A negative electrode sheet, its preparation method and application

Abstract

This invention provides a negative electrode sheet, its preparation method, and its application, relating to the field of lithium battery technology. The preparation method includes the step of forming an active material layer on a carbon-coated copper foil using a negative electrode slurry containing a sulfide electrolyte. Before forming the active material layer, adhesive tape is applied to both sides of the carbon-coated copper foil at the locations where the tabs are to be welded. This invention isolates the areas of the carbon-coated copper foil where the tabs are to be welded by applying adhesive tape before coating the negative electrode slurry containing the sulfide electrolyte onto the carbon-coated copper foil. This tape not only prevents the sulfide electrolyte from directly contacting the carbon-coated copper foil at the locations where the tabs are to be welded, preventing oxidation of the copper foil and tabs, but also avoids the risk of poor tab soldering, thus improving cell performance.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a negative electrode sheet, its preparation method, and its application. Background Technology

[0002] With technological advancements, various fields are placing higher demands on the battery life of lithium batteries. For the same mass or volume, lithium batteries with higher energy density offer better range. However, the risk of thermal runaway in high-energy-density liquid lithium batteries is also gradually increasing. The flammable, easily oxidized, and chemically reactive properties of the electrolyte make lithium batteries highly susceptible to combustion and explosion should thermal runaway occur.

[0003] Solid-state lithium batteries use solid electrolytes instead of liquid electrolytes, offering significant advantages in safety. Compared to liquid electrolytes, solid electrolytes are non-flammable, heat-resistant, and chemically less reactive. They also possess a certain degree of mechanical strength, which can withstand external stress and reduce the risk of thermal runaway, thus greatly improving battery safety. Solid electrolytes are mainly of three types: polymers, oxides, and sulfides. Among them, sulfides have the highest lithium-ion conductivity, with some sulfides even achieving or exceeding the conductivity of traditional liquid electrolytes. Therefore, the application of sulfides in solid-state batteries is highly anticipated.

[0004] Currently, the mainstream application of sulfide electrolytes is to mix them with positive and negative electrode materials and then coat them onto the current collector. Existing negative electrode current collectors are mainly copper foil, but copper foil reacts with sulfides, causing it to oxidize and affecting its function as a current collector. This, in turn, affects the capacity of the negative electrode and ultimately the performance of the battery cell.

[0005] CN113764625A discloses a method for preparing a sulfide silicon-based negative electrode sheet. This method involves depositing a graphite buffer layer on the surface of a copper foil to prevent direct contact between the copper foil and the sulfide electrolyte. However, when coating the slurry containing the sulfide electrolyte, the area on the copper foil to be welded is also coated. This poses a risk of incomplete soldering during subsequent electrode welding, leading to abnormal current during cell charging and discharging and reducing cell performance. Summary of the Invention

[0006] This invention provides a negative electrode sheet and its preparation method to solve the above-mentioned problems existing in the prior art.

[0007] In a first aspect, the present invention provides a method for preparing a negative electrode sheet, comprising the step of forming an active material layer on a carbon-coated copper foil using a negative electrode slurry containing a sulfide electrolyte, wherein before forming the active material layer, adhesive tape is respectively attached to the portions on both sides of the carbon-coated copper foil used for welding tabs, and the active material layer is formed on the carbon-coated copper foil located between the adhesive tapes.

[0008] Before coating the negative electrode slurry containing sulfide electrolyte onto the carbon-coated copper foil, the present invention isolates the area of ​​the carbon-coated copper foil to be welded by applying adhesive tape. This tape can prevent the sulfide electrolyte from directly contacting the area of ​​the carbon-coated copper foil to be welded, thus preventing oxidation of the copper foil and the tab at that location. It can also avoid the risk of poor soldering, improve cell performance, and prevent current disturbances during charging and discharging. Furthermore, the tape can be removed during subsequent processing, facilitating subsequent production.

[0009] Furthermore, during the coating process, the negative electrode slurry is coated onto the tape and onto the side close to the active material layer, so that the interface between the tape and the active material layer is coated with the negative electrode slurry, and a tape coating layer is formed on the tape.

[0010] This invention has discovered that, during coating, the absence of adhesive application not only fails to effectively prevent oxidation of the carbon-coated copper foil at the electrode tabs to be welded, but also results in the edge thickness of the coated copper foil being less than that of the center. This means the edge thickness of the active material layer is less than its central thickness, creating a thinned area at the edge of the copper foil coated with the negative electrode paste. This compromises the consistency of the electrode thickness and areal density, reducing cell performance. However, this invention addresses this problem by applying adhesive before coating and further coating the negative electrode paste onto the tape close to the active material layer. After removing the tape in subsequent processing, the thickness at the interface between the active material layer and the tape (i.e., the edge of the active material layer) is consistent with the central thickness. This eliminates the thinned edges, improves the consistency of the active material layer density, and ensures the consistency of the electrode thickness and areal density, thereby improving cell performance.

[0011] Furthermore, the width of the tape coating layer is 2-5 mm. This invention has found that when the width of the tape coating layer is controlled within the aforementioned reasonable range, it can achieve a better effect in removing thinned edges, essentially eliminating the thinned coating area, ensuring uniform electrode surface density, and improving cell performance. However, excessive width may lead to slurry waste and increased costs.

[0012] Furthermore, the thickness of the carbon-coated copper foil is 3~6 μm. Controlling the thickness of the carbon-coated copper foil within this range can effectively reduce the risk of the carbon-coated copper foil being torn when the tape is removed later. After the tape is removed, the electrode can still remain intact, ensuring good cell performance.

[0013] Furthermore, the width of the carbon-coated copper foil is greater than the total width of the tape and the active material layer, which can effectively reduce the risk of the carbon-coated copper foil being torn when the tape is removed later. After the tape is removed, the electrode can still remain intact, ensuring good cell performance.

[0014] Furthermore, the negative electrode slurry includes an active material, the content of which is 40-90 wt% of the slurry.

[0015] Preferably, the active material comprises silicon carbon material and the sulfide electrolyte, wherein the mass ratio of the silicon carbon material to the sulfide electrolyte is 1:9 to 9:1. Controlling the mass ratio of silicon carbon material to sulfide electrolyte within the above range can effectively prevent the phenomenon of sulfide electrolyte seeping into the tape.

[0016] Preferably, the sulfide electrolyte has a mass percentage of 1 to 50% in the active material.

[0017] Furthermore, the sulfide electrolyte includes Li 10 GeP2S 12 Li6PS5Cl, Li 3x La 2 / 3-x TiO3 and Li 1+ y Al y Ti 2-y One or more of (PO4)3, wherein 0.04≤x≤0.167, 0≤y≤1; And / or, the silicon-carbon material is a material having one or more of the following structures: silicon-carbon core-shell structure, silicon-carbon porous structure, and silicon-carbon composite microspheres.

[0018] In one optional embodiment of the invention, the negative electrode slurry comprises, by mass percentage, 40-90% of the active material, 5-30% of the binder, and 5-30% of the conductive agent.

[0019] In one optional embodiment of the invention, the active material comprises, by mass percentage, 1-50% of the sulfide electrolyte and 50-99% of the silicon-carbon material.

[0020] Furthermore, the thickness of the adhesive tape is less than or equal to the thickness of the active material layer. If the thickness of the adhesive tape is greater than the thickness of the active layer, it will cause uneven coating density and reduce cell performance.

[0021] Preferably, the thickness of the tape is 1~300 μm.

[0022] Preferably, the tape is made of a high-temperature resistant material that does not react with sulfides, including but not limited to polyester, polyimide and polytetrafluoroethylene.

[0023] Furthermore, the preparation method also includes the steps of drying the coated product, removing the tape, and then rolling and forming the tabs.

[0024] In a second aspect, the present invention provides a negative electrode sheet, which is prepared by the preparation method described above.

[0025] The beneficial effects of the negative electrode sheet, its preparation method, and its application provided by this invention are as follows: Before coating the negative electrode slurry containing sulfide electrolyte onto the carbon-coated copper foil, the location of the electrode tab to be welded on the carbon-coated copper foil is isolated by applying adhesive tape. This tape can prevent the sulfide electrolyte from directly contacting the location of the electrode tab to be welded on the carbon-coated copper foil, preventing the copper foil and electrode tab at this location from being oxidized. It can also avoid the risk of poor electrode tab soldering, improve cell performance (such as charge and discharge capacity, cycle performance, rate performance, etc.), and prevent current disturbances during the charge and discharge process. Furthermore, the tape can be removed during subsequent processing, facilitating subsequent production. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0027] Figure 1 This is a flowchart of the coating process in Example 1.

[0028] Figure 2 This is a structural diagram of the electrode sheet in Example 1.

[0029] Figure 3 This is a structural diagram of the electrode sheet in Example 9.

[0030] Figure 4 This is a structural diagram of the electrode sheet in Example 10.

[0031] Figure label: 1: Carbon-coated copper foil; 2: Active material layer; 3: Adhesive tape; 4: Adhesive tape coating layer. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0033] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0034] The following are the raw material information used in the examples and comparative examples: Silicon-carbon material: SL450B-SC nano silicon-carbon composite material, purchased from Shenzhen Kejing Zhida Technology Co., Ltd.

[0035] Sulfide electrolyte: LPSCl-Li6PS5Cl 5-10um sulfide solid electrolyte, purchased from Shenzhen Kejing Zhida Technology Co., Ltd.

[0036] Example 1 This embodiment provides a method for preparing a negative electrode sheet, the steps of which are as follows: (1) Preparation of negative electrode slurry: Thermoplastic styrene-butadiene rubber (SEBS) and butyl butyrate solution were weighed and mixed at a mass ratio of 10:90 to obtain a slurry; then the above slurry, conductive agent (VGCF) and active material were weighed and stirred at a mass ratio of 5:5:90. The active material is composed of silicon carbon material and sulfide electrolyte at a mass ratio of 50:50. (2) Coating: The flowchart of the coating process is as follows Figure 1 As shown, the carbon-coated copper foil 1 is unwound by the foil unwinding mechanism; the tape unwinding mechanism is used to smooth and stretch the tape 3, and the carbon-coated foil and tape (made of polyester material) are compounded at the clamping rollers. The tape is then pasted onto the two sides of the carbon-coated copper foil where the electrode tabs are to be welded; then the negative electrode slurry is coated onto the carbon-coated copper foil located between the tapes using the coating mechanism to form the active material layer 2. The carbon-coated copper foil has a thickness of 6 μm and a width of 220 mm, the tape has a width of 20 mm and a thickness of 40 μm, and the active material layer has a width of 160 mm and a thickness of 150 μm. (3) Drying: The coated product is dried in ovens at 50℃, 60℃ and 70℃ in sequence, and the conveyor belt speed during product transportation is 1 m / min, to obtain the following results: Figure 2 The electrode shown; (4) Rolling: First, remove the tape on the electrode sheet, then roll and form the tabs to obtain the negative electrode sheet.

[0037] Example 2 This embodiment is basically the same as Embodiment 1, except that the mass ratio of silicon-carbon material to sulfide electrolyte in the active material is 60:40.

[0038] Example 3 This embodiment is basically the same as Embodiment 1, except that the mass ratio of silicon-carbon material to sulfide electrolyte in the active material is 70:30.

[0039] Example 4 This embodiment is basically the same as Embodiment 1, except that the mass ratio of silicon carbide material to sulfide electrolyte in the active material is 80:20.

[0040] Example 5 This embodiment is basically the same as Embodiment 1, except that the mass ratio of silicon carbide material to sulfide electrolyte in the active material is 90:10.

[0041] Example 6 This embodiment is basically the same as Embodiment 1, except that the thickness of the carbon-coated copper foil is 4.5 μm.

[0042] Example 7 This embodiment is basically the same as Embodiment 1, except that the thickness of the carbon-coated copper foil is 3.5 μm.

[0043] Example 8 This embodiment is basically the same as Embodiment 1, except that the thickness of the carbon-coated copper foil is 3 μm.

[0044] Example 9 This embodiment is basically the same as Embodiment 1, except that adhesive tape is also pasted in the middle of the carbon-coated copper foil. The width of the carbon-coated copper foil is 380 mm, and two active material layers are applied, each with a width of 80 mm. The width of each adhesive tape is 20 mm. The formed electrode sheet is as follows: Figure 3 As shown.

[0045] Example 10 This embodiment is basically the same as Embodiment 1, except that during the coating process, the negative electrode slurry is also coated on the tape and on the side close to the active material layer, so that the negative electrode slurry is coated at the junction of the tape and the active material layer, and a tape coating layer 4 is formed on the tape.

[0046] The tape is 25 mm wide, and the tape coating is 5 mm wide. The resulting electrode is as follows: Figure 4 As shown.

[0047] Example 11 This embodiment is basically the same as embodiment 10, except that the width of the tape is 24 mm and the width of the tape coating layer is 4 mm.

[0048] Example 12 This embodiment is basically the same as embodiment 10, except that the width of the tape is 23 mm and the width of the tape coating layer is 3 mm.

[0049] Example 13 This embodiment is basically the same as embodiment 10, except that the width of the tape is 22 mm and the width of the tape coating layer is 2 mm.

[0050] Comparative Example This comparative example is basically the same as Example 1, except that: no adhesive is applied before coating, and the negative electrode slurry is directly coated onto the carbon-coated copper foil.

[0051] The performance of the negative electrode sheets prepared in the embodiments and comparative examples of this invention was measured, and the test methods are as follows: Electrode oxidation detection: Visual inspection is performed. Oxidized electrodes appear black, while unoxidized electrodes appear the normal color of copper foil.

[0052] Electrode integrity inspection: Visual inspection is performed. If the electrode is cracked or the active material layer is peeled off, the electrode is damaged. Otherwise, it is not damaged.

[0053] Tab resistance test: A four-probe resistance meter is used for testing. Take a 50mm section of the tab to be tested, lay it flat on the sample stage, and test every 10mm. After 5 tests, take the average reading.

[0054] Battery short circuit rate: The number of short-circuited cells (i.e., abnormal voltage drop) is counted after battery assembly and calculated by comparing it with the total number of cells.

[0055] Maximum thinning thickness at the edge of the electrode: Take the coated electrode and test the thickness 30mm from the boundary of the active material. Take the minimum value and repeat the above operation three times to get the average value.

[0056] Normal area thickness: The thickness of the middle area of ​​the same electrode coating area as above is measured, and the average value is taken after three measurements.

[0057] The results are shown in Table 1: Comparing Examples 1-8 and Comparative Example 1, the adhesive application step introduced before coating not only effectively prevents oxidation of the negative electrode tabs of the solid-state battery cell, but is also applicable to negative electrode slurries composed of different thicknesses of carbon-coated copper foil and different mass ratios of silicon-carbon materials to sulfide electrolytes; after the adhesive tape is removed, the negative electrode sheet remains intact. This indicates that the preparation method of the present invention can effectively avoid the oxidation of the tabs, further improve the battery cell performance, and prevent current disturbances during charging and discharging.

[0058] Compared with Examples 10-13 and Example 1, simultaneously coating the negative electrode slurry onto the tape during coating can, to some extent, improve the presence of the thinned active material area of ​​the negative electrode sheet, thereby ensuring the consistency of the negative electrode sheet's surface density and further guaranteeing the subsequent cycle performance of the battery cell. Table 1 shows that the percentage of the maximum thinned edge thickness of the negative electrode sheet prepared in Examples 10-13 to the normal area thickness is close to 100%. The closer this percentage is to 100%, the smaller the thickness difference between the edge of the negative electrode sheet and other parts. This indicates that when the width of the tape coating layer is 2-5 mm, the width of the tape coating layer is 1 / 11 to 1 / 5 of the tape width, which can essentially eliminate the thinned area (the edge region of the active material layer), ensuring the consistency of the negative electrode sheet's thickness and surface density. This allows the battery cell to remain flat under high pressure, ensuring complete contact between the positive and negative electrodes, guaranteeing ion conduction, reducing the battery's short-circuit rate, and improving battery cell performance.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a negative electrode sheet, characterized in that, The process includes the step of forming an active material layer on a carbon-coated copper foil using a negative electrode slurry containing a sulfide electrolyte. Before forming the active material layer, adhesive tape is applied to the portions of the carbon-coated copper foil on both sides for welding the tabs.

2. The method for preparing the negative electrode sheet according to claim 1, characterized in that, During the coating process, the negative electrode slurry is applied to the tape and to the side close to the active material layer, so that the interface between the tape and the active material layer is coated with the negative electrode slurry, and a tape coating layer is formed on the tape.

3. The method for preparing the negative electrode sheet according to claim 2, characterized in that, The width of the tape coating layer is 2~5 mm.

4. The method for preparing the negative electrode sheet according to any one of claims 1 to 3, characterized in that, The thickness of the carbon-coated copper foil is 3~15 μm; Preferably, the width of the carbon-coated copper foil is greater than the total width of the tape and the active material layer.

5. The method for preparing the negative electrode sheet according to any one of claims 1 to 4, characterized in that, The negative electrode slurry includes an active material, the content of which is 40-90 wt% of the slurry. Preferably, the active material comprises silicon-carbon material and the sulfide electrolyte, wherein the mass ratio of the silicon-carbon material to the sulfide electrolyte is 1 to 9:1; Preferably, the sulfide electrolyte has a mass percentage of 1 to 50% in the active material.

6. The method for preparing the negative electrode sheet according to claim 5, characterized in that, The sulfide electrolyte comprises Li 10 GeP2S 12 , Li6PS5Cl, Li 3x La 2 / 3-x TiO3and Li 1+y Al y Ti 2-y (PO4)3, wherein 0.04≤x≤0.167, 0≤y≤1; And / or, the silicon-carbon material is a material having one or more of the following structures: silicon-carbon core-shell, silicon-carbon porous, and silicon-carbon composite microspheres.

7. The method for preparing the negative electrode sheet according to any one of claims 1 to 6, characterized in that, The thickness of the tape is less than or equal to the thickness of the active material layer; Preferably, the thickness of the tape is 1~300 μm.

8. The method for preparing the negative electrode sheet according to any one of claims 1 to 7, characterized in that, It also includes steps such as drying the coated product, removing the tape, and then rolling and forming the tabs.

9. A negative electrode sheet, characterized in that, It is prepared by the preparation method described in any one of claims 1 to 8.

10. The negative electrode sheet prepared by the preparation method according to any one of claims 1 to 8 or the negative electrode sheet according to claim 9, in the preparation of lithium batteries.

Images