermore, the thickness of the first lithium-containing material layer is in the range of 5 μm to 30 μm, and / or the thickness of the second material layer is in the range of 10 μm to 60 μm.

[0015] Furthermore, the average particle size (D50) of the second material is in the range of 1 μm to 40 μm.

[0016] Further, in step S3, hot pressing is performed at a pressure of 10 MPa to 40 MPa and a temperature of 60°C to 100°C.

[0017] Furthermore, in step S3, the hot pressing time lasts from 5 min to 30 min.

[0018] Furthermore, the method further includes pre-coating a lithium-containing material onto a PET film in step S2 to form a lithium-containing film layer, and placing the lithium-containing film layer on a second material layer, and removing the PET film after hot pressing in step S3.

[0019] Furthermore, the ratio of the thickness H1 of the second material layer to the thickness H2 of the first lithium-containing material layer satisfies 1 < H1 / H2 ≤ 10.

[0020] Furthermore, the ratio of the area S1 of the second material layer to the area S2 of the first lithium-containing material layer satisfies 1.0 < S1 / S2 ≤ 1.5.

[0021] Another aspect of the present invention provides a lithium metal battery, comprising: a positive electrode, an electrolyte, and a composite negative electrode of the present invention or a composite negative electrode prepared by the method of the present invention. Attached Figure Description

[0022] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0023] Figure 1 A schematic diagram of the gradient structure of a composite negative electrode according to an embodiment of the present invention is shown;

[0024] Figure 2 A schematic diagram of a method for preparing a composite negative electrode according to an embodiment of the present invention is shown; and

[0025] Figure 3 The changes in discharge specific capacity and coulombic efficiency during charge-discharge cycles according to Embodiment 3 of the present invention are shown. Detailed Implementation

[0026] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0027] As described in the background section, existing lithium metal batteries suffer from lithium dendrite formation and dead lithium formation during charge-discharge cycles. To address these technical problems, one embodiment of this application provides a composite negative electrode comprising a substrate and a negative electrode coating disposed on the substrate. The negative electrode coating comprises a first material and a second material, wherein the first material is a lithium-containing material, and the content of the lithium-containing material in at least a portion of the composite negative electrode gradually increases along the direction away from the substrate.

[0028] The composite anode disclosed herein includes a substrate and an anode coating disposed on the substrate. The anode coating comprises a lithium-containing material and a second material (such as silicon, graphite, and carbon nanotubes). Along the direction away from the substrate, at least a portion of the lithium-containing material in the composite anode exhibits a concentration gradient distribution, with its content gradually increasing. The innermost layer of the coating, in contact with the substrate, has a lower lithium-containing material content, typically representing the lowest point of lithium-containing material content in the coating. This ensures the coating is firmly bonded to the substrate while preventing lithium dendrites from forming in the early stages of battery charging and discharging. Along the thickness direction of the coating, i.e., away from the substrate, the lithium-containing material content gradually increases.

[0029] This structural design allows for a more uniform distribution of lithium ions during charging and discharging, effectively suppressing the growth of lithium dendrites and preventing internal short circuits caused by lithium dendrites piercing the separator. Simultaneously, because the outer coating is rich in lithium-containing materials, the battery maintains stable performance even under high-rate charging and discharging conditions, improving cycle stability and rate performance.

[0030] Furthermore, along the direction away from the substrate in the composite negative electrode, the content of the second material gradually decreases in at least a portion of the composite negative electrode. According to the composite negative electrode of the present invention, the content of the second material (such as carbon or silicon) is relatively high in the coating region near the substrate, which helps buffer the growth of lithium dendrites while providing structural support and improving the conductivity of the electrode. In the coating region away from the substrate, the content of lithium-containing material increases, making this region more suitable for lithium-ion storage and release, helping to withstand higher charge and discharge current densities, thereby further improving the rate performance of the battery.

[0031] Furthermore, in some composite anodes, the mass ratio of lithium-containing materials in the anode coating varies with coating thickness in a gradient of (1-5 wt%) / μm. This gradient includes values ​​of 1 wt% / μm, 2 wt% / μm, 3 wt% / μm, 4 wt% / μm, and 5 wt% / μm. This gradient of lithium-containing material mass ratio in the anode coating with coating thickness, within the scope of this disclosure, can effectively suppress lithium dendrite growth, reduce the risk of short circuits, and thus further improve battery safety. Additionally, it can improve the migration path of lithium ions, reduce disordered diffusion of lithium ions at the interface, lower interface impedance, and further improve the rate performance and charge / discharge efficiency of the battery.

[0032] Furthermore, the thickness of the negative electrode coating is d. In the thickness direction of the negative electrode coating, within the range of 2%d-25%d from the substrate, the mass of the lithium-containing material is 10%-20% of the total mass of the lithium-containing material in the negative electrode coating; within the range of 25%d-40%d from the substrate, the mass of the lithium-containing material is 20%-50% of the total mass of the lithium-containing material in the negative electrode coating; and within the range of 40%-100%d from the substrate, the mass of the lithium-containing material is 40%-70% of the total mass of the lithium-containing material in the negative electrode coating.

[0033] Preferably, the thickness of the negative electrode coating is at most 50 μm. In the thickness direction of the negative electrode coating, within the range of 1-12 μm from the substrate, the mass of the lithium-containing material is 10%-20% of the total mass of the lithium-containing material in the negative electrode coating; within the range of 12-20 μm from the substrate, the mass of the lithium-containing material is 20%-50% of the total mass of the lithium-containing material in the negative electrode coating; and within the range of 20-45 μm from the substrate, the mass of the lithium-containing material is 40%-70% of the total mass of the lithium-containing material in the negative electrode coating.

[0034] Furthermore, the lithium-containing material includes one or more of lithium metal and lithium alloys, and the lithium alloys include one or more of lithium-silicon alloys and lithium-carbon alloys. Within the scope of this disclosure, the lithium element can provide the battery with lithium-ion storage capacity, which can help improve the battery's initial capacity, cycle performance, and safety. The lithium-containing material is preferably a lithium-silicon alloy or a lithium-carbon alloy. Lithium-silicon alloys have a higher theoretical specific capacity and a lower electrochemical potential, which can further increase the battery's energy density and control the material's expansion rate, reducing the risk of structural damage during charging and discharging. Lithium-carbon alloys can stabilize the surface structure of the lithium metal anode, reduce side reactions with the electrolyte, improve the battery's cycle efficiency, and improve the lithium-ion insertion and extraction process, further enhancing the battery's rate performance.

[0035] Furthermore, the second material includes carbon materials and / or silicon materials, wherein the carbon materials include one or more of natural graphite, artificial graphite, soft carbon, hard carbon, carbon black, Ketten carbon, acetylene black, Super P, graphene, and single-walled carbon nanotubes, and wherein the silicon materials include one or more of monocrystalline silicon, polycrystalline silicon, and amorphous silicon. Within the scope of this disclosure, carbon materials not only serve as excellent lithium-ion storage media but also provide good conductivity and structural stability. Within the scope of this disclosure, silicon materials can act as advantageous anode materials for high-energy-density batteries, reducing expansion and contraction during lithiation / delithiation processes, minimizing structural damage to the electrodes and degradation of battery performance.

[0036] Furthermore, the negative electrode coating further includes a binder. For example, the binder includes one or more of polyacrylic acid (PAA), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and carboxymethyl cellulose (CMC). For example, the mass ratio of the second material to the binder is 1.5-55.0. For example, the mass ratio of the second material to the binder is 1.5, 3.0, 5.0, 10.0, 15.0, 20.0, 25.0, 30.0, 40.0, 50.0, 55.0, etc. The type of binder and its mass ratio, within the scope of this disclosure, can maximize the utilization rate of the active material while ensuring the structural stability of the negative electrode coating, thereby further improving the overall performance of the battery. A mass ratio that is too low may increase the internal impedance of the electrode, reduce the utilization rate of the active material, and thus affect the battery's capacity and rate performance. A mass ratio that is too high may result in insufficient adhesion between the negative electrode coating and the substrate, making the electrode prone to detachment during charge and discharge, affecting the battery's cycle stability and safety.

[0037] Furthermore, the negative electrode coating further includes a conductive agent, including vapor-grown carbon fiber (VGCF). By adding a conductive agent, such as VGCF, to the negative electrode coating, not only can the electronic and ionic conductivity of the negative electrode coating be improved, but its structural stability and overall performance can also be enhanced.

[0038] Furthermore, the second material includes silicon, graphite, and carbon nanotubes, and / or the mass ratio of silicon to graphite is 0.1-5.0, and the mass ratio of silicon to carbon nanotubes is 0.5-20.0. For example, the mass ratio of silicon to graphite is 0.1, 0.5, 1.0, 3.0, 5.0, etc. For example, the mass ratio of silicon to carbon nanotubes is 0.5, 1.0, 5.0, 10.0, 15.0, 20.0, etc. Within the scope of this disclosure, the mass ratios of silicon to graphite and silicon to carbon nanotubes can suppress volume expansion during charging and discharging, improve the mechanical strength and electronic conductivity of the negative electrode coating, effectively prevent the growth of lithium dendrites, reduce direct contact between the electrolyte and lithium metal, and ensure the stability and long cycle life of the battery cell under high-rate charging and discharging conditions.

[0039] Another embodiment of this application provides a method for preparing the above-mentioned composite negative electrode, the method comprising the following steps: step S1, providing a first material and a second material, wherein the first material is a lithium-containing material, and disposing the second material on a substrate to form a second material layer; step S2, disposing the first material on the second material layer to form a first lithium-containing material layer; and step S3, hot-pressing the first lithium-containing material layer and the second material layer to obtain a composite negative electrode in which the content of lithium-containing material gradually increases in at least a portion of the composite negative electrode in a direction away from the substrate.

[0040] By further depositing a first lithium-containing material layer on a second material layer on a substrate, and then hot-pressing the bonded first and second material layers, a composite anode with a gradually increasing lithium content in at least a portion of the composite anode can be obtained. Bonding the first and second material layers and then hot-pressing them achieves close contact between the materials and the formation of a gradient structure. The hot-pressing process promotes the chemical reaction between the first lithium-containing material and the second material (carbon or silicon), thereby forming a structure with a gradually increasing lithium content along the direction away from the substrate in the composite anode. This concentration variation in the composite anode effectively prevents the formation of lithium dendrites, suppresses dead lithium, reduces interfacial impedance, and ultimately improves the cycle performance and rate performance of the battery. Furthermore, the preparation method of this invention is simple and easy to operate, thereby improving production efficiency.

[0041] For example, the second material includes one or more silicon materials selected from monocrystalline silicon, polycrystalline silicon, and amorphous silicon. For example, the substrate is selected from copper foil (e.g., electrolytic copper foil) and composite copper foil. Within the scope of this disclosure, the types of silicon materials can serve as advantageous anode materials for high-energy-density batteries, reducing expansion and contraction during lithiation / delithiation processes, minimizing structural damage to the electrodes and degradation of battery performance.

[0042] Furthermore, the thickness of the first lithium-containing material layer is in the range of 5 μm to 30 μm, and / or the thickness of the second material layer is in the range of 10 μm to 60 μm. For example, the thickness of the first lithium-containing material layer is 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 22 μm, 25 μm, 28 μm, or 30 μm. For example, the thickness of the second material layer is 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, or 60 μm. If the thickness of the first lithium-containing material layer is too thin, it may not be able to provide sufficient lithium-ion storage, thus limiting the battery capacity. Conversely, if the thickness is too thick, it may increase the internal impedance of the battery, affecting the rate performance and charge / discharge efficiency of the battery. A thinner second material layer may not effectively suppress lithium dendrite growth, nor provide sufficient buffer space to accommodate volume changes in silicon. An excessively thick second material layer may increase the cell thickness, reduce energy density, and increase the battery's internal impedance, affecting rate performance.

[0043] Further, the average particle size (D50) of the second material is in the range of 1 μm to 40 μm. Preferably, the second material is silicon and the average particle size (D50) of the silicon material is in the range of 1 μm to 20 μm. For example, the average particle size (D50) of the second material is 1 μm, 5 μm, 8 μm, 10 μm, 12 μm, 15 μm, 18 μm, 20 μm, 25 μm, 30 μm, 35 μm, or 40 μm. The average particle size (D50) of the second material within the scope of this disclosure can provide sufficient surface area and volume ratio, which helps the rapid insertion and extraction of lithium ions, thereby further improving the rate performance of the battery. Too small an average particle size (D50) will increase the manufacturing cost and handling difficulty of the material, while too large an average particle size (D50) will reduce the lithium ion diffusion rate of the electrode, affecting the cycle stability and rate performance of the battery.

[0044] Further, in step S3, hot pressing is performed at a pressure of 10 MPa to 40 MPa and a temperature of 60°C to 100°C.

[0045] For example, the pressure selected during hot pressing is 10 MPa, 15 MPa, 20 MPa, 25 MPa, 30 MPa, 35 MPa, 40 MPa, etc. The pressure values ​​during hot pressing, within the range disclosed herein, can effectively promote close contact and bonding between the first lithium-containing material layer and the second material layer (including materials such as silicon, graphite, and carbon nanotubes), forming a stable interface. It also ensures that the material layers form a gradient structure in the thickness direction, while avoiding damage to the internal structure of the material layers due to excessive pressure. This can avoid lithium dendrite formation and dead lithium problems during the charge-discharge cycle of lithium metal batteries, further improving the cycle stability and rate performance of the battery.

[0046] For example, the selected temperatures for hot pressing are 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, and 100°C. Within the range of this disclosure, the hot pressing temperature can promote diffusion and reaction within the material, facilitate the formation of lithium-silicon alloys and lithium-carbon alloys, contribute to the formation of gradient structures, improve the adhesion of material layers, and enhance the mechanical strength of the composite material. The temperature of this invention can balance the thermal stability of the material with the reactivity during the hot pressing process, ensuring that the formation of gradient structures does not cause degradation of material performance. This avoids the problems of lithium dendrite formation and dead lithium during charge-discharge cycles in lithium metal batteries, further improving the cycle stability and rate performance of the battery.

[0047] Furthermore, in step S3, the hot-pressing time lasts from 5 min to 30 min. For example, the hot-pressing time can last for 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 15 min, 20 min, 25 min, 30 min, etc. Within the scope of this disclosure, the hot-pressing time can improve the bonding strength between material layers, the formation of changes in active material concentration, control production costs, and ultimately further improve battery performance.

[0048] Furthermore, the method further includes pre-coating a lithium-containing material onto a PET (polyethylene terephthalate) film in step S2 to form a lithium-containing film layer, and placing the lithium-containing film layer on a second material layer, and removing the PET film after hot pressing in step S3.

[0049] Furthermore, the ratio of the thickness H1 of the second material layer to the thickness H2 of the first lithium-containing material layer satisfies 1 < H1 / H2 ≤ 10. For example, H1 / H2 can be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc. Within the scope of this disclosure, the thickness of the second material layer (e.g., a material containing carbon or silicon) is greater than that of the first lithium-containing material layer, which can further increase the effective capacity of the negative electrode and the energy density of the battery, thereby improving the overall capacity of the battery.

[0050] Furthermore, the ratio of the area S1 of the second material layer to the area S2 of the first lithium-containing material layer satisfies 1.0 < S1 / S2 ≤ 1.5. For example, S... C / S Li The values ​​are 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, etc. Within the scope of this disclosure, the area of ​​the second material layer S1 / S2 can be appropriately expanded to ensure the uniform distribution and reaction of lithium ions during charging and discharging, avoid excessively high local lithium ion concentration and current density, thereby further reducing the formation of lithium dendrites and the occurrence of dead lithium, and thus improving the cycle stability and safety of the battery.

[0051] Another embodiment of this application provides a lithium metal battery, which includes a positive electrode, an electrolyte, and the above-described composite negative electrode or a composite negative electrode prepared by the above-described method.

[0052] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0053] In the following embodiments, the lithium content in the negative electrode coating was determined by XRF (i.e., X-ray fluorescence spectrometry). Specifically, the prepared negative electrode sheet was taken and cut along the thickness direction to obtain test slices of different thickness regions of the negative electrode sheet. The slices were then pressed to make their surfaces flat. The size of the test slices was 40-50 mm and the thickness was 30-40 μm. The standard sample was a lithium-silicon alloy slice with a lithium content of 50%. The lithium content of the test slices was quantitatively analyzed by testing the energy and intensity of XRF.

[0054] Example 1

[0055] Step S1: Graphite, polyacrylic acid (PAA), and deionized water are added to micron-sized single-crystal silicon and single-walled carbon nanotubes with an average particle size (D50) of approximately 1 μm, and homogenized to obtain a uniformly mixed silicon-carbon material. In this silicon-carbon material, the mass ratio of micron-sized silicon to graphite is 0.1, the mass ratio of micron-sized silicon to carbon nanotubes is 0.5, and the mass ratio of the second material (i.e., a mixture of micron-sized silicon, graphite, and carbon nanotubes, which will not be described further below) to PAA is 1.5. The uniformly mixed silicon-carbon material is then uniformly coated onto a copper foil to form a silicon-carbon layer with a thickness of 10 μm.

[0056] Step S2: After drying the silicon carbon layer obtained in step S1, lithium coated on the PET (polyethylene terephthalate) film is imprinted onto the surface of the silicon carbon layer, the PET film is removed, and a lithium layer with a thickness of 5 μm is formed.

[0057] Step S3: The silicon-carbon layer and the lithium layer are hot-pressed, with the pressure controlled at 10 MPa, the temperature at 60°C, and the duration at 30 min, to obtain a composite anode with a gradually increasing lithium content in the direction away from the copper foil. In this composite anode, the thickness of the anode coating is 12 μm. In the thickness direction of the anode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium is 15% of the total lithium mass in the anode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium is 25% of the total lithium mass in the anode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium is 60% of the total lithium mass in the anode coating.

[0058] Example 2

[0059] Step S1: Graphite, carboxymethyl cellulose (CMC), and deionized water are added to micron-sized single-crystal silicon and single-walled carbon nanotubes with an average particle size (D50) of approximately 1 μm, and homogenized to obtain a uniformly mixed silicon-carbon material. In this silicon-carbon material, the mass ratio of micron-sized silicon to graphite is 1, the mass ratio of micron-sized silicon to carbon nanotubes is 2, and the mass ratio of the second material to CMC is 5. The uniformly mixed silicon-carbon material is then uniformly coated onto a copper foil to form a silicon-carbon layer with a thickness of 60 μm.

[0060] Step S2: After drying the silicon-carbon layer obtained in step S1, lithium coated on the PET film is imprinted onto the surface of the silicon-carbon layer, the PET film is removed, and a lithium layer with a thickness of 30 μm is formed.

[0061] Step S3: The silicon-carbon layer and the lithium layer are hot-pressed, with the pressure controlled at 20 MPa, the temperature at 100°C, and the duration at 5 min, to obtain a composite anode with a gradually increasing lithium content in the direction away from the copper foil. In this composite anode, the thickness of the anode coating is 72 μm. In the thickness direction of the anode coating, within the range of 1.44–18 μm from the copper foil, the mass of lithium accounts for 18% of the total lithium mass in the anode coating; within the range of 18–28.8 μm from the copper foil, the mass of lithium accounts for 31% of the total lithium mass in the anode coating; and within the range of 28.8–72 μm from the copper foil, the mass of lithium accounts for 51% of the total lithium mass in the anode coating.

[0062] Example 3

[0063] Step S1: Graphite, polyacrylic acid (PAA), and deionized water are added to micron-sized single-crystal silicon and single-walled carbon nanotubes with an average particle size (D50) of approximately 1 μm, and homogenized to obtain a uniformly mixed silicon-carbon material. In this silicon-carbon material, the mass ratio of micron-sized silicon to graphite is 3, the mass ratio of micron-sized silicon to carbon nanotubes is 8, and the mass ratio of the second material to PAA is 10. The uniformly mixed silicon-carbon material is then uniformly coated onto a copper foil to form a silicon-carbon layer with a thickness of 40 μm.

[0064] Step S2: After drying the silicon-carbon layer obtained in step S1, lithium coated on the PET film is imprinted onto the surface of the silicon-carbon layer, the PET film is removed, and a lithium layer with a thickness of 20 μm is formed.

[0065] Step S3: The silicon-carbon layer and the lithium layer are hot-pressed, with the pressure controlled at 40 MPa, the temperature at 80°C, and the duration at 15 min, to obtain a composite anode with a gradually increasing lithium content in the direction away from the copper foil. In this composite anode, the thickness of the anode coating is 48 μm. In the thickness direction of the anode coating, within the range of 0.96–12 μm from the copper foil, the mass of lithium is 12% of the total lithium mass in the anode coating; within the range of 12–19.2 μm from the copper foil, the mass of lithium is 33% of the total lithium mass in the anode coating; and within the range of 19.2–48 μm from the copper foil, the mass of lithium is 55% of the total lithium mass in the anode coating.

[0066] Example 4

[0067] Step S1: Graphite, polyvinylidene fluoride (PVDF), and deionized water are added to micron-sized single-crystal silicon and single-walled carbon nanotubes with an average particle size (D50) of approximately 1 μm, and homogenized to obtain a uniformly mixed silicon-carbon material. The mass ratio of micron-sized silicon to graphite in this silicon-carbon material is 4, the mass ratio of micron-sized silicon to carbon nanotubes is 15, and the mass ratio of the second material to PVDF is 20. The uniformly mixed silicon-carbon material is then uniformly coated onto a copper foil to form a silicon-carbon layer with a thickness of 30 μm.

[0068] Step S2: After drying the silicon-carbon layer obtained in step S1, lithium coated on the PET film is imprinted onto the surface of the silicon-carbon layer, the PET film is removed, and a lithium layer with a thickness of 20 μm is formed.

[0069] Step S3: The silicon-carbon layer and the lithium layer are hot-pressed, with the pressure controlled at 30 MPa, the temperature at 85°C, and the duration at 10 min, to obtain a composite anode with a gradually increasing lithium content in the direction away from the copper foil. In this composite anode, the thickness of the anode coating is 40 μm. In the thickness direction of the anode coating, within the range of 0.8–10 μm from the copper foil, the mass of lithium is 10% of the total lithium mass in the anode coating; within the range of 10–16 μm from the copper foil, the mass of lithium is 23% of the total lithium mass in the anode coating; and within the range of 16–40 μm from the copper foil, the mass of lithium is 67% of the total lithium mass in the anode coating.

[0070] Example 5

[0071] Step S1: Graphite, styrene-butadiene rubber (SBR), and deionized water are added to micron-sized single-crystal silicon and single-walled carbon nanotubes with an average particle size (D50) of approximately 1 μm, and homogenized to obtain a uniformly mixed silicon-carbon material. In this silicon-carbon material, the mass ratio of micron-sized silicon to graphite is 5:1, the mass ratio of micron-sized silicon to carbon nanotubes is 2:0, and the mass ratio of the second material to SBR is 5:5. The uniformly mixed silicon-carbon material is then uniformly coated onto a copper foil to form a silicon-carbon layer with a thickness of 40 μm.

[0072] Step S2: After drying the silicon-carbon layer obtained in step S1, lithium coated on the PET film is imprinted onto the surface of the silicon-carbon layer, the PET film is removed, and a lithium layer with a thickness of 10 μm is formed.

[0073] Step S3: The silicon-carbon layer and the lithium layer are hot-pressed, with the pressure controlled at 40 MPa, the temperature at 70°C, and the duration at 20 min, to obtain a composite anode with a gradually increasing lithium content in the direction away from the copper foil. In this composite anode, the thickness of the anode coating is 40 μm. In the thickness direction of the anode coating, within the range of 0.8–10 μm from the copper foil, the mass of lithium accounts for 20% of the total lithium mass in the anode coating; within the range of 10–16 μm from the copper foil, the mass of lithium accounts for 36% of the total lithium mass in the anode coating; and within the range of 16–40 μm from the copper foil, the mass of lithium accounts for 44% of the total lithium mass in the anode coating.

[0074] Example 6

[0075] The composite electrode was prepared in the same manner as in Example 1, except that the hot-pressing process of step S3 was not applied to the lithium layer and silicon-carbon layer; instead, a rolling process at 2 MPa was applied for 5 min at room temperature. In the resulting composite negative electrode, the thickness of the negative electrode coating was 14 μm. In the thickness direction of the negative electrode coating, within the range of 0.28–3.5 μm from the copper foil, the mass of lithium was 0% of the total lithium mass in the negative electrode coating; within the range of 3.5–5.6 μm from the copper foil, the mass of lithium was 0% of the total lithium mass in the negative electrode coating; and within the range of 5.6–14 μm from the copper foil, the mass of lithium was 100% of the total lithium mass in the negative electrode coating.

[0076] Example 7

[0077] The composite electrode was prepared using the same method as in Example 1, except that in step S1, the mass ratio of micron-sized silicon to graphite was 0.05, the mass ratio of micron-sized silicon to carbon nanotubes was 0.1, and the mass ratio of the second material to PAA was 1. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium was 8% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium was 10% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium was 81% of the total lithium mass in the negative electrode coating.

[0078] Example 8

[0079] The composite electrode was prepared using the same method as in Example 1, except that in step S1, the mass ratio of micron-sized silicon to graphite was 8, the mass ratio of micron-sized silicon to carbon nanotubes was 25, and the mass ratio of the second material to PAA was 60. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium accounted for 5% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium accounted for 6% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium accounted for 89% of the total lithium mass in the negative electrode coating.

[0080] Example 9

[0081] The composite electrode was prepared in the same manner as in Example 1, except that in step S3, the pressure during hot pressing was controlled at 5 MPa, the temperature at 50°C, and the duration at 3 min. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium accounted for 7% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium accounted for 11% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium accounted for 82% of the total lithium mass in the negative electrode coating.

[0082] Example 10

[0083] The composite electrode was prepared in the same manner as in Example 1, except that in step S3, the pressure during hot pressing was controlled at 8 MPa, the temperature at 80°C, and the duration at 3 min. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium accounted for 8% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium accounted for 9% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium accounted for 83% of the total lithium mass in the negative electrode coating.

[0084] Example 11

[0085] The composite electrode was prepared in the same manner as in Example 1, except that in step S3, the applied pressure was controlled at 10 MPa, the temperature at room temperature, and the duration at 5 min. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium accounted for 6% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium accounted for 10% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium accounted for 84% of the total lithium mass in the negative electrode coating.

[0086] Example 12

[0087] The composite electrode was prepared in the same manner as in Example 1, except that in step S3, the pressure during hot pressing was controlled at 2 MPa, the temperature at 60°C, and the duration at 5 min. In the resulting composite negative electrode, the thickness of the negative electrode coating was 12 μm. Along the thickness direction of the negative electrode coating, within the range of 0.24–3 μm from the copper foil, the mass of lithium accounted for 6% of the total lithium mass in the negative electrode coating; within the range of 3–4.8 μm from the copper foil, the mass of lithium accounted for 9% of the total lithium mass in the negative electrode coating; and within the range of 4.8–12 μm from the copper foil, the mass of lithium accounted for 85% of the total lithium mass in the negative electrode coating.

[0088] Performance testing

[0089] I. Battery manufacturing

[0090] Fabrication of coin cells: A ternary cathode material was coated onto the surface of carbon-coated aluminum foil with a coating thickness of 30 μm, forming a cathode coin cell with a diameter of d = 16 mm. An electrolyte membrane with a diameter of d = 18 mm and a negative cathode coin cell with a diameter of d = 19 mm were then fabricated. The cathode and electrolyte membrane were subjected to hydraulic pressing at 50 MPa for 15 min, followed by hot pressing at 50 T for 30 min. A silicon-carbon composite negative electrode was then attached, and the coin cell was fabricated using the same process of hydraulic pressing at 50 MPa for 15 min, followed by hot pressing at 50 T for 30 min. This coin cell is model 2016. All performance tests were conducted at 55°C.

[0091] The above-mentioned negative electrode sheet is a composite negative electrode sheet obtained by any of the processes in Examples 1-12.

[0092] II. Performance Testing

[0093] (1) Cyclic performance test

[0094] The battery was activated by constant current charge-discharge at 0.1 C for the first two cycles, and then cycle performance was tested at 0.33 C. A significant fluctuation in the specific capacity curve and a cell capacity retention rate below 80% indicate that the battery has failed.

[0095] (2) Ratio performance test

[0096] The battery was activated by performing two charge-discharge cycles at a current of 0.05 C, and then the rate performance of the battery prepared in the example was tested at currents of 0.5 C and 1 C, respectively.

[0097] A battery is considered valid if it retains 80% or more of its capacity after at least 80 charge-discharge cycles at different rates. A battery is considered faulty if its capacity retention falls below 80%.

[0098] (3) First Coulomb efficiency test

[0099] The prepared all-solid-state coin cell was charged to 4.25 V at 0.1 C under constant current, and then discharged to 2.5 V at 0.1 C under constant current. The initial coulombic efficiency was obtained by calculating the ratio of the initial discharge capacity to the initial charge capacity.

[0100] The performance test results of the above embodiments are shown in Table 1.

[0101] Table 1

[0102]

[0103] The cycle count (0.33C) data shown in Table 1 above represents the number of charge-discharge cycles performed on the battery at a current of 0.33C when the battery retains 80% of its capacity.

[0104] As can be seen from the above description, the battery made with the composite electrode prepared by the method of the present invention has excellent battery cycle performance, first coulombic efficiency and rate performance.

[0105] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0106] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite negative electrode, characterized in that, The composite negative electrode includes a substrate and a negative electrode coating disposed on the substrate. The negative electrode coating includes a first material and a second material, wherein the first material is a lithium-containing material, and the content of the lithium-containing material in at least a portion of the composite negative electrode gradually increases in the direction away from the substrate.

2. The composite negative electrode according to claim 1, characterized in that, Along the direction away from the substrate along the composite negative electrode, the content of the second material in at least a portion of the composite negative electrode gradually decreases.

3. The composite negative electrode according to claim 1 or 2, characterized in that, In some of the composite anodes, the mass ratio of the lithium-containing material in the anode coating varies with the coating thickness at a gradient of (1-5 wt%) / μm.

4. The composite negative electrode according to claim 1 or 2, characterized in that, The thickness of the negative electrode coating is d. In the thickness direction of the negative electrode coating, within the range of 2%d-25%d from the substrate, the mass of the lithium-containing material is 10%-20% of the total mass of the lithium-containing material in the negative electrode coating; within the range of 25%d-40%d from the substrate, the mass of the lithium-containing material is 20%-50% of the total mass of the lithium-containing material in the negative electrode coating; and within the range of 40%-100%d from the substrate, the mass of the lithium-containing material is 40%-70% of the total mass of the lithium-containing material in the negative electrode coating.

5. The composite negative electrode according to claim 1 or 2, characterized in that, The lithium-containing material includes one or more of lithium metal and lithium alloys, and the lithium alloys include one or more of lithium silicon alloys and lithium carbon alloys.

6. The composite negative electrode according to claim 1 or 2, characterized in that, The second material includes carbon materials and / or silicon materials. The carbon materials mentioned include one or more of the following: natural graphite, artificial graphite, soft carbon, hard carbon, carbon black, Ketten carbon, acetylene black, Super P, graphene, and single-walled carbon nanotubes. The silicon material mentioned includes one or more of monocrystalline silicon, polycrystalline silicon, and amorphous silicon.

7. The composite negative electrode according to claim 1 or 2, characterized in that, The negative electrode coating further includes an adhesive.

8. A method for preparing a composite negative electrode according to any one of claims 1 to 7, characterized in that, The method includes the following steps: Step S1: Provide a first material and a second material, wherein the first material is a lithium-containing material, and dispose of the second material on the substrate to form a second material layer; Step S2, depositing the first material onto the second material layer to form a first lithium-containing material layer; and Step S3: Hot-press the first lithium-containing material layer and the second material layer to obtain a composite negative electrode in which the content of the lithium-containing material gradually increases in at least a portion of the composite negative electrode in a direction away from the substrate.

9. The method according to claim 8, characterized in that, The substrate is selected from copper foil and composite copper foil.

10. The method according to claim 8 or 9, characterized in that, The thickness of the first lithium-containing material layer is in the range of 5 μm to 30 μm, and / or the thickness of the second material layer is in the range of 10 μm to 60 μm.

11. The method according to claim 8 or 9, characterized in that, The average particle size (D50) of the second material is in the range of 1 μm to 40 μm.

12. The method according to claim 8 or 9, characterized in that, In step S3, the hot pressing is performed at a pressure of 10 MPa to 40 MPa and a temperature of 60°C to 100°C.

13. The method according to claim 8 or 9, characterized in that, In step S3, the hot pressing time lasts from 5 min to 30 min.

14. The method according to claim 8 or 9, characterized in that, The method further includes pre-coating the lithium-containing material onto a PET film in step S2 to form a lithium-containing film layer, and placing the lithium-containing film layer on the second material layer, and removing the PET film after hot pressing in step S3.

15. The method according to claim 8 or 9, characterized in that, The ratio of the thickness H1 of the second material layer to the thickness H2 of the first lithium-containing material layer satisfies 1 < H1 / H2 ≤ 10.

16. The method according to claim 8 or 9, characterized in that, The ratio of the area S1 of the second material layer to the area S2 of the first lithium-containing material layer satisfies 1.0 < S1 / S2 ≤ 1.

5.

17. A lithium metal battery, characterized in that, The lithium metal battery includes: positive electrode, Electrolytes, and The composite negative electrode according to any one of claims 1 to 7.

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