An electrode sheet, a method for manufacturing an electrode sheet, and a battery

Abstract

The application discloses a kind of pole piece, the preparation method of pole piece and battery, belong to battery technical field.The pole piece includes current collector and liquid retaining layer, current collector and liquid retaining layer are equipped with active layer between, active layer is equipped with micropore, liquid retaining layer covers micropore;The hole spacing D between any two adjacent holes is =kd+w, wherein k is safety factor, the value range of k is 3~5, d is pore size, d is 10~100 μm, w is heat affected zone width, w is not more than 100 μm;Pore depth is 40%~80% of active layer thickness.The application considers the influence of heat affected zone generated when micropore is processed by hot processing on the structure strength around micropore when confirming hole spacing, establishes the empirical formula between hole spacing and pore size, heat affected zone width these two parameters, and sets pore depth as 40%~80% of active layer thickness, so that purple stain elimination rate reaches more than 85%, highest can reach 92.5%, simultaneously, the capacity retention of battery can also be improved to more than 90%.

Description

Technical Field

[0001] This invention belongs to the field of battery technology, and more specifically, relates to an electrode, a method for preparing the electrode, and a battery. Background Technology

[0002] In the use of wound lithium-ion batteries, purple spots often appear in the outer R-corner area (battery corner) of the negative electrode sheet. This phenomenon manifests as purplish-black or dark purple patches on the surface of the negative electrode active material. The purple spots are essentially localized areas of graphite where lithium cannot be properly intercalated, accompanied by the precipitation of metallic lithium (lithium plating). Purple spots at the R-corner not only lead to cell capacity loss and reduced cycle life, but may also cause localized micro-short circuits and thermal runaway risks.

[0003] Existing technical analysis shows that the main causes of purple spots at the R-corner include: (1) the bending stress of the electrode at the outer R-corner after winding is large, and the diaphragm and the electrode form a micro gap, making it difficult for the electrolyte to fully wet; (2) during charge and discharge cycles, the electrolyte consumption rate in the R-corner area is much higher than in other areas due to the concentrated current density, resulting in a "dry zone" in the later stage; (3) the surface of traditional electrode is dense and lacks a liquid storage buffer structure, which cannot provide a continuous electrolyte replenishment for the R-corner area.

[0004] Existing technologies often mitigate the formation of purple spots by improving the electrode structure. For example, Chinese invention patent application CN116742165A discloses an electrode and an electrochemical device containing the same. The electrode includes a current collector and an active layer disposed on one or both sides of the current collector. Grooves are provided in local areas of the active layer. By providing grooves, the liquid retention per unit area of ​​the electrode is increased, which delays the formation of purple spots and lithium plating, thereby improving the cycle life of the battery.

[0005] However, the aforementioned and similar patents suffer from a decrease in the strength of the active layer around the groove, which can easily lead to breakage and peeling of the active layer, making it difficult to fully realize the effect of setting the groove in improving the purple spot elimination rate and capacity retention rate. Summary of the Invention

[0006] 1. The problem to be solved To address the problem that the reduced strength of the active layer around the micropores in existing battery electrodes may lead to difficulties in fully improving the purple spot elimination rate and capacity retention rate, this invention provides an electrode that proposes an empirical formula relating the pore spacing to the pore diameter and the width of the heat-affected zone, and controls the pore depth within a reasonable range to obtain the optimal purple spot elimination rate and capacity retention rate.

[0007] 2. Technical Solution To solve the above problems, the technical solution adopted by the present invention is as follows: The first aspect of the present invention provides an electrode, comprising a current collector and a liquid retention layer, wherein an active layer is disposed between the current collector and the liquid retention layer, the active layer having micropores, and the liquid retention layer covering the micropores, wherein the spacing between any two adjacent pores is D=kd+w, where k is a safety factor, the value of k ranges from 3 to 5, d is the pore diameter, d is from 10 to 100 μm, and w is the width of the heat-affected zone, w is not greater than 100 μm; the pore depth is 40% to 80% of the thickness of the active layer.

[0008] When setting micropores within the active layer, the porosity is commonly used to investigate its impact on results. The porosity reflects the relationship between pore spacing and pore diameter. However, when using thermal processing to create micropores, the significant changes in the microstructure surrounding the micropores caused by heat, forming a heat-affected zone (HAZ), are often overlooked. The decreased mechanical strength within the HAZ increases the risk of cracking in the active layer during cycling. If the pore spacing is improperly set, cracking of the active layer can occur, making it difficult to fully realize the benefits of micropores in improving purpura elimination and capacity retention. Therefore, when using thermal processing, the pore spacing should be determined by considering both the width of the HAZ and the pore diameter, rather than solely considering pore diameter as in cold processing. With an appropriate pore spacing, a reasonable ratio between pore depth and active layer thickness can increase the liquid retention capacity of the active layer, further improving purpura elimination and capacity retention.

[0009] More preferably, k is any value taken from the following range: 3~3.3, 3~3.5, 3~4, 3~4.5, 3.3~3.5, 3.3~4, 3~4.3, 3.3~4.5, 3.3~5, 4~4.3, 4~5, 4.3~5.

[0010] More preferably, d is any value taken from the following range: 10~30μm, 10~35μm, 10~45μm, 10~60μm, 10~70μm, 10~80μm, 20~35μm, 20~45μm, 20~60μm, 20~70μm, 30~35μm, 30~45μm, 30~60μm, 30~70μm.

[0011] More preferably, w is any value taken from the following range: not greater than 90μm, not greater than 80μm, not greater than 70μm, not greater than 60μm, and not greater than 50μm.

[0012] More preferably, the pore depth is 40%, 50%, 57%, 63%, 65%, 75%, 79% or 80% of the active layer thickness.

[0013] Preferably, the micropores are formed by laser etching.

[0014] Laser etching is a precision machining process that uses a high-energy laser beam to remove materials. It has a fast processing speed and can focus a micron-sized spot, making it suitable for the preparation of small-diameter micropores.

[0015] Preferably, the micropores are in the shape of an inverted frustum.

[0016] The electrolyte injection process is driven by both capillary wetting and external pressure. The large pore size of the inverted frustum-shaped micropores reduces the inlet resistance of the electrolyte into the pores, shortens the injection time, and improves the pore filling rate after vacuum settling. If the surface pore size is too small, air bubbles are easily trapped in the pores during injection, forming "dead pores" (no electrolyte filling) and losing their electrolyte retention function. More importantly, the small bottom pore size makes the bottom of the pore closer to the "point source," and when the electrolyte is released from the bottom into the depth of the coating, the flow path is more concentrated, which is beneficial for replenishing electrolyte at the interface between the active layer and the current collector (the point of highest current density).

[0017] Preferably, the ratio of the pore depth to the original N / P ratio of the battery is 35~50μm.

[0018] The original N / P ratio of a battery is an important parameter when designing a battery. This ratio can be used to quickly deduce the appropriate range for hole depth settings, reducing the experimental exploration process.

[0019] More preferably, the ratio of the pore depth to the original N / P of the battery is taken from any of the following numerical ranges: 35~41μm, 35~45μm, 35~47μm, 40~45μm, 40~47μm, 40~50μm, 45~47μm, 45~50μm.

[0020] Preferably, the active layer has a liquid storage channel that communicates with micropores.

[0021] The electrolyte storage channels connect multiple micropores and transport electrolyte from the edge of the electrode to the center of the radius (R-angle), alleviating localized electrolyte shortage. These channels can be created by adding a pore-forming agent to the active material slurry used to prepare the active layer. After the active material slurry is dried, the pore-forming agent decomposes, leaving interconnected channels.

[0022] More preferably, the pore-forming agent is polystyrene.

[0023] Preferably, the electrolyte-retaining layer has localized protrusions located within the micropores. This structure enhances the interaction between the micropores and the electrolyte-retaining layer, thereby increasing the electrolyte content within the micropores.

[0024] Preferably, the thickness of the liquid-retaining layer is 2~10μm and the porosity is 60%~90%.

[0025] The high-porosity liquid-retaining layer not only serves as a surface liquid reservoir, but also continuously introduces the electrolyte into the depths of the micropores through capillary action, forming a three-dimensional continuous liquid phase channel between the surface and the interior.

[0026] Preferably, the liquid-retaining layer is made of one or both of polyvinylidene fluoride and polyimide. The liquid-retaining layer is formed by fabricating the above materials into nanofibers and then attaching them to the surface of the active layer. The diameter of the nanofibers is preferably 50-500 nm.

[0027] A second aspect of the present invention provides a method for preparing an electrode sheet according to any embodiment of the first aspect of the present invention, comprising the following steps: S1. An active material slurry is coated on the current collector, and after drying and rolling, an active layer is formed. S2. Micropores are fabricated by etching on the active layer. The spacing between any two adjacent pores is D = kd + w, where k is a safety factor, and the value of k ranges from 3 to 5; d is the pore diameter, which is 10 to 100 μm; w is the width of the heat-affected zone, which is not greater than 100 μm; and the pore depth is 40% to 80% of the thickness of the active layer. S3. Prepare a liquid-retaining layer on the surface of the active layer using spraying, rolling, or electrospinning processes, so that the liquid-retaining layer covers the micropores.

[0028] Preferably, in step S2, the method for preparing the micropores is to use a nanosecond, picosecond, or femtosecond pulsed laser for etching, with a laser power density of 10. 6 ~10 8 W / cm², laser scanning speed is 100~1000mm / s.

[0029] Nanosecond, picosecond, or femtosecond pulsed lasers have a smaller heat-affected zone, which can minimize the damage to the crystal structure of the active layer. At the same time, the hole depth and shape can be precisely controlled by adjusting the laser energy density.

[0030] Preferably, in step S3, the liquid-retaining layer is prepared using an electrospinning process, wherein the solvent of the spinning solution is one or more of N-methylpyrrolidone, N,N-dimethylformamide, or deionized water, the solute of the spinning solution is 8-15 wt.% of one or two of polyvinylidene fluoride or polyimide, the solid content of the spinning solution is 1%-5%, the receiving distance is 10-20 cm, the voltage is 15-25 kV, and the spinning time is 30-120 s.

[0031] A third aspect of the present invention provides a battery comprising a casing, a top cover, and a battery cell. The casing has an opening and is sealed to the top cover. The battery cell includes a positive electrode, a negative electrode, and a separator located between the positive and negative electrode. The battery cell is wound and placed inside the casing. The casing is filled with an electrolyte. The negative electrode is an electrode prepared according to any embodiment of the first aspect of the present invention or an electrode prepared according to any embodiment of the second aspect of the present invention, and the micropores on the wound negative electrode are located in the R-corner region of the battery cell.

[0032] Preferably, the height of the region containing the micropores is 70% to 100% of the cell height, for example, 75%, 80%, 85%, 90% or 95%; and / or, the ratio between the length of the region containing the micropores and the cell thickness is 3 to 4, for example, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8 or 3.9.

[0033] The height and length of the area containing the micropores can be reasonably set according to the location and area of ​​the purple spots on the battery, so that the micropore area that can increase the liquid storage capacity can cover the purple spot area as much as possible, thereby maximizing the elimination rate of purple spots and the capacity retention rate of the battery.

[0034] 3. Beneficial effects Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The electrode provided by the present invention establishes an electrolyte reservoir by setting micropores in the liquid retention layer and the active layer. When confirming the hole spacing, the influence of the heat-affected zone generated when processing the micropores by hot processing on the structural strength around the micropores is taken into consideration. An empirical formula is established between the hole spacing and the hole diameter and the width of the heat-affected zone. The hole depth is set to 40%~80% of the active layer thickness, so that the purple spot elimination rate reaches more than 85%, and can reach up to 92.5%. At the same time, the battery capacity retention rate can be increased to more than 90%.

[0035] (2) The ratio of the pore depth of the micropores in the electrode provided by the present invention to the original N / P of the battery is 35~50μm. The appropriate setting range of the pore depth can be quickly derived using this ratio, reducing the experimental exploration process.

[0036] (3) The electrolyte retention layer of the electrode provided by the present invention has a protrusion in a certain part, so that the protrusion is located in the micropore, which can enhance the interaction between the micropore and the electrolyte retention layer, increase the electrolyte content in the micropore, and further improve the purple spot elimination rate and capacity retention rate. Attached Figure Description

[0037] Figure 1 This is a cross-sectional schematic diagram of the electrode sheet of the present invention; Figure 2 This is a schematic diagram of the micropores in the active layer of the present invention when the electrode is laid flat; Figure 3 This is a top view of the battery of the present invention; In the diagram: 1. Current collector; 11. Tab; 2. Active layer; 21. Micropore; 22. Liquid storage channel; 3. Liquid retention layer; 31. Protrusion; 4. Battery; 41. Rounded corner area; 42. Flat area. Detailed Implementation

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains; the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0039] Unless otherwise specified in the examples, the procedures should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0040] As used herein, the term “about” is used to provide for the flexibility and imprecision associated with a given term, measure, or value. Those skilled in the art can readily determine the degree of flexibility for a particular variable.

[0041] like Figures 1-2 As shown, the present invention provides an electrode sheet, including a current collector 1 and a liquid retention layer 3. An active layer 2 is provided between the current collector 1 and the liquid retention layer 3. Micropores 21 are provided in the active layer 2. The liquid retention layer 3 covers the micropores 21. The spacing between any two adjacent pores is D=kd+w, where k is a safety factor, the value of k is in the range of 3~5, d is the pore diameter, d is 10~100μm, w is the width of the heat-affected zone, w is not greater than 100μm, and the pore depth is 40%~80% of the thickness of the active layer 2.

[0042] When thermal processing is used, such as laser etching of micropores 21, heat will cause significant changes in the microstructure of the area around micropores 21. Microscopic observation can confirm the extension distance of the area with significant changes along the surface of the active layer 2, and the average value of the extension distance is taken as the width w of the heat-affected zone.

[0043] In some embodiments, the micropore 21 is cylindrical or inverted frustum-shaped. Preferably, the micropore 21 is inverted frustum-shaped, in which case d = (surface pore diameter + bottom pore diameter) / 2. The surface pore diameter is the pore diameter of the micropore 21 at the interface between the active layer 2 and the liquid-retaining layer 3, and the bottom pore diameter is the pore diameter of the end face of the micropore 21 closest to the current collector 1, and the surface pore diameter is larger than the bottom pore diameter.

[0044] In some embodiments, the ratio of the pore depth to the original N / P of the battery 4 is 35~50μm. Here, N / P is the ratio of the negative electrode capacity to the positive electrode capacity of the battery 4, and the original N / P of the battery 4 refers to the N / P of the battery 4 before the micropores 21 are processed into the active layer 2 of the electrode sheet.

[0045] In some embodiments, the thickness of the current collector 1 is preferably 3~9μm, and the material is preferably copper. Lithium ions are difficult to form a lithium intercalation alloy with copper, and copper is stable at low potentials and is not easily oxidized. The current collector 1 is the core component of the battery 4 that carries the active layer 2 and collects the output current. It includes the current collector 1 body and the tab 11 connected to the current collector 1 body. The active layer 2 is located on the surface of the current collector 1 body.

[0046] In some embodiments, the active layer 2 comprises one or more of graphite and silicon-based materials. The function of the active layer 2 is to enable the reversible insertion and extraction of lithium ions during charging and discharging, thereby achieving energy storage and release.

[0047] In some embodiments, the active layer 2 is provided with a liquid storage channel 22, which is connected to the micropore 21. The diameter of the liquid storage channel 22 is 20~50μm, and the spacing between the liquid storage channels 22 is 10~50μm.

[0048] In some embodiments, the liquid-retaining layer 3 is partially provided with protrusions 31, which are located within the micropores 21.

[0049] This invention forms an electrode with a multi-level electrolyte storage structure by providing micropores 21 within the active layer 2 and a liquid-retaining layer 3 covering the micropores 21 on the surface of the active layer 2. The micropores 21 absorb and store electrolyte from the external electrolyte or the liquid-retaining layer 3 through capillary action, increasing the electrolyte retention of the electrode. The liquid-retaining layer 3 acts as a reservoir on the surface of the micropores 21, providing an electrolyte source for the micropores 21. This multi-level electrolyte storage structure improves the wettability of the R-corner region 41, effectively delaying electrolyte depletion during cycling.

[0050] This invention provides a method for preparing the above-mentioned electrode sheet, comprising the following steps: S1. An active material slurry is coated on the current collector 1, and after drying and rolling, an active layer 2 is formed. S2. Micropores 21 are prepared on the active layer 2. The spacing between any two adjacent pores is D = kd + w, where k is a safety factor, the value of k ranges from 3 to 5, d is the pore diameter, d is 10 to 100 μm, w is the width of the heat-affected zone, w is not greater than 100 μm, and the pore depth is 40% to 80% of the thickness of the active layer 2. S3. Prepare a liquid-retaining layer 3 on the surface of the active layer 2 by spraying, rolling or electrospinning, so that the liquid-retaining layer 3 covers the micropores 21.

[0051] In some embodiments, in step S2, the method for preparing the micropore 21 is to use a nanosecond, picosecond, or femtosecond pulsed laser for etching, with a laser power density of 10. 6 ~10 8 W / cm², scanning speed is 100~1000mm / s.

[0052] In some embodiments, in step S3, the liquid-retaining layer 3 is prepared by electrospinning, wherein the solvent of the spinning solution is one or more of N-methylpyrrolidone, N,N-dimethylformamide or deionized water, the solute of the spinning solution is one or two of polyvinylidene fluoride or polyimide at 8-15 wt.%, the solid content of the spinning solution is 1%-5%, the receiving distance is 10-20 cm, the voltage is 15-25 kV, the spinning time is 30-120 s, and after spraying, it is vacuum dried at 60-80 °C to form a nanofiber membrane with controllable thickness, so that the fibers and the interior of the micropores 21 form capillary bridges.

[0053] The present invention provides a battery 4, including a casing, a top cover and a battery cell. The casing has an opening and is sealed to the top cover. The battery cell includes a positive electrode, a negative electrode and a separator located between the positive electrode and the negative electrode. The battery cell is wound and placed inside the casing, and the casing is filled with an electrolyte. The negative electrode is an electrode prepared by any of the above embodiments or by any of the above preparation methods, and the micropores 21 on the wound negative electrode are located in the R-corner region 41 of the battery cell.

[0054] After winding, the cross-section of the cell perpendicular to the height of battery 4 is approximately rectangular (e.g., Figure 3 As shown, due to limitations in the winding process, the four corners of the rectangle are not right angles, but rather rounded corners. Therefore, the areas with rounded corners at both ends of battery 4 are the R-corner areas 41, while the remaining areas are flat areas 42. Micropores 21 are located in the R-corner area 41 after the cell is wound. During cycling, a multi-stage electrolyte storage structure continuously replenishes the R-corner area 41 with electrolyte, eliminating purple spots and lithium plating caused by localized drying.

[0055] The present invention will be further described below with reference to specific embodiments.

[0056] It should be noted that, in this invention, before the electrode is wound, the width direction of the electrode is the direction of its short side, and the length direction of the electrode is the direction of its long side. After the electrode is wound, the height direction of the cell and the battery are both the original width direction before the electrode is wound, the thickness direction of the cell and the battery are both the direction of the short side of the cross-section perpendicular to its height direction, and the length direction of the cell and the battery are both the direction of the long side of the cross-section perpendicular to its height direction.

[0057] Example 1 This embodiment provides a negative electrode sheet and its preparation method, the preparation method including the following steps: S1. Graphite, conductive carbon black (Super P Li-2060), styrene-butadiene rubber, and sodium carboxymethyl cellulose are mixed in a mass ratio of 96:2:0.8:1.2 and water is added to prepare an aqueous active material slurry. The aqueous active material slurry is then coated onto a copper foil that is 6μm thick and 221mm wide, with a coating thickness of 120μm. After drying at 90℃ and rolling, the active layer formed has a thickness of 80μm. S2. Determine several target regions that will be located in the R-corner area after the electrode is wound. Each target region has a width of 50 mm and a length of 30 mm. A nanosecond laser is used to fabricate a microporous rectangular array on the active layer of the target region, where the laser wavelength is 1030 nm, the pulse width is 300 fs, and the power density is 5 × 10⁻⁶. 7 W / cm², scanning speed of 500mm / s, heat-affected zone width of 50μm, micropores are inverted frustum shape, pore spacing of 150μm, surface pore diameter of 50μm, bottom pore diameter of 10μm, and pore depth of 50μm; S3. An electrospinning process was used to prepare a liquid-retaining layer on the surface of the active layer. The spinning solution was an N,N-dimethylformamide solution containing 10 wt.% polyvinylidene fluoride, with a solid content of 2.14%. The receiving distance was 15 cm, the voltage was 20 kV, and the spinning time was 60 s. After spraying, the solution was vacuum dried at 80 °C to form a liquid-retaining layer composed of a 5 μm thick nanofiber membrane with a porosity of 75%, wherein the diameter of the nanofibers was 200 nm. The liquid-retaining layer covered micropores.

[0058] Example 2 The method is basically the same as in Example 1, except that the hole spacing is 200 μm, the surface hole diameter is 55 μm, the bottom hole diameter is 15 μm, and the hole depth is 45 μm.

[0059] Example 3 The method is basically the same as in Example 1, except that the hole spacing is 180μm, the surface hole diameter is 55μm, the bottom hole diameter is 10μm, and the hole depth is 52μm.

[0060] Example 4 This embodiment provides a battery and its preparation method: A1. Lithium iron phosphate, conductive carbon black, and polyvinylidene fluoride are mixed in a mass ratio of 95:2:3. Then, N-methylpyrrolidone is added to make a positive electrode slurry. The positive electrode slurry is coated on an aluminum foil with a thickness of 12μm and a width of 235mm, and the coating thickness is 120μm. After drying at 90°C and rolling, the coating thickness is 80μm, and a positive electrode sheet is obtained. A2. Cut the separator, the positive electrode sheet prepared in step A1, and the negative electrode sheet prepared in Example 1 into 54mm wide (including a 2mm high tab) and 430mm long dimensions. Then stack and wind them into a core in the order of negative electrode sheet-separator-positive electrode sheet-separator. The winding needle diameter is 3.5mm, the winding tension is 1.2N, the core thickness after winding is 8.2mm, and the core height is 52mm (excluding the tab height). Place the core into an 8.5mm deep aluminum-plastic film punch pit and perform top sealing (temperature 190℃, time 3s, pressure 0.4MPa) and side sealing (temperature 180℃, time 3s, pressure 0.4MPa), leaving a liquid injection port. The encapsulated battery dimensions are 8.2mm thick, 52mm high, and 72mm long. A3. In a glove box with a dew point temperature ≤ -40°C, inject 3.8g of electrolyte (designed electrolyte retention coefficient 3.5g / Ah) through the injection port; after injection, evacuate to -95kPa at 25°C, let stand for 30min, and repeat the above evacuation-standing operation 3 times to ensure the electrolyte is fully wetted; at 45°C, charge to 3.0V using a constant current of 0.05C, let stand for 10min, charge to 3.3V using a constant current of 0.1C, let stand for 10min, charge to 3.6V using a constant current of 0.2C, let stand for 10min, and charge to 3.8V using a constant current of 0.2C; then evacuate to -90kPa in the glove box to remove gas, and heat-seal the battery at 190°C for 3s.

[0061] Example 5 The process is basically the same as in Example 4, except that in step A2, the negative electrode sheet prepared in Example 2 is used.

[0062] Example 6 The process is basically the same as in Example 4, except that in step A2, the negative electrode sheet prepared in Example 3 is used.

[0063] Comparative Example 1 It is basically the same as Example 1, except that step S2 is not included, that is, the active layer does not have micropores.

[0064] Comparative Example 2 The basic structure is the same as in Example 1, except that the hole spacing is 300 μm, the surface hole diameter is 55 μm, the bottom hole diameter is 35 μm, and the hole depth is 20 μm.

[0065] Comparative Example 3 It is basically the same as Example 1, except that the hole spacing is 250μm, the surface hole diameter is 55μm, the bottom hole diameter is 30μm, and the hole depth is 18μm.

[0066] Comparative Example 4 The basic structure is the same as in Example 1, except that the hole spacing is 200 μm, the surface hole diameter is 70 μm, the bottom hole diameter is 45 μm, and the hole depth is 36 μm.

[0067] Comparative Example 5 It is basically the same as Example 1, except that the hole spacing is 220μm, the surface hole diameter is 55μm, the bottom hole diameter is 30μm, and the hole depth is 25μm.

[0068] Comparative Example 6 The basic structure is the same as in Example 1, except that the hole spacing is 200 μm, the surface hole diameter is 80 μm, the bottom hole diameter is 60 μm, and the hole depth is 60 μm.

[0069] Comparative Example 7 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 1 is used as the negative electrode. The N / P ratio of the battery prepared in this comparative example is 1.12.

[0070] Comparative Example 8 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 2 is used as the negative electrode.

[0071] Comparative Example 9 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 3 is used as the negative electrode.

[0072] Comparative Example 10 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 4 is used as the negative electrode.

[0073] Comparative Example 11 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 5 is used as the negative electrode.

[0074] Comparative Example 12 The process is basically the same as in Example 4, except that in step A2, the electrode prepared in Comparative Example 6 is used as the negative electrode.

[0075] Test Example 1 The batteries prepared in Examples 4-6 and Comparative Examples 7-12 were placed in a constant temperature chamber at 45°C, and then charged to 4.2V using a constant current and constant voltage of 1C (cutoff current of 0.05C), left to stand for 10 minutes, and then discharged to 2.75V using a constant current of 1C, left to stand for 10 minutes. The above charge and discharge steps were repeated 500 times. Standard capacity calibration was performed after the first and 500th cycles.

[0076] The standard capacity calibration procedure is as follows: Place the battery in a 25°C environment, charge it with a constant current and constant voltage of 0.5C to 4.2V (cutoff current is 0.02C), switch to constant voltage charging until the cutoff current is 0.02C, let it stand for 30 minutes, and then discharge it with a constant current of 0.5C to 2.75V and record the discharge capacity.

[0077] After 500 cycles, the battery capacity retention rate = C500 / C1 × 100%, where C500 is the discharge capacity after the 500th cycle and C1 is the discharge capacity after the first cycle. The capacity retention rate results of the batteries prepared in Examples 4-6 and Comparative Examples 7-12 are shown in Table 1.

[0078] Test Example 2 After the capacity retention rate of the battery in Test Example 1 was determined, it was transferred to an inert atmosphere glove box protected by high-purity argon gas for disassembly. The dew point temperature of the high-purity argon gas in the glove box was ≤-50℃, and the oxygen content was <1ppm. The aluminum-plastic film was cut open with ceramic scissors, the core was carefully removed, and the negative electrode sheet was separated after unfolding. The negative electrode sheet was immersed in a beaker containing dimethyl carbonate (battery grade, water content ≤20ppm) and ultrasonically cleaned for 30s (power 100W, frequency 40kHz) to remove residual electrolyte, lithium salt, and by-products from the surface. After removal, it was dried with high-purity nitrogen gas (99.999%) and laid flat on a black matte background, avoiding folding.

[0079] A high-resolution digital camera (Canon EOS R5 with a 100mm macro lens) was fixed on a copy stand. The lens was perpendicular to the surface of the negative electrode to capture images. The image acquisition parameters were: aperture f / 8, ISO 100, shutter speed 1 / 125s, and the light source was a ring LED cold light source (color temperature 5500K, illuminance 2000 lux). Shadowless shooting was performed.

[0080] Five rounded corner areas were photographed for each negative electrode sheet, with three duplicate photos taken for each area. The photos were imported into ImageJ 1.53k image analysis software. The software automatically identified and calculated the pixel area of ​​the purple areas, and simultaneously calculated the effective coating pixel area of ​​the negative electrode sheet based on its dimensions. During the calculation, interference from the tabs, exposed foil areas, and wrinkles was deducted. The software's color threshold settings were as follows: Hue 240~300 (corresponding to blue-violet to magenta), Saturation 30~100%, and Value 20~80%.

[0081] The percentage of purple spot area in each photo = purple pixel area / effective coating pixel area of ​​the negative electrode × 100%. The arithmetic mean of the percentage of purple spot area in each of the 5 R-corner regions of each electrode, and the 3 photos of each region, for a total of 15 samples, is taken to obtain the average percentage of purple spot area S of a single battery.

[0082] The purple spot elimination rate of a single battery is (1-S / S0)×100%, where S0 is the average purple spot area percentage of Comparative Example 7.

[0083] For each group of batteries prepared in Examples 4-6 and Comparative Examples 7-12, 12 parallel samples were set up. The average value of 10 valid data samples (excluding the highest and lowest values) was taken as the final purple spot elimination rate of the group. The results are shown in Table 1.

[0084] Table 1. Purple spot elimination rate and capacity retention rate of batteries prepared in Examples 4-6 and Comparative Examples 7-12

[0085] As shown in Table 1, the safety factors k of batteries 4 prepared in Examples 4, 5, and 6 are 3.3, 4.3, and 4.0, respectively, and the ratios of pore depth to active layer 2 thickness (i.e., pore depth ratios) are 62.5%, 56.3%, and 65.0%, respectively. The purple spot elimination rates of the above three examples are between 85.3% and 92.5%, and the capacity retention rates are between 90.8% and 94.2%. In contrast, the purple spot elimination rates of batteries 4 prepared in Comparative Examples 8 to 12 are between 31.2% and 78.9%, and the capacity retention rates are between 80.3% and 86.9%, which are lower than the corresponding values ​​of Examples 4 to 6. Although the safety factors k of Comparative Examples 9 and 11 are between 3 and 5, the smaller pore depth ratios and smaller liquid storage volumes reduce the effectiveness of the liquid storage structure in improving the purple spot elimination rate and capacity retention rate.

[0086] The battery 4 prepared in Examples 4-6 showed significantly better capacity retention and purple spot elimination rates than the batteries 4 prepared in Comparative Examples 8-12. This demonstrates that when using laser etching of the micropores 21, the hole spacing D = kd + w, where k ranges from 3 to 5, and the hole depth ratio is 40% to 80%, can fully utilize the function of the liquid storage structure. The battery 4 prepared in Comparative Example 7 showed a capacity retention rate of only 78.5%, and a large area of ​​purple spots appeared in the R-corner region 41, verifying the necessity of setting up the liquid storage structure.

[0087] The micropores 21 in the active layer 2 can be considered as discrete point sources on a two-dimensional plane. The electrolyte diffuses from the inside of the pores outwards, forming an approximately circular wetted area. When the pore spacing is too large, the wetted areas of adjacent pores cannot overlap, resulting in "liquid-retaining blind zones." These blind zones are the first to lack liquid in the later stages of cycling, becoming nucleation sites for lithium plating. Furthermore, based on production experience, the effective diffusion distance L of the electrolyte in the porous active layer 2 of the electrode is related to the pore spacing D as: L≈D / 2. According to Fick's diffusion law, the diffusion time t∝L. 2Therefore, when the aperture spacing increases to twice, the diffusion time increases to four times. Under rapid charge and discharge (such as 1C rate), the liquid phase diffusion of electrodes with large aperture spacing cannot compensate for local consumption in time, resulting in intensified concentration polarization and accelerated capacity decay.

[0088] However, if the pore spacing is too small, although the areal density of the micropores 21 will increase, which is beneficial to improving the purpura elimination rate and capacity retention rate, it should also be considered that the heat-affected zone will cause the mechanical strength of some areas around the micropores 21 to decrease, causing the active layer 2 between the micropores 21 to form "islands". The risk of cracking of the active layer 2 during cycling will increase, which will reduce the capacity retention rate.

[0089] Furthermore, the micropores 21 are not actually points on a two-dimensional plane, but rather circles with a specific area. In the embodiments and comparative examples of the present invention, since the micropores 21 have the same size and shape and are uniformly distributed in a matrix, when the pore spacing is a fixed value, as the pore diameter of the micropores 21 increases, the volume of the micropores 21 also increases, thus storing more electrolyte, while the structural stability of the active layer 2 between the micropores 21 decreases. Therefore, the pore spacing should be determined by comprehensively considering the influence of the heat-affected zone width and the pore diameter.

[0090] In this invention, the remaining thickness is defined as the difference between the thickness of the active layer 2 and the pore depth, representing the bonding and anchoring thickness between the active layer 2 at the bottom of the pore and the current collector 1. Since electron transport in the graphite anode relies on the conductive path between the particles and the current collector 1, the remaining thickness also represents the thickness of the "conductive bridge" from the bottom of the pore to the current collector 1. When the pore depth is relatively large, the volume of the micropore 21 is larger, allowing for the storage of more electrolyte. However, this results in a smaller remaining thickness, making the active layer 2 at the bottom of the pore prone to peeling off from the current collector 1 under cyclic stress, causing active material detachment (capacity decay) and lithium plating on the exposed current collector 1 (safety hazard). Simultaneously, the conductive network in the pore bottom region breaks, leading to the loss of electrochemical activity in that region, macroscopically manifested as capacity loss. Conversely, when the pore depth is relatively small, although the structure and conductivity of the pore bottom region are more stable, the volume of the micropore 21 in the active layer 2 is smaller, storing less electrolyte, which is detrimental to the elimination of purple spots.

[0091] Therefore, the pore depth ratio should be within a suitable range to maximize the liquid storage capacity of the active layer 2 while maintaining the structural and electrical stability of the pore bottom region. As shown in Examples 4-6, provided the pore spacing is within a suitable range, the pore depth should be 40% to 80% of the thickness of the active layer 2. Depths above or below this range will not result in good capacity retention and purple spot elimination rates.

[0092] The core mechanism of capacity retention decay lies in the continuous loss of active lithium and the deterioration of the electrode interface during cycling. Due to the concentrated current density in the R-corner region 41, the electrolyte in this region is preferentially consumed, leading to local electrolyte depletion, purple spot defects, irreversible loss of active lithium, and increased interfacial impedance. The electrode in this invention, comprising micropores 21 and a liquid-retaining layer 3, establishes a distributed electrolyte reservoir and a rapid wetting channel. The electrolyte stored in the micropores 21 acts as a "slow-release electrolyte source" during cycling. When the electrolyte concentration in the R-corner region 41 of the electrode decreases due to electromigration and decomposition, the electrolyte inside the pores diffuses outward under capillary action, maintaining the wettability of the active layer 2 between the pores.

[0093] The above description provides an illustrative overview of the present invention and its embodiments. This description is not restrictive, and the embodiments shown are merely one example of the invention's implementation. Actual implementations are not limited to these examples. Therefore, if those skilled in the art are inspired by this description and design similar implementations and examples without departing from the spirit of the invention, such designs should fall within the scope of protection of the present invention.

Claims

1. An electrode, comprising a current collector (1) and a liquid-retaining layer (3), wherein an active layer (2) is disposed between the current collector (1) and the liquid-retaining layer (3), the active layer (2) having micropores (21) therein, and the liquid-retaining layer (3) covering the micropores (21), characterized in that: The hole spacing between any two adjacent holes is D=kd+w, where k is the safety factor, the value of k ranges from 3 to 5, d is the hole diameter, d is from 10 to 100 μm, and w is the width of the heat-affected zone, w is not greater than 100 μm; The pore depth is 40% to 80% of the thickness of the active layer (2).

2. The electrode sheet according to claim 1, characterized in that: The micropores (21) are formed by laser etching.

3. The electrode sheet according to claim 1, characterized in that: The micropore (21) is shaped like an inverted frustum.

4. The electrode sheet according to claim 1, characterized in that: The ratio of the pore depth to the original N / P of the battery (4) is 35~50μm.

5. The electrode sheet according to any one of claims 1 to 4, characterized in that: The active layer (2) is provided with a liquid storage channel (22), which is connected to the micropore (21).

6. The electrode sheet according to any one of claims 1 to 4, characterized in that: The liquid-retaining layer (3) has a protrusion (31) in a certain part, and the protrusion (31) is located in the micropore (21).

7. The electrode sheet according to any one of claims 1 to 4, characterized in that: The thickness of the liquid-retaining layer (3) is 2~10μm, and the porosity is 60%~90%.

8. The electrode sheet according to any one of claims 1 to 4, characterized in that: The liquid-retaining layer (3) is made of one or two of polyvinylidene fluoride and polyimide.

9. A method for preparing an electrode sheet according to any one of claims 1 to 8, characterized in that, Includes the following steps: S1. An active material slurry is coated on the current collector (1), and after drying and rolling, an active layer (2) is formed. S2. Micropores (21) are prepared on the active layer (2). The spacing between any two adjacent pores is D = kd + w, where k is the safety factor, the value of k is 3~5, d is the pore diameter, d is 10~100μm, w is the width of the heat-affected zone, w is not greater than 100μm, and the pore depth is 40%~80% of the thickness of the active layer (2). S3. Prepare a liquid-retaining layer (3) on the surface of the active layer (2) by spraying, rolling or electrospinning process, so that the liquid-retaining layer (3) covers the micropores (21).

10. The method for preparing the electrode according to claim 9, characterized in that: In step S2, the method for preparing the micropores (21) is to use nanosecond, picosecond, or femtosecond pulsed lasers for etching, with a laser power density of 10. 6 ~10 8 W / cm², laser scanning speed is 100~1000mm / s.

11. The method for preparing the electrode according to claim 9 or 10, characterized in that: In step S3, the liquid-retaining layer (3) is prepared by electrospinning. The solvent of the spinning solution is one or more of N-methylpyrrolidone, N,N-dimethylformamide or deionized water; The solute in the spinning solution is 8-15 wt.% of one or both of polyvinylidene fluoride or polyimide; The solid content of the spinning solution is 1%~5%; The receiving distance is 10~20cm, the voltage is 15~25kV, and the spinning time is 30~120s.

12. A battery comprising a casing, a top cover, and a battery cell, wherein the casing has an opening and is sealed to the top cover, the battery cell comprising a positive electrode, a negative electrode, and a separator located between the positive and negative electrode, the battery cell being wound and placed inside the casing, and the casing being filled with an electrolyte, characterized in that: The negative electrode sheet is prepared by any one of the electrode sheets described in claims 1 to 8 or by any one of the preparation methods described in claims 9 to 11, and the micropores (21) on the negative electrode sheet after winding are located in the R-corner region (41) of the cell.

13. The battery according to claim 12, characterized in that: The height of the region containing the micropores (21) is 70% to 100% of the cell height; and / or, the ratio between the length of the region containing the micropores (21) and the cell thickness is 3 to 4.

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