Gradient solid-state electrolyte negative electrode sheet, preparation method thereof, lithium supplementing method, and battery

Abstract

This invention relates to a gradient solid-state electrolyte negative electrode, its preparation method, lithium replenishment method, and battery in the field of lithium-ion battery technology. The invention comprises a first active material layer and a second active material layer on the surface of a current collector, with a lithium metal replenishment layer on the surface of the second active material layer. The first active material layer has a solid electrolyte content of 0%–5%, a compaction density of 1.6–2.0 g / cm³, and a porosity of 15%–25%. The second active material layer has a solid electrolyte content of 10%–30%, a compaction density of 1.0–1.4 g / cm³, and a porosity of 30%–50%. A three-dimensional ion-conducting network is constructed through the high solid electrolyte content in the surface layer, allowing the replenished lithium metal to rapidly diffuse into the interior of the electrode, achieving rapid and uniform lithium replenishment. The high compaction density and low solid electrolyte content in the inner layer ensure efficient electron collection and electrode structural strength. This invention is mainly used in high-energy-density lithium-ion batteries and solid-state lithium batteries.

Description

Technical Field

[0001] This invention belongs to the technical field of lithium-ion batteries, and in particular to a gradient solid electrolyte negative electrode sheet, its preparation method, lithium replenishment method, and battery. Background Technology

[0002] With the rapid development of the new energy industry, the market is placing increasingly higher demands on the energy density and safety performance of lithium-ion batteries. Dry electrode technology, with its advantages of being environmentally friendly, low-cost, and capable of producing thick electrodes, has received widespread attention for improving energy density. However, when using calendering or physical vapor deposition for negative electrode lithium replenishment, the lithium metal layer is typically placed on the electrode surface, requiring lithium ions to travel a relatively long solid-state diffusion distance to embed into the active material. For negative electrode sheets with high compaction density and low porosity, electrolyte wetting is difficult, and the lithium-ion transport channels are extremely limited. The added lithium metal cannot react fully with the active material in a short time, leading to uneven lithium replenishment, low efficiency, and residual lithium metal, which in turn affects the battery's initial coulombic efficiency, cycle life, and safety.

[0003] Dry electrode technology offers an effective process route to address the aforementioned lithium diffusion challenges. Compared to wet electrode technology, dry electrode technology avoids the use of organic solvents, reducing environmental pollution and lowering production costs, while also increasing electrode compaction density. In the dry electrode process, a three-dimensional network structure is formed using fibrous binders, anchoring solid electrolyte particles and active material particles within the fiber network to create a mechanically stable composite electrode structure.

[0004] All-solid-state lithium batteries use non-flammable solid electrolytes instead of electrolytes and separators, significantly reducing safety hazards. The ionic conductivity of solid electrolyte materials is a key indicator determining battery performance: sulfide-based solid electrolytes exhibit the highest room-temperature ionic conductivity, reaching 10. - The electrochemical efficiency is on the order of 2S / cm; halide electrolytes have a wider electrochemical window; while oxide electrolytes have better air stability. In recent years, the appropriate doping of solid electrolytes into high-energy-density negative electrode sheets has attracted attention. This approach can construct an internal ion-conducting network, improve the ion transport characteristics of the electrode sheet, and at the same time utilize the high mechanical strength of solid electrolytes to enhance the structural stability and safety of the electrode sheet.

[0005] Against this backdrop, the organic integration of solid-state electrolyte doping strategies with lithium replenishment technologies holds promise for accelerating lithium replenishment reactions and improving replenishment efficiency by constructing rapid lithium-ion conduction channels. This technological direction has recently attracted widespread attention. However, current technologies lack specific solutions for effectively combining solid-state electrolyte gradient distribution with external lithium replenishment technologies.

[0006] Chinese patent publication CN119297429A employs a multi-layer stacking method in which a negative electrode sheet, a lithium replenishment layer, and a solid electrolyte membrane are stacked sequentially. Both the lithium replenishment layer and the solid electrolyte membrane are surface covering structures, which fails to solve the problem of the long transport path of replenished lithium inside the electrode, and also fails to introduce a solid electrolyte ion conductive network inside the negative electrode sheet.

[0007] Chinese patent publication numbers CN120637390A and CN115148960B both incorporate solid electrolytes into the negative electrode material or perform intrinsic pre-lithiation of the material. However, both involve uniform doping at a single concentration and do not consider the differences in ion transport requirements along the electrode thickness direction. Their technical purpose is to buffer volume expansion stress rather than accelerate the lithium replenishment reaction rate, which is fundamentally different from the technical concept of this invention.

[0008] Chinese patent publication numbers CN121687870A and CN121812476A focus on the material type distribution and particle size gradient design of the bilayer active layer, respectively, but neither involves the gradient doping of solid electrolyte, nor does it coordinate the gradient distribution of solid electrolyte with the external lithium replenishment process.

[0009] US Patent Publication No. US20230095801A1 discloses a solid-state battery with a solid electrolyte gradient composite structure, but it does not combine the gradient electrode with external lithium replenishment technology and lacks specific bifunctional layer parameter design and industrial fabrication process.

[0010] In summary, there is currently a lack of specific solutions in the existing technology that can effectively combine the gradient distribution of solid electrolyte with external lithium replenishment technology. The fundamental reason for the limited lithium replenishment efficiency is that the transport rate of replenished lithium inside the electrode is slow, and this problem has not yet been effectively solved. Summary of the Invention

[0011] The technical problem to be solved by the present invention is to provide a gradient solid electrolyte negative electrode sheet, its preparation method, lithium replenishment method and battery. By constructing a double-layer structure with a gradient distribution of solid electrolyte content in the thickness direction of the negative electrode sheet, and combining it with external lithium replenishment technology, the rapid and uniform transport and reaction of added metallic lithium is realized, which effectively improves the lithium replenishment efficiency and the overall performance of the battery.

[0012] According to an embodiment of the present invention, a gradient solid-state electrolyte negative electrode sheet includes a current collector, a first active material layer, a second active material layer, and a lithium metal supplementation layer; the first active material layer is disposed on the surface of the current collector, and the first active material layer contains a negative electrode active material and a conductive agent, the content of solid electrolyte is 0% to 5%, the compaction density is 1.6 to 2.0 g / cm³, and the porosity is 15% to 25%; the second active material layer is disposed on the surface of the first active material layer, and the second active material layer contains a negative electrode active material, a conductive agent, and a solid electrolyte, the content of solid electrolyte is 10% to 30%, the compaction density is 1.0 to 1.4 g / cm³, and the porosity is 30% to 50%; the lithium metal supplementation layer is disposed on the surface of the second active material layer.

[0013] Furthermore, the solid electrolyte is one or more of the following: sulfide system solid electrolyte, halide system solid electrolyte, or oxide system solid electrolyte; the thickness of the first active material layer is 20–80 μm, and the thickness of the second active material layer is 30–150 μm.

[0014] The present invention also provides a method for replenishing lithium in a gradient solid electrolyte negative electrode sheet, comprising the following steps: Lithium replenishment step: introducing a metallic lithium layer on the surface of the second active material layer by means of rolling lithium replenishment or physical vapor deposition lithium replenishment, forming a metallic lithium replenishment layer; Shelving step: shelving the lithium-replenished negative electrode sheet in a vacuum or inert atmosphere for a period of time.

[0015] Furthermore, the method of calendering for lithium replenishment includes: applying a lithium strip or foil with a thickness of 1 to 20 μm to the surface of the second active material layer using a rolling equipment, with a rolling line pressure of 50 to 200 N / mm.

[0016] Furthermore, physical vapor deposition lithium replenishment methods include: in a vacuum degree ≤ 5 × 10 -4 Lithium metal layers are deposited in an environment of Pa at a deposition rate of 0.1–0.5 nm / s and a thickness of 0.1–5 μm.

[0017] This invention also provides a method for preparing a gradient solid electrolyte negative electrode sheet, comprising the following steps: S1: preparing a first dry powder mixture, the first dry powder mixture comprising 80%–95% negative electrode active material, 1%–10% conductive agent, 0%–5% solid electrolyte, and 1%–5% binder; S2: preparing a second dry powder mixture, the second dry powder mixture comprising 60%–85% negative electrode active material, 1%–10% conductive agent, 10%–30% solid electrolyte, and 1%–5% binder; S3: mixing the first dry powder... The mixture is fibroinated and then molded to form a first active material layer. The compacted density of the first active material layer is 1.6-2.0 g / cm³, and the porosity is 15%-25%. S4: A second dry powder mixture is applied to the surface of the first active material layer and molded to form a second active material layer. The compacted density of the second active material layer is 1.0-1.4 g / cm³, and the porosity is 30%-50%. S5: The first and second active material layers are combined on the surface of the current collector and rolled to obtain a negative electrode sheet with a double-layer gradient structure.

[0018] Further, steps S3 and S4 specifically include: rolling the first dry powder mixture to a compaction density of 1.6-2.0 g / cm³ after fiberization treatment to form a first active material layer self-supporting film; rolling the second dry powder mixture to a compaction density of 1.0-1.4 g / cm³ after fiberization treatment to form a second active material layer self-supporting film; and stacking the first and second active material layer self-supporting films and then performing light-pressure composite bonding with a linear pressure of 10-50 N / mm and a temperature of 40-60℃.

[0019] Further, step S4 specifically includes: spreading the first dry powder mixture on the surface of the current collector and lightly pressing it to pre-shape; spreading the second dry powder mixture on the surface of the first dry powder mixture; and performing a single roller press with a linear pressure of 100-200 N / mm to compact the first dry powder mixture to 1.6-2.0 g / cm³ and the second dry powder mixture to 1.0-1.4 g / cm³.

[0020] The present invention also provides a battery comprising a positive electrode, a separator or solid electrolyte membrane, an electrolyte, and a gradient solid electrolyte negative electrode.

[0021] The present invention also provides a battery comprising a positive electrode, a separator or solid electrolyte membrane, an electrolyte, and a gradient solid electrolyte negative electrode prepared by a preparation method.

[0022] The beneficial effects of this invention are as follows: the three-dimensional ion-conducting network rich in solid electrolyte on the surface provides a low-impedance, rapid diffusion channel for the added lithium metal, enabling lithium ions to diffuse deep into the electrode in a short time, significantly improving the lithium replenishment rate; at the same time, the three-dimensional ion-conducting network ensures the uniform distribution of lithium ions in the electrode thickness direction, avoiding the problem of excessive lithium replenishment on the surface and insufficient lithium replenishment in the inner layer, and reducing lithium metal residue; in addition, the surface solid electrolyte is in direct contact with the lithium metal replenishment layer, and its high mechanical strength can effectively suppress the growth of lithium dendrites, enhancing the safety of the battery. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the structure of the gradient solid electrolyte negative electrode sheet of the present invention; Figure 2 This is a process flow diagram of the preparation method of the present invention; Figure 3 This is a process flow diagram of the lithium replenishment method of the present invention.

[0024] The labels in the figure are: a) lithium metal layer; b) high-content solid electrolyte negative electrode layer; c) low-content solid electrolyte negative electrode layer; d) current collector. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] The gradient solid electrolyte negative electrode sheet, its preparation method, lithium replenishment method, and battery of the present invention are described in detail below with reference to the accompanying drawings.

[0027] Figure 1This is a schematic diagram of the structure of a gradient solid-state electrolyte negative electrode sheet provided in an embodiment of the present invention. A low-content solid-state electrolyte negative electrode layer c is disposed on the surface of the current collector d, a high-content solid-state electrolyte negative electrode layer b is disposed on the surface of the low-content solid-state electrolyte negative electrode layer c, and a lithium metal layer a is disposed on the surface of the high-content solid-state electrolyte negative electrode layer b. The low-content solid-state electrolyte negative electrode layer c has a solid-state electrolyte content of 0%–5%, a compaction density of 1.6–2.0 g / cm³, and a porosity of 15%–25%, corresponding to the first active material layer in the present invention. The high-content solid-state electrolyte negative electrode layer b has a solid-state electrolyte content of 10%–30%, a compaction density of 1.0–1.4 g / cm³, and a porosity of 30%–50%, corresponding to the second active material layer in the present invention. The lithium metal layer a corresponds to the lithium metal supplementation layer in the present invention, and is formed on the surface of the high-content solid-state electrolyte negative electrode layer b by lithium supplementation through rolling or physical vapor deposition.

[0028] All raw materials used in the embodiments of this invention are commercially available. The negative electrode active material is selected from one or more of graphite, silicon-carbon, silicon-oxygen, or hard carbon. The graphite can be artificial or natural graphite with a particle size D50 of 5–20 μm; the silicon-carbon material is a composite material of nano-silicon and a carbon matrix, with a silicon content of 5%–30% and a particle size D50 of 5–15 μm; the silicon-oxygen material is SiOx, where 0.8 ≤ x ≤ 1.2, and is composite with carbon, with a particle size D50 of 5–15 μm; the hard carbon material has a particle size D50 of 5–20 μm. The conductive agent is selected from one or more of conductive carbon black such as Super P or acetylene black, carbon nanotubes, or graphene. When carbon nanotubes are used, their diameter is 5–20 nm and their length is 5–30 μm; when graphene is used, its sheet diameter is 1–10 μm and its number of layers is 1–10. The solid electrolyte is selected from one or more of the following: sulfide system, halide system, or oxide system solid electrolytes, wherein the ionic conductivity of the sulfide system solid electrolyte is 10. - ³~10 - ²S / cm, the ionic conductivity of the halide system solid electrolyte is 10 - ³~10 - ²S / cm, the ionic conductivity of the oxide system solid electrolyte is 10 -4 ~10 - ³S / cm. The binder is polytetrafluoroethylene (PTFE) with a particle size of 100–500 μm, used to form a three-dimensional network structure through fiberization under high-speed shearing. The current collector d is copper foil with a thickness of 6–12 μm.

[0029] Example 1: See Figure 2 The method for preparing the gradient solid electrolyte negative electrode sheet provided in this embodiment of the invention includes the following steps: Step 1: Prepare the first layer of dry powder mixture. Weigh the following raw materials by mass percentage: graphite as the negative electrode active material, with a D50 of 12μm and a content of 88%; Super P as the conductive agent, with a content of 5%; Li6PS5Cl as the solid electrolyte, with a content of 3%; and PTFE as the binder, with a content of 4%. Put the above raw materials into a high-speed mixer and perform high-speed shear mixing for 10-30 minutes at a speed of 1000-3000 rpm and a temperature of 20-40℃ to fully fibrose the PTFE and obtain the first layer of dry powder mixture.

[0030] Step 2: Prepare the second layer of dry powder mixture. Weigh the following raw materials by mass percentage: graphite as the negative electrode active material, with a D50 of 12μm and a content of 70%; Super P as the conductive agent and a content of 5%; Li6PS5Cl as the solid electrolyte and a content of 20%; and PTFE as the binder and a content of 5%. Put the above raw materials into a high-speed mixer and perform high-speed shear mixing for 10 to 30 minutes at a speed of 1000 to 3000 rpm and a temperature of 20 to 40°C to fully fibrose the PTFE and obtain the second layer of dry powder mixture.

[0031] Step 3: Preparation of the first active material layer self-supporting membrane. The first layer of dry powder mixture is rolled through the first pair of hot roller presses. The rolling pressure is 50-150 N / mm, the rolling temperature is 40-60℃, and the rolling is carried out until the compaction density is 1.8 g / cm³, to obtain the first active material layer self-supporting membrane with a thickness of 50 μm and a porosity of 20%.

[0032] Step 4: Preparation of the second active material layer self-supporting membrane. The second layer of dry powder mixture is rolled through a second pair of hot roller presses. The rolling pressure is 10-50 N / mm, the rolling temperature is 40-60℃, and the rolling is carried out until the compaction density is 1.2 g / cm³, to obtain the second active material layer self-supporting membrane with a thickness of 80 μm and a porosity of 40%.

[0033] Step 5: Light Press Lamination. The first and second active material layer self-supporting films are stacked sequentially, with the first layer on the bottom and the second layer on top. Light pressing lamination is then performed using a third pair of heated rollers at a linear pressure of 10–50 N / mm and a temperature of 40–60°C. Under this linear pressure, the PTFE fibers at the interface of the two layers interpenetrate and fuse, forming a transition zone that ensures interlayer bonding strength, while the compaction density within each layer remains essentially unchanged.

[0034] Step 6: Composite with current collector. The composite double-layer film and an 8μm thick copper foil are passed together through the fourth set of hot rollers and thermally composited at a linear pressure of 10-50 N / mm and a temperature of 40-60℃ to obtain a semi-finished negative electrode sheet with a double-layer gradient structure.

[0035] Example 2: Step 1: Prepare the first layer of dry powder mixture. Weigh the following raw materials by mass percentage: the negative electrode active material is silicon-carbon material, with a silicon content of 15% and a D50 of 10μm, and a content of 85%; the conductive agent is carbon nanotubes, with a content of 5%; the solid electrolyte is Li6PS5Cl, with a content of 5%; and the binder is PTFE, with a content of 5%. Put the above raw materials into a high-speed mixer and perform high-speed shear mixing for 10-30 minutes at a speed of 1000-3000 rpm and a temperature of 20-40℃ to fully fibrose the PTFE and obtain the first layer of dry powder mixture.

[0036] Step 2: Prepare the second layer of dry powder mixture. Weigh the following raw materials by mass percentage: the negative electrode active material is silicon-carbon material, with a silicon content of 15% and a D50 of 10μm, comprising 65%; the conductive agent is carbon nanotubes, comprising 5%; the solid electrolyte is LLZO, comprising 25%; and the binder is PTFE, comprising 5%. Add the above raw materials to a high-speed mixer and perform high-speed shear mixing for 10–30 minutes at a speed of 1000–3000 rpm and a temperature of 20–40℃ to fully fibrose the PTFE, obtaining the second layer of dry powder mixture. The second layer uses hard oxide solid electrolyte LLZO, whose compressibility is much lower than that of the sulfide solid electrolyte in the first layer, providing a structural basis for achieving differentiated compaction density through subsequent one-time roll pressing.

[0037] Step 3: Lamination and Pre-setting. A first layer of dry powder mixture is spread onto the surface of an 8μm thick copper foil. It is then lightly pressed with a linear pressure of 5–20 N / mm to initially fix it onto the current collector surface, but not to the final density. A second layer of dry powder mixture is then spread on top of the first layer, controlling the powder thickness to be uniform.

[0038] Step 4: One-time high-pressure rolling. The current collector carrying the double-layer powder is passed through a high-pressure rolling mill in one pass, undergoing a single rolling process at a linear pressure of 100–200 N / mm and a temperature of 40–60 °C. Because the second layer contains 25% LLZO hard solid electrolyte and has a lower initial bulk density, its compressibility is much lower than the first layer. Therefore, under the same high pressure, the first layer is compacted to 1.8 g / cm³, while the second layer is only compacted to 1.2 g / cm³. Both layers simultaneously undergo compaction and interfacial bonding during the rolling process, resulting in a double-layer gradient negative electrode semi-finished product. In this electrode, the first active material layer has a thickness of 50 μm and a porosity of 20%; the second active material layer has a thickness of 80 μm and a porosity of 40%.

[0039] Example 2 demonstrates that the key to achieving differentiated compaction density lies in the difference in compressibility between the two layers. The surface layer achieves lower compressibility than the bottom layer by increasing the solid electrolyte content, selecting hard oxide or halide electrolytes, controlling the particle size distribution of active materials, and adjusting the binder content. Under the same high-pressure rolling conditions, the bottom layer is compacted to the target density, while the surface layer maintains a higher porosity.

[0040] If process stability and significant density differences are desired, the layered pre-compression-light compression composite method of Example 1 can be preferred. If the process needs to be simplified to the greatest extent, the compressibility difference control method of Example 2 can be used, but preliminary experimental verification is required for specific solid electrolytes and active materials. Generally, when the surface solid electrolyte is a hard material such as LLZO, Example 2 performs better; when the solid electrolyte is a soft material such as sulfides, Example 1 is more reliable.

[0041] Example 3: The gradient solid electrolyte negative electrode sheet was prepared according to the method in Example 1, with the difference being the raw material ratio and process parameters of each layer as follows.

[0042] In the first layer of dry powder mixture, by mass percentage: the negative electrode active material is graphite, with a content of 92%; the conductive agent is Super P, with a content of 4%; the solid electrolyte is Li6PS5Cl, with a content of 1%; and the binder is PTFE, with a content of 3%. The compaction density of the first active material layer is 1.6 g / cm³, the porosity is 25%, and the thickness is 20 μm.

[0043] In the second layer of dry powder mixture, by mass percentage: the negative electrode active material is graphite, with a content of 80%; the conductive agent is Super P, with a content of 4%; the solid electrolyte is Li6PS5Cl, with a content of 12%; and the binder is PTFE, with a content of 4%. The compaction density of the second active material layer is 1.0 g / cm³, the porosity is 50%, and the thickness is 30 μm.

[0044] A bilayer gradient negative electrode sheet was prepared according to the steps in Example 1, and the lithium-added and battery assembled were performed according to the method in Example 8. Test results showed that the electrode sheet had an intact structure, good interlayer bonding, and a peel strength of not less than 0.4 N / cm; the initial coulombic efficiency of the battery was 88%, which was significantly improved compared to the initial coulombic efficiency of 82% in Comparative Example 1 without solid electrolyte.

[0045] Example 4: The gradient solid electrolyte negative electrode sheet was prepared according to the method in Example 1, with the difference being the raw material ratio and process parameters of each layer as follows.

[0046] In the first layer of dry powder mixture, by mass percentage: the negative electrode active material is graphite, with a content of 83%; the conductive agent is Super P, with a content of 8%; the solid electrolyte is Li6PS5Cl, with a content of 5%; and the binder is PTFE, with a content of 4%. The compaction density of the first active material layer is 2.0 g / cm³, the porosity is 15%, and the thickness is 80 μm.

[0047] In the second layer of dry powder mixture, by mass percentage: the negative electrode active material is graphite, with a content of 62%; the conductive agent is Super P, with a content of 5%; the solid electrolyte is Li6PS5Cl, with a content of 28%; and the binder is PTFE, with a content of 5%. The compaction density of the second active material layer is 1.4 g / cm³, the porosity is 30%, and the thickness is 150 μm.

[0048] A bilayer gradient negative electrode sheet was prepared according to the steps in Example 1, and the lithium-added and battery assembled according to the method in Example 8. Test results show that the electrode sheet has an intact structure, good interlayer bonding, and a peel strength of not less than 0.5 N / cm; the initial coulombic efficiency of the battery is 91%, and the capacity retention rate is not less than 87% after 500 cycles.

[0049] Example 5: A gradient solid-state electrolyte negative electrode sheet was prepared according to the method of Example 1, except that the solid electrolyte in the first and second layers was replaced with a halide-based solid electrolyte, Li3YCl6. In the first layer of dry powder mixture, by mass percentage: the negative electrode active material was graphite (88%); the conductive agent was Super P (5%); the solid electrolyte was Li3YCl6 (3%); and the binder was PTFE (4%). In the second layer of dry powder mixture, by mass percentage: the negative electrode active material was graphite (70%); the conductive agent was Super P (5%); the solid electrolyte was Li3YCl6 (20%); and the binder was PTFE (5%). The first active material layer had a compaction density of 1.8 g / cm³, a porosity of 20%, and a thickness of 50 μm; the second active material layer had a compaction density of 1.2 g / cm³, a porosity of 40%, and a thickness of 80 μm. The remaining process parameters were the same as in Example 1. Lithium replenishment and battery assembly were performed according to the method of Example 8, and the resulting battery had an initial coulombic efficiency of 90%.

[0050] Example 6: A gradient solid-state electrolyte negative electrode sheet was prepared according to the method of Example 1, the difference being that the first solid-state electrolyte layer was LATP and the second solid-state electrolyte layer was LLZO. In the first layer of dry powder mixture, by mass percentage: the negative electrode active material was graphite (88%); the conductive agent was Super P (5%); the solid-state electrolyte was LATP (3%); and the binder was PTFE (4%). In the second layer of dry powder mixture, by mass percentage: the negative electrode active material was graphite (70%); the conductive agent was Super P (5%); the solid-state electrolyte was LLZO (20%); and the binder was PTFE (5%). The compacted density of the first active material layer was 1.8 g / cm³, the porosity was 20%, and the thickness was 50 μm; the compacted density of the second active material layer was 1.2 g / cm³, the porosity was 40%, and the thickness was 80 μm.

[0051] Since both layers use oxide-based solid electrolytes, the difference in compressibility is relatively small. Therefore, this embodiment uses the layered pre-compression-light compression composite method of Example 1. The first layer is pre-compressed to 1.8 g / cm³, and the second layer is pre-compressed to 1.2 g / cm³, followed by light compression composite at a linear pressure of 30 N / mm and a temperature of 50 °C. The resulting electrode structure is intact, with good interfacial bonding. After lithium replenishment according to the method of Example 8, the battery's initial coulombic efficiency is 89%.

[0052] Example 7: A gradient solid-state electrolyte negative electrode sheet was prepared according to the method of Example 1, except that the negative electrode active material in the first and second layers was replaced with hard carbon, with a D50 of 15 μm. The hard carbon content in the first layer dry powder mixture was 88%, and the hard carbon content in the second layer dry powder mixture was 70%, with the remaining components and proportions the same as in Example 1. The compaction density of the first active material layer was 1.8 g / cm³, the porosity was 20%, and the thickness was 50 μm; the compaction density of the second active material layer was 1.2 g / cm³, the porosity was 40%, and the thickness was 80 μm. Lithium replenishment and battery assembly were performed according to the method of Example 8, and the resulting battery was suitable for fast-charging lithium-ion batteries.

[0053] Example 8: Take the semi-finished double-layer gradient negative electrode sheet obtained in Example 1. This semi-finished product has not yet been provided with a lithium metal supplement layer. See Figure 3 An ultrathin lithium foil with a thickness of 10 μm is laminated onto the surface of the second active material layer (the surface layer) using a roll forming device. The roll forming line pressure is 50–200 N / mm, and the operation is carried out at room temperature. Since the second active material layer is rich in solid electrolyte (20% content), it has formed a three-dimensional ion-conducting network. The added metallic lithium can quickly diffuse into the electrode through the efficient ion transport channels of the second layer, fully contacting and reacting with the internal active material.

[0054] After lithium replenishment during rolling, the lithium-replenished negative electrode sheet is placed in an argon-atmosphere glove box at 25°C for 24 hours. The oxygen content and water content in the atmosphere are not higher than 1 ppm, which promotes further diffusion and reaction of lithium into the electrode sheet. After resting, the negative electrode sheet is cut according to the cell design size and dried under vacuum at 100°C for 8 hours to obtain the finished graded solid electrolyte negative electrode sheet after lithium replenishment.

[0055] Example 9: The semi-finished double-layer gradient negative electrode sheet obtained in Example 2 was used. This semi-finished product has not yet been provided with a lithium metal supplementation layer. See Figure 3 Lithium replenishment is performed via physical vapor deposition in a vacuum lithium plating apparatus. The vacuum level of the apparatus is evacuated to no higher than 5 × 10⁻⁶. -4 Pa evaporates metallic lithium by resistance heating and deposits a metallic lithium layer a on the surface of the second active material layer, i.e., the surface layer, at a deposition rate of 0.3 nm / s and a deposition thickness of 2 μm.

[0056] After PVD lithium replenishment, the lithium-replenished negative electrode sheet is placed in a vacuum environment at 30°C for 12 hours, with the vacuum degree not exceeding -90kPa, to promote further diffusion and reaction of lithium into the electrode sheet. After resting, the negative electrode sheet is cut according to the cell design size and dried in a vacuum at 100°C for 8 hours to obtain the finished graded solid electrolyte negative electrode sheet after lithium replenishment.

[0057] Example 10: The method of Example 8 was followed for calendering and lithium replenishment, except that the ultrathin lithium foil was replaced with an ultrathin lithium strip with a thickness of 5 μm, which was then laminated onto the surface of the second active material layer by a rolling equipment with a rolling line pressure of 100 N / mm. The remaining process parameters were the same as in Example 8.

[0058] Example 11: PVD lithium replenishment was performed according to the method of Example 9, except that the deposition thickness of the lithium metal layer a was 0.5 μm and the deposition rate was 0.2 nm / s, while the other process parameters were the same as in Example 9.

[0059] Example 12: The lithium-added gradient solid electrolyte negative electrode obtained in Example 8 is combined with a positive electrode such as a lithium iron phosphate positive electrode or a ternary positive electrode, a separator such as a polypropylene separator or a polyethylene separator, and an electrolyte such as a LiPF6 / EC+DEC electrolyte to form a button cell or a pouch cell according to conventional processes. After formation and aging treatment, electrochemical performance is tested.

[0060] Example 13: The lithium-added gradient solid electrolyte negative electrode sheet obtained in Example 9 is stacked, hot-pressed and packaged with the positive electrode sheet and solid electrolyte membrane such as sulfide solid electrolyte membrane according to the assembly process of all-solid-state battery to obtain an all-solid-state battery.

[0061] Test Example 1: The lithium-added gradient solid electrolyte negative electrode sheet prepared in Example 8 was sliced ​​along its thickness direction. X-ray photoelectron spectroscopy (XPS) or time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to characterize the lithium distribution at different depths of the cross-section. The test results showed that the lithium element was uniformly distributed along the electrode thickness direction, with no excessive lithium addition on the surface or insufficient lithium addition in the inner layer. The lithium content deviation between the surface and inner layers was less than 5%.

[0062] Test Example 2: The lithium-replenished gradient solid electrolyte negative electrode sheet prepared in Example 8 was assembled with the positive electrode sheet to form a coin cell, and its first charge-discharge test was conducted. The test conditions were: constant current charging at 0.1C to 4.2V, then constant voltage charging at 4.2V until the current drops to 0.05C, and then constant current discharging at 0.1C to 2.5V. The test results showed that the battery using the gradient solid electrolyte negative electrode sheet of the present invention had an initial coulombic efficiency of no less than 90%, which was no more than 85% compared to the control battery without the gradient structure, indicating a significant improvement in lithium replenishment efficiency.

[0063] The mechanism behind the improved lithium replenishment efficiency lies in the fact that this invention transforms the lithium replenishment reaction from the traditional slow diffusion in the solid phase to accelerated transport by an ion conductor. The three-dimensional ion-conducting network formed by the solid electrolyte on the surface provides a continuous ion transport channel for the added lithium metal, allowing lithium ions to bypass the solid-solid interface impedance between active material particles and diffuse rapidly along the ion-conducting network to all parts of the electrode, thus fundamentally solving the technical problem of slow lithium replenishment reaction rate.

[0064] Test Example 3: The lithium-added gradient solid electrolyte negative electrode sheet prepared in Example 8 was assembled with the positive electrode sheet to form a pouch battery, and cycle life testing was conducted. The test conditions were: 1C / 1C charge / discharge, voltage range of 2.5–4.2V, 500 cycles. The test results showed that the battery retained no less than 85% of its capacity after 500 cycles, exhibiting excellent cycle stability.

[0065] Test Example 4: Gradient solid electrolyte negative electrode sheets were prepared using the methods of Examples 1 and 2, respectively, and lithium replenishment and battery assembly were performed according to the method of Example 8. Peel strength and lithium replenishment uniformity tests were conducted on the electrodes prepared by both methods. The results showed that the interlayer peel strength of the electrode prepared in Example 1 was not less than 0.5 N / cm, and the interlayer peel strength of the electrode prepared in Example 2 was not less than 0.4 N / cm. Regarding lithium replenishment uniformity, the deviation of lithium content between the surface and inner layers was less than 5% for both methods. Example 1 offers greater density controllability, while Example 2 has a shorter process; the choice can be made based on actual production line conditions.

[0066] Comparative Example 1: The negative electrode sheet was prepared according to the method of Example 1, except that no solid electrolyte was added to either the first or second layer, i.e., the solid electrolyte content was 0%. All other process parameters were the same as in Example 1. Lithium replenishment and battery assembly were performed according to the method of Example 8. Test results showed that, due to the lack of a three-dimensional ionic conductive network constructed by a solid electrolyte, the diffusion rate of the added metallic lithium inside the electrode sheet was significantly reduced. After lithium replenishment, the lithium element distribution in the electrode sheet cross-section was uneven, with a lithium content deviation between the surface and inner layers exceeding 20%. The initial coulombic efficiency of the battery was only 82%, lower than the 90% of Example 8.

[0067] Comparative Example 2: The negative electrode sheet was prepared according to the method of Example 1, the difference being that the content of solid electrolyte in the first and second layers was the same, both at 10%, without forming a content gradient. The remaining process parameters were the same as in Example 1. Lithium-addition and battery assembly were performed according to the method of Example 8. Test results showed that although the incorporation of solid electrolyte improved ion conduction to some extent, the lack of gradient design and the excessively high content of solid electrolyte in the first layer (inner layer) reduced electronic conductivity and electrode compaction density. Consequently, the rate performance and cycle stability of the battery were lower than in Example 8.

[0068] Comparative Example 3: The gradient solid electrolyte negative electrode sheet was prepared according to the method of Example 1, and lithium replenishment was performed by rolling according to the method of Example 8, except that no resting treatment was performed after lithium replenishment, and the battery was directly assembled. The test results showed that, due to the insufficient diffusion and reaction of the added metallic lithium, there was residual metallic lithium on the electrode surface, and the initial coulombic efficiency of the battery was only 86%, lower than 90% in Example 8, and the cycle stability was significantly reduced.

[0069] In summary, this invention constructs a double-layer structure with a gradient distribution of solid electrolyte content along the thickness of the negative electrode sheet. Specifically, the high-content solid electrolyte on the surface forms a three-dimensional ion-conducting network, while the low-content solid electrolyte on the inner layer ensures high electronic conductivity and mechanical strength. Combined with external rolling or PVD lithium replenishment technology, this achieves rapid and uniform transport and reaction of added metallic lithium, effectively solving the technical problems of slow lithium replenishment reaction rate, uneven lithium replenishment, and residual metallic lithium in existing lithium replenishment technologies. The gradient solid electrolyte negative electrode sheet provided by this invention has advantages such as high lithium replenishment efficiency, high initial coulombic efficiency, excellent cycle performance, and good safety. Its preparation process is highly compatible with existing dry electrode production lines, making it suitable for large-scale industrial production and showing promising application prospects.

[0070] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A gradient solid-state electrolyte negative electrode sheet, characterized in that, include: current collector; A first active material layer is disposed on the surface of the current collector. The first active material layer contains a negative electrode active material and a conductive agent. The content of solid electrolyte is 0% to 5%, the compaction density is 1.6 to 2.0 g / cm³, and the porosity is 15% to 25%. A second active material layer is disposed on the surface of the first active material layer. The second active material layer comprises a negative electrode active material, a conductive agent, and a solid electrolyte. The content of the solid electrolyte is 10% to 30%, the compaction density is 1.0 to 1.4 g / cm³, and the porosity is 30% to 50%. A lithium metal replenishment layer is disposed on the surface of the second active material layer.

2. The gradient solid-state electrolyte negative electrode sheet according to claim 1, characterized in that, The solid electrolyte is one or more of a sulfide system solid electrolyte, a halide system solid electrolyte, or an oxide system solid electrolyte; the thickness of the first active material layer is 20-80 μm, and the thickness of the second active material layer is 30-150 μm.

3. A method for lithium replenishment of the gradient solid electrolyte negative electrode sheet as described in claim 1 or 2, characterized in that, Includes the following steps: Lithium replenishment step: A lithium metal layer is introduced on the surface of the second active material layer by means of rolling lithium replenishment or physical vapor deposition lithium replenishment to form a lithium metal replenishment layer. Resting procedure: After lithium replenishment, the negative electrode sheet is placed in a vacuum or inert atmosphere for a period of time.

4. The lithium replenishment method according to claim 3, characterized in that, The method of calendering for lithium replenishment includes: applying a lithium strip or foil with a thickness of 1 to 20 μm to the surface of the second active material layer using a rolling equipment, wherein the rolling pressure is 50 to 200 N / mm.

5. The lithium replenishment method according to claim 3, characterized in that, The physical vapor deposition lithium replenishment method includes: in a vacuum degree ≤ 5 × 10 -4 A lithium metal layer is deposited at a Pa environment with a deposition rate of 0.1–0.5 nm / s and a thickness of 0.1–5 μm.

6. A method for preparing a gradient solid electrolyte negative electrode sheet, characterized in that, Includes the following steps: S1: Prepare a first dry powder mixture, wherein the first dry powder mixture contains 80% to 95% of negative electrode active material, 1% to 10% of conductive agent, 0% to 5% of solid electrolyte and 1% to 5% of binder; S2: Prepare a second dry powder mixture, which contains 60% to 85% of negative electrode active material, 1% to 10% of conductive agent, 10% to 30% of solid electrolyte and 1% to 5% of binder. S3: The first dry powder mixture is shaped after being treated with fiber to form a first active material layer. The compacted density of the first active material layer is 1.6 to 2.0 g / cm³, and the porosity is 15% to 25%. S4: Apply the second dry powder mixture to the surface of the first active material layer and form it to form a second active material layer. The compaction density of the second active material layer is 1.0 to 1.4 g / cm³, and the porosity is 30% to 50%. S5: The first active material layer and the second active material layer are composited on the surface of the current collector and rolled to obtain a negative electrode sheet with a double-layer gradient structure.

7. The preparation method according to claim 6, characterized in that, Steps S3 and S4 specifically include: rolling the first dry powder mixture to a compaction density of 1.6-2.0 g / cm³ after fiberization treatment to form a first active material layer self-supporting film; rolling the second dry powder mixture to a compaction density of 1.0-1.4 g / cm³ after fiberization treatment to form a second active material layer self-supporting film; and stacking the first active material layer self-supporting film and the second active material layer self-supporting film together, and then performing light pressing composite with a linear pressure of 10-50 N / mm and a temperature of 40-60℃.

8. The preparation method according to claim 6, characterized in that, Step S4 specifically includes: spreading the first dry powder mixture on the surface of the current collector and lightly pressing it to shape it; spreading the second dry powder mixture on the surface of the first dry powder mixture; and performing a single roller press with a linear pressure of 100-200 N / mm to compact the first dry powder mixture to 1.6-2.0 g / cm³ and the second dry powder mixture to 1.0-1.4 g / cm³.

9. A battery, characterized in that, It includes a positive electrode, a separator or solid electrolyte membrane, an electrolyte, and a gradient solid electrolyte negative electrode as described in any one of claims 1 to 2.

10. A battery, characterized in that, It includes a positive electrode, a separator or solid electrolyte membrane, an electrolyte, and a gradient solid electrolyte negative electrode prepared by the preparation method of any one of claims 6 to 8.

Images