Negative pole piece, secondary battery and electronic device

Abstract

The invention provides a negative pole piece, a secondary battery and an electronic device. The negative electrode piece comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the thickness of the negative electrode material layer is H [mu] m, and H is larger than or equal to 85 and smaller than or equal to 250; the negative electrode material layer comprises a negative electrode active material, and Dv50 [mu] m and H of the negative electrode active material meet the condition that Dv50 / H is larger than or equal to 0.4 and smaller than or equal to 0.8. According to the present invention, the H and the Dv50 / H of the negative electrode plate are regulated to meet the relationship, such that the processing performance of the negative electrode plate can be easily improved, and the cycle performance of the secondary battery can be improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to a negative electrode, a secondary battery, and an electronic device. Background Technology

[0002] Secondary batteries, such as lithium-ion batteries, are widely used in smartphones, wearable devices, consumer drones, and electric vehicles due to their advantages such as high energy density, long cycle life, and no memory effect. With the widespread application of lithium-ion batteries in these fields, the market demands increasingly higher cycle performance from them.

[0003] However, when the thickness of the negative electrode material layer of the negative electrode sheet is too large, such as greater than 85 μm, the problem of drying and cracking is likely to occur when coating the negative electrode material layer during the preparation of the negative electrode sheet, which will affect the cycle performance of the lithium-ion battery. Summary of the Invention

[0004] The purpose of this application is to provide a negative electrode sheet, a secondary battery, and an electronic device to improve the processing performance of thick negative electrode sheets and enhance the cycle performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The thickness of the negative electrode material layer is H μm, where 85 ≤ H ≤ 250. The negative electrode material layer comprises a negative electrode active material, and the Dv50 μm of the negative electrode active material and H satisfy: 0.4 ≤ Dv50 / H ≤ 0.8. By controlling the value of the thickness H of the negative electrode material layer within the above range, this application helps to reduce the manufacturing cost of the secondary battery and increase its energy density. However, a large thickness of the negative electrode material layer can also easily lead to drying cracking during the coating process of the negative electrode sheet, affecting the processing performance of the negative electrode sheet and the cycle performance of the secondary battery. By controlling the Dv50 and H of the negative electrode active material to satisfy the above relationship, it is beneficial to increase the critical thickness for drying cracking of the negative electrode material layer coating, improve the drying cracking problem during the coating process of the negative electrode sheet, thereby improving the processing performance of thick negative electrode sheets and simultaneously improving the cycle performance of the secondary battery.

[0006] In some embodiments of this application, 34 ≤ Dv50 ≤ 200. By adjusting the value of Dv50 within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer coating. This improves the drying cracking problem during the coating of the negative electrode material layer when the negative electrode material layer is thicker, thereby improving the processing performance of thick negative electrode sheets and enhancing the cycle performance of secondary batteries.

[0007] In some embodiments of this application, 0.5 ≤ Dv50 / H ≤ 0.8. By adjusting the value of Dv50 / H to satisfy the above relationship, it is beneficial to further increase the critical thickness for drying cracking of the negative electrode material layer coating, improve the drying cracking problem when coating the negative electrode material layer during the preparation of the negative electrode sheet, thereby further improving the processing performance of thick negative electrode sheets and improving the cycle performance of secondary batteries.

[0008] In some embodiments of this application, the Dv99μm and H of the negative electrode active material satisfy the condition: 0.6≤Dv99 / H≤0.9. By controlling the value of Dv99 / H within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer coating. This improves the drying cracking problem during the coating process of the negative electrode sheet when the negative electrode material layer is thicker, thereby improving the processing performance of thick negative electrode sheets and enhancing the cycle performance of the secondary battery.

[0009] In some embodiments of this application, 0.7 ≤ Dv99 / H ≤ 0.9. By adjusting the value of Dv99 / H within the above range, the particle size of the negative electrode active material is larger, which is beneficial to further increase the critical thickness for drying cracking of the negative electrode material layer coating. When the negative electrode material layer is thicker, the drying cracking problem during the coating of the negative electrode material layer in the preparation of the negative electrode sheet is improved, thereby further improving the processing performance of thick negative electrode sheets. At the same time, it improves the cycle performance of secondary batteries, which is beneficial to further improve their cycle performance.

[0010] In some embodiments of this application, 51 ≤ Dv99 ≤ 225. By adjusting the value of Dv99 within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer coating. This improves the drying cracking problem during the coating of the negative electrode material layer in the case of a larger negative electrode material layer, thereby improving the processing performance of thick negative electrode sheets and enhancing the cycle performance of secondary batteries.

[0011] In some embodiments of this application, the particles of the negative electrode active material have through-pores with an average pore size of D μm, where 1 ≤ D ≤ 10. Having through-pores and controlling the value of D within the aforementioned range facilitates the transport of electrolyte from the surface of the negative electrode active material particles to the bottom of the negative electrode material layer, even with a large negative electrode material layer thickness. This improves the wettability of the electrolyte to the negative electrode sheet and also helps to shorten the lithium-ion transport distance, thereby improving the kinetic performance of the secondary battery. Therefore, its application in secondary batteries is beneficial for improving their cycle performance and storage performance.

[0012] In some embodiments of this application, 1 ≤ D ≤ 2. By adjusting the value of D within the above range, it is beneficial to further improve the wettability of the electrolyte to the negative electrode sheet, and also to further shorten the lithium ion transport distance, thereby improving the kinetic performance of the secondary battery. Thus, its application in secondary batteries is beneficial to further improve its cycle performance and storage performance.

[0013] In some embodiments of this application, the negative electrode active material includes at least one of graphite, silicon, or silicon-carbon. Using the aforementioned negative electrode active material in secondary batteries is beneficial for improving their energy density and cycle performance.

[0014] In some embodiments of this application, the negative electrode sheet satisfies at least one of the following characteristics: (1) the mass percentage of the negative electrode active material is 80% to 98% based on the mass of the negative electrode material layer; (2) the coating crack width of the negative electrode material layer is less than or equal to 0.3 mm; (3) the compaction density of the negative electrode sheet is 1.2 g / cm³. 3 Up to 1.8 g / cm 3 (4) The porosity of the negative electrode material layer is 20% to 60%. The negative electrode sheet meets the above characteristics and is beneficial to improving its energy density, cycle performance and storage performance when applied to secondary batteries.

[0015] The second aspect of this application provides a secondary battery including the negative electrode sheet provided in the first aspect of this application. The negative electrode sheet provided in this application has a high volume and mass ratio of negative electrode active material, and also helps to improve the drying cracking problem during the coating of the negative electrode material layer in the preparation process of the negative electrode sheet, resulting in good processing performance of the negative electrode sheet. Therefore, its application in secondary batteries is beneficial to giving it high energy density and good cycle performance.

[0016] A third aspect of this application provides an electronic device that includes the secondary battery provided in the second aspect of this application. The secondary battery provided in this application has high energy density and good cycle performance, thereby providing the electronic device with a long service life and good performance.

[0017] The beneficial effects of this application are:

[0018] This application provides a negative electrode sheet, a secondary battery, and an electronic device. The negative electrode sheet includes a negative current collector and a negative electrode material layer disposed on at least one surface of the negative current collector. The thickness of the negative electrode material layer is H μm, where 85 ≤ H ≤ 250. The negative electrode material layer includes a negative electrode active material, and the Dv50 μm of the negative electrode active material and H satisfy the condition: 0.4 ≤ Dv50 / H ≤ 0.8. By controlling the value of the negative electrode material layer thickness H within the above range, this application helps to reduce the manufacturing cost of the secondary battery and increase its energy density. However, a large thickness of the negative electrode material layer can also easily lead to drying cracking during the coating process of the negative electrode sheet, affecting the processing performance of the negative electrode sheet and the cycle performance of the secondary battery. By controlling the Dv50 and H of the negative electrode active material to satisfy the above relationship, it is beneficial to increase the critical thickness for drying cracking of the negative electrode material layer coating, improve the drying cracking problem during the coating process of the negative electrode sheet, thereby improving the processing performance of thick negative electrode sheets and simultaneously improving the cycle performance of the secondary battery.

[0019] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of this application 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 only some embodiments of this application. For those skilled in the art, other embodiments can be obtained based on these drawings.

[0021] Figure 1 This is a cross-sectional view of the negative electrode sheet along its thickness direction according to one embodiment of this application;

[0022] Figure 2 This is a cross-sectional view along the thickness direction of the negative electrode sheet of another embodiment of this application;

[0023] Figure 3 This is a schematic diagram of the through-holes in the negative electrode active material according to one embodiment of this application;

[0024] Figure 4 This is a schematic diagram of the through-holes in the negative electrode active material according to another embodiment of this application.

[0025] Reference numerals: negative electrode 100, negative electrode material layer 110, negative electrode active material 111, negative electrode current collector 120, first through hole 111a, second through hole 111b. Detailed Implementation

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0027] It should be noted that, in the following explanation, lithium-ion batteries are used as an example of secondary batteries to illustrate this application; however, the secondary batteries in this application are not limited to lithium-ion batteries. The specific technical solution is as follows:

[0028] This application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The thickness of the negative electrode material layer is H μm, and 85 ≤ H ≤ 250. For example, the value of H can be 85, 100, 120, 134, 150, 175, 180, 200, 213, 230, 250, or any range of two of the above values. The negative electrode material layer includes a negative electrode active material, and the Dv50 μm of the negative electrode active material and H satisfy: 0.4 ≤ Dv50 / H ≤ 0.8, preferably 0.5 ≤ Dv50 / H ≤ 0.8. For example, the value of Dv50 / H can be 0.4, 0.45, 0.5, 0.53, 0.55, 0.58, 0.6, 0.62, 0.67, 0.7, 0.75, 0.8, or any range of two of the above values. In this application, the thickness of the negative electrode material layer refers to the thickness of one side of the negative electrode material layer after cold pressing.

[0029] For ease of understanding, in this application, the length direction of the negative electrode sheet is defined as Y, and its thickness direction as Z. The negative electrode sheet typically has a long side and a short side; the aforementioned length direction refers to the extension direction of the long side of the negative electrode sheet. It should be understood that the above definitions of directions are for the purpose of conveniently describing this application. Figure 1 As shown, the negative electrode 100 includes a negative electrode current collector 120 and a negative electrode material layer 110 disposed on one surface of the negative electrode current collector 120. The negative electrode material layer 110 includes a negative electrode active material 111. In this application, the negative electrode material layer can be disposed on one surface or on two surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here can be the entire area of ​​the negative electrode current collector or a part of the negative electrode current collector. This application has no particular limitation, as long as the purpose of this application can be achieved.

[0030] The negative electrode material layer of this application has a relatively large thickness. When the size of the secondary battery is fixed, this is beneficial for reducing the volume and mass ratio of the negative current collector in the negative electrode sheet, increasing the volume and mass ratio of the negative electrode material layer, thereby increasing the volume and mass ratio of the negative electrode active material, and thus increasing the energy density of the secondary battery. It can also reduce the number of negative electrode layers in multi-layer wound / stacked secondary batteries, reducing the amount of negative current collector, positive current collector, and separator, thereby lowering the manufacturing cost of the secondary battery. However, the large thickness of the negative electrode material layer also makes it prone to drying and cracking during the coating process. On the one hand, this increases the risk of "powder shedding" from the negative electrode sheet, increasing the probability of internal short circuits in the secondary battery. On the other hand, it can cause localized loss of negative electrode active material, making the negative electrode sheet more susceptible to lithium plating, increasing the risk of thermal runaway in the secondary battery, and affecting the processing performance of the negative electrode sheet as well as the safety and cycle performance of the secondary battery. This application achieves the above-mentioned relationship by adjusting the Dv50 and H of the negative electrode active material. The particle size of the negative electrode active material is relatively large, which is beneficial to increasing the critical thickness of the negative electrode material layer coating drying crack, improving the drying crack problem when coating the negative electrode material layer during the preparation of the negative electrode sheet, thereby improving the processing performance of the thick negative electrode sheet and improving the cycle performance of the secondary battery.

[0031] When the value of H is too small, for example, less than 85, when the size of the secondary battery is fixed, the volume and mass proportion of the negative electrode current collector in the negative electrode sheet is too large, while the volume and mass proportion of the negative electrode material layer and the negative electrode active material are too small, affecting the energy density of the secondary battery. Furthermore, for multi-layer wound / stacked secondary batteries, the required number of negative electrode layers is excessive, increasing the amount of negative electrode current collector, positive electrode current collector, and separator, thus raising the manufacturing cost of the secondary battery. When the value of H is too large, for example, greater than 250, drying cracking is more likely to occur during the coating of the negative electrode material layer in the preparation of the negative electrode sheet, affecting the processing performance of the negative electrode sheet and the cycle performance of the secondary battery. When the value of Dv50 / H is too small, for example, less than 0.4, the Dv50 of the negative electrode active material and the thickness of the negative electrode material layer are mismatched, resulting in a too small critical thickness for drying cracking of the negative electrode material layer coating. This makes the negative electrode material layer more prone to drying cracking, affecting the processing performance of the negative electrode sheet and the cycle performance of the secondary battery. When the Dv50 / H value is too large, for example, greater than 0.8, the Dv50 of the negative electrode active material and the thickness of the negative electrode material layer are mismatched. This results in an excessively long transport path for lithium ions in the negative electrode material layer, affecting the kinetic performance of the secondary battery and thus its cycle performance. Furthermore, with a fixed battery size, the volume and mass ratio of the negative electrode active material is too small, impacting the energy density of the secondary battery. Additionally, the increased usage of negative electrode current collectors, positive electrode current collectors, and separators leads to higher manufacturing costs. Moreover, the negative electrode sheet is more prone to bumps after cold pressing, affecting its processing performance and the appearance of the secondary battery. Therefore, by controlling the H and Dv50 / H of the negative electrode sheet within the scope of this application, it is beneficial to reduce the manufacturing cost of the secondary battery and the risk of drying cracking during the coating of the negative electrode material layer, improve the processing performance of thick negative electrode sheets, and increase the energy density and cycle performance of the secondary battery. In this application, energy density refers to volumetric energy density.

[0032] In some embodiments of this application, 34 ≤ Dv50 ≤ 200. For example, the value of Dv50 can be 34, 50, 100, 130, 160, 200, or any combination of two of the above values. By adjusting the value of Dv50 within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer coating. This improves the drying cracking problem during the coating process of the negative electrode sheet when the negative electrode material layer is thicker. It also helps reduce the risk of agglomeration of negative electrode active material particles during the preparation of the negative electrode slurry, resulting in a more uniform distribution of the negative electrode active material, improving the processing stability of the negative electrode sheet, and reducing the contact interface between the negative electrode active material particles and the electrolyte. This reduces the occurrence of side reactions between the negative electrode sheet and the electrolyte, thereby improving the processing performance of thick negative electrode sheets and enhancing the cycle performance of the secondary battery.

[0033] In some embodiments of this application, the Dv99μm and H of the negative electrode active material satisfy the following: 0.6≤Dv99 / H≤0.9, preferably 0.7≤Dv99 / H≤0.9. For example, the value of Dv99 / H can be 0.6, 0.64, 0.7, 0.72, 0.77, 0.8, 0.83, 0.85, 0.87, 0.9, or any combination of the above values. By controlling the value of Dv99 / H within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer. This improves the drying cracking problem during the coating of the negative electrode material layer when the negative electrode material layer is thicker, thereby improving the processing performance of thick negative electrode sheets and simultaneously enhancing the cycle performance of the secondary battery.

[0034] In some embodiments of this application, 51 ≤ Dv99 ≤ 225. For example, the value of Dv99 can be 51, 70, 90, 100, 120, 140, 160, 200, 225, or any combination of two of the above values. By adjusting the value of Dv99 within the above range, the particle size of the negative electrode active material is larger, which is beneficial to increasing the critical thickness for drying cracking of the negative electrode material layer coating. This improves the drying cracking problem during the coating process of the negative electrode sheet when the negative electrode material layer is thicker. It also helps reduce the risk of agglomeration of negative electrode active material particles during the preparation of the negative electrode slurry, resulting in a more uniform distribution of the negative electrode active material, improving the processing stability of the negative electrode sheet, and reducing the contact interface between the negative electrode active material particles and the electrolyte. This reduces the occurrence of side reactions between the negative electrode sheet and the electrolyte, thereby improving the processing performance of thick negative electrode sheets and enhancing the cycle performance of the secondary battery.

[0035] In this application, Dv50 refers to the particle size that, in the volumetric particle size distribution of a material, reaches 50% of the cumulative volume from the smallest particle size. Dv99 refers to the particle size that, in the volumetric particle size distribution of a material, reaches 99% of the cumulative volume from the smallest particle size. Typically, negative electrode active materials with different Dv50 and Dv99 values ​​can be obtained through mechanical crushing (e.g., ball milling). For example, the Dv50 and Dv99 of the negative electrode active material can be controlled by adjusting the ball milling time. When other conditions remain constant, extending the ball milling time decreases the Dv50 of the negative electrode active material; shortening the ball milling time increases the Dv50. When other conditions remain constant, extending the ball milling time decreases the Dv99 of the negative electrode active material; shortening the ball milling time increases the Dv99.

[0036] In this application, negative electrode active materials with different Dv50 and Dv99 can be purchased, and the Dv50 and Dv99 of the negative electrode active materials can be tested using the "particle size test" method provided in this application, and the desired Dv50 and Dv99 negative electrode active materials can be selected.

[0037] In some embodiments of this application, such as Figure 2 As shown, the negative electrode 100 includes a negative electrode current collector 120 and a negative electrode material layer 110 disposed on one surface of the negative electrode current collector 120. The negative electrode material layer 110 includes a negative electrode active material 111. The particles of the negative electrode active material 111 have through holes with an average pore size of D μm, where 1 ≤ D ≤ 10, preferably 1 ≤ D ≤ 2. For example, the value of D can be 1, 1.2, 1.5, 1.7, 2, 2.3, 2.7, 3, 5, 6, 7.8, 9, 10, or any two of the above values. When the thickness of the negative electrode material layer in the negative electrode is too large, the electrolyte does not easily flow to the bottom of the negative electrode material layer, i.e., the area near the negative electrode current collector. The electrolyte has poor wettability on the negative electrode, and it is difficult to replenish the electrolyte in time after it is consumed during cycling. This leads to the secondary battery being prone to cycle "diving" phenomenon, affecting the cycle performance and storage performance of the secondary battery. The presence of pores in the particles of the negative electrode active material, along with the control of the D value within the aforementioned range, facilitates the transport of electrolyte from the surface of the particles to the bottom of the negative electrode material layer via these pores. This improves the wettability of the electrolyte on the negative electrode sheet. Simultaneously, metal ions dissolved from the positive electrode sheet (e.g., Mn, Co, Ni ions) can be deposited into the interior of the negative electrode active material through these pores, or transported and deposited at the bottom of the negative electrode material layer. This reduces the amount of metal ions deposited per unit area on the surface of the negative electrode active material particles, thus minimizing the damage caused by metal ions to the negative electrode sheet. This helps improve lithium plating and internal short-circuit issues in secondary batteries. Furthermore, it shortens the lithium ion transport distance, enhancing the kinetic performance of the secondary battery. Consequently, its application in secondary batteries improves both cycle performance and storage performance.

[0038] In this application, the particles of the negative electrode active material may or may not have through-pores. When the negative electrode active material has through-pores, this application does not impose any particular restrictions on the shape of the through-pores, as long as they achieve the purpose of this application. For example, Figure 3 and Figure 4 A cross-sectional view of the negative electrode active material 111 along the thickness direction of the negative electrode sheet is shown. Figure 3 As shown, the negative electrode active material 111 may include a straight first through hole 111a; as Figure 4 As shown, the negative electrode active material 111 may include a bent second through hole 111b.

[0039] This application does not impose any particular restrictions on the preparation method of the through-pore negative electrode active material, as long as the purpose of this application can be achieved. For example, the through-pore negative electrode active material can be prepared by a template method. The template method preparation process may include, but is not limited to: using a template material, such as polystyrene microspheres or silica, mixing it with the negative electrode active material, sintering it at high temperature, causing the template material to detach, and preparing the through-pore negative electrode active material. The template material is cylindrical in shape, and this application does not impose any particular restrictions on its bottom diameter and height. Those skilled in the art can adjust it according to the average pore size of the through-pores in the negative electrode active material to be prepared, but the height of the template material must be greater than the Dv99 value of the negative electrode active material. This application does not impose any particular restrictions on the process parameters for preparing the above-mentioned through-pore negative electrode active material, and those skilled in the art can choose them according to actual needs, as long as the purpose of this application can be achieved.

[0040] Typically, the average pore size D of the negative electrode active material can be controlled by adjusting the particle size of the template material. For example, when other conditions remain constant, increasing the particle size of the template material increases the D value; decreasing the particle size of the template material decreases the D value.

[0041] In this application, negative electrode active materials with different D values ​​can be purchased, and the D value of the negative electrode active material is tested in conjunction with the test method "Test of the average pore size D of the through-hole of the negative electrode active material particles" provided in this application, and the negative electrode active material with the required D value is selected.

[0042] In some embodiments of this application, the negative electrode active material includes at least one of graphite, silicon, or silicon-carbon. Using the aforementioned negative electrode active material results in a larger particle size, which is beneficial for increasing the critical thickness for drying cracking of the negative electrode material layer. This improves the drying cracking problem during the coating process of the negative electrode sheet when the negative electrode material layer is thicker, and also helps to increase the energy density of the secondary battery. Therefore, its application in secondary batteries is beneficial for improving their energy density and cycle performance.

[0043] In some embodiments of this application, the mass percentage of the negative electrode active material is between 80% and 98%, based on the mass of the negative electrode material layer. For example, the mass percentage of the negative electrode active material can be 80%, 82%, 85%, 88%, 90%, 92%, 96%, 98%, or any combination of two of the above values. By controlling the mass percentage of the negative electrode active material within the above range, a larger mass proportion of the negative electrode active material in the negative electrode material layer is beneficial to improving the energy density of the negative electrode sheet. Therefore, its application in secondary batteries is beneficial to improving their energy density, while also exhibiting good cycle performance.

[0044] In this application, the negative electrode material layer may further include a negative electrode binder and a negative electrode conductive agent, or the negative electrode material layer may further include a negative electrode binder, a negative electrode conductive agent, or a thickener. Based on the mass of the negative electrode material layer, the mass percentage content of the negative electrode binder may be 0.5% to 19%, the mass percentage content of the negative electrode conductive agent may be 0.5% to 19%, and the mass percentage content of the thickener may be 0% to 18%.

[0045] This application does not impose any particular limitation on the types of negative electrode binders and negative electrode conductive agents, as long as they can achieve the purpose of this application. For example, the negative electrode binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon. The negative electrode conductive agent may include, but is not limited to, at least one of carbon-based materials, metal-based materials, or conductive polymers. For example, carbon-based materials may include at least one of natural graphite, artificial graphite, conductive carbon black (Super P), or carbon fiber; metal-based materials may include, but are not limited to, at least one of metal powder, metal fiber, copper, nickel, aluminum, or silver; and conductive polymers may include, but are not limited to, polyphenylene derivatives. This application does not impose any particular limitation on the types of thickeners, as long as they can achieve the purpose of this application. For example, thickeners may include, but are not limited to, at least one of sodium carboxymethyl cellulose or carboxymethyl cellulose.

[0046] In some embodiments of this application, the coating crack width of the negative electrode material layer is less than or equal to 0.3 mm. For example, the coating crack width of the negative electrode material layer can be 0 mm, 0.05 mm, 0.1 mm, 0.14 mm, 0.2 mm, 0.26 mm, 0.3 mm, or any combination of two of the above values. When the coating crack width of the negative electrode material layer is within the above range, the negative electrode sheet does not experience coating cracking problems, or the coating crack width is very small. This helps reduce the probability of "powder shedding" on the negative electrode sheet, reduces the probability of internal short circuits in the secondary battery, and improves the processing performance of the negative electrode sheet. Therefore, its application in secondary batteries is beneficial for improving their cycle performance and safety performance.

[0047] In some embodiments of this application, the compaction density of the negative electrode sheet is 1.2 g / cm³. 3 Up to 1.8 g / cm 3 For example, the compaction density of the negative electrode sheet can be 1.2 g / cm³. 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm3 1.7g / cm 3 1.8g / cm 3 Or any two of the above values ​​within a range. By adjusting the compaction density of the negative electrode sheet within the above range, it is beneficial to keep the porosity of the negative electrode material layer within a suitable range, improve the wettability of the electrolyte to the negative electrode sheet, and also improve the capacity of the negative electrode active material per unit volume. Therefore, when applied to secondary batteries, this is beneficial to improve their energy density and cycle performance.

[0048] In some embodiments of this application, the porosity of the negative electrode material layer is 20% to 60%. For example, the porosity of the negative electrode material layer can be 20%, 25%, 30%, 36%, 40%, 45%, 50%, 56%, 60%, or any combination of two of the above values. By controlling the porosity of the negative electrode material layer within the above range, when the thickness of the negative electrode material layer is large, it is beneficial to transport the electrolyte to the bottom of the negative electrode material layer, improve the wettability of the electrolyte to the negative electrode sheet, and at the same time, it is beneficial to reduce the metal ions deposited per unit area on the surface of the negative electrode active material particles, reduce the damage of metal ions to the negative electrode sheet, improve the lithium plating and internal short circuit problems of the secondary battery, and further shorten the lithium ion transport distance, improve the kinetic performance of the secondary battery, thereby improving its cycle performance and storage performance when applied to secondary batteries.

[0049] Generally, the porosity of the negative electrode material layer can be controlled by adjusting the porosity of the negative electrode active material and the cold pressing pressure. For example, when other conditions remain constant, increasing the porosity of the negative electrode active material increases the porosity of the negative electrode material layer; conversely, decreasing the porosity of the negative electrode active material decreases the porosity of the negative electrode material layer.

[0050] This application does not impose any particular limitation on the negative electrode current collector, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or composite current collectors (e.g., carbon-copper composite current collector, nickel-copper composite current collector, titanium-copper composite current collector, etc.). In this application, there is no particular limitation on the thickness of the negative electrode current collector, as long as it achieves the purpose of this application; for example, the thickness of the negative electrode current collector can be from 4 μm to 12 μm.

[0051] This application does not impose any particular restrictions on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, the preparation method of the negative electrode sheet includes, but is not limited to, the following steps: (1) preparing a negative electrode slurry; (2) coating the negative electrode slurry onto one surface of the negative electrode current collector, drying it, and forming a negative electrode material layer on one surface of the negative electrode current collector; (3) coating the negative electrode slurry onto the other surface of the negative electrode current collector, drying it, and forming a negative electrode material layer on each of the two surfaces of the negative electrode current collector; (4) obtaining the negative electrode sheet by cold pressing, cutting, and welding the negative electrode tabs. This application does not impose any particular restrictions on the solid content of the negative electrode slurry in step (1) above, as long as it can achieve the purpose of this application. This application does not impose any particular restrictions on the drying time and temperature in steps (2) and (3) above, as long as it can achieve the purpose of this application.

[0052] Generally, the thickness of the negative electrode material layer can be controlled by adjusting the coating surface density. For example, when other conditions remain unchanged, increasing the coating surface density of the negative electrode material layer increases its thickness; conversely, decreasing the coating surface density decreases its thickness.

[0053] Normally, the compaction density of the negative electrode sheet can be controlled by adjusting the cold pressing pressure. For example, when other conditions remain unchanged, increasing the cold pressing pressure increases the compaction density of the negative electrode sheet, while decreasing the cold pressing pressure decreases the compaction density of the negative electrode sheet.

[0054] The second aspect of this application provides a secondary battery including the negative electrode sheet provided in the first aspect of this application. The negative electrode sheet provided by this application helps to reduce the manufacturing cost of the secondary battery and also helps to improve the drying and cracking problem during the coating of the negative electrode material layer in the process of preparing the negative electrode sheet, resulting in good processing performance of the negative electrode sheet. Therefore, when applied to a secondary battery, it can give the battery high energy density and good cycle performance.

[0055] The secondary battery of this application also includes a positive electrode sheet. This application does not impose any particular limitation on the positive electrode sheet, as long as it achieves the purpose of this application. For example, the positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. In this application, the positive electrode material layer can be disposed on one surface of the positive current collector along its thickness direction, or it can be disposed on two surfaces of the positive current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of ​​the positive current collector or a part of the positive current collector; this application does not impose any particular limitation, as long as it achieves the purpose of this application.

[0056] This application does not impose any particular limitation on the positive electrode current collector, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, aluminum foil, aluminum alloy foil, or composite current collectors (e.g., aluminum-carbon composite current collectors). This application does not impose any particular limitation on the thickness of the positive electrode current collector and the positive electrode material layer, as long as it achieves the purpose of this application. For example, the thickness of the positive electrode current collector may be 6 μm to 12 μm, and the thickness of the positive electrode material layer may be 30 μm to 500 μm. This application does not impose any particular limitation on the thickness of the positive electrode sheet, as long as it achieves the purpose of this application; for example, the thickness of the positive electrode sheet may be 50 μm to 1012 μm.

[0057] The cathode material layer of this application includes a cathode active material, which comprises a substance capable of reversibly inserting and extracting active ions such as lithium ions. The cathode material layer can be one or more layers, and each layer in a multilayer cathode material layer can contain the same or different cathode active materials. This application does not impose any particular limitation on the cathode active material, as long as it can achieve the purpose of this application. For example, the cathode active material can include, but is not limited to, at least one of lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. The chemical formula of the aforementioned lithium-rich manganese-based material is γLi2MnO3·(1-γ)LiGO2, 0<γ<1, and G is a transition metal such as nickel, cobalt, or iron.

[0058] The positive electrode material layer may further include a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive electrode binder may include, but is not limited to, at least one of the aforementioned negative electrode binders; the positive electrode conductive agent may include, but is not limited to, at least one of the aforementioned negative electrode conductive agents. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in the positive electrode material layer; they can be selected according to actual needs, as long as the purpose of this application is achieved.

[0059] The secondary battery of this application also includes an electrolyte, which comprises a lithium salt and a non-aqueous solvent. The lithium salt may include various lithium salts commonly used in the art, such as at least one selected from LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. This application does not impose any particular limitation on the concentration of the lithium salt in the electrolyte, as long as it achieves the purpose of this application. This application also does not impose any particular limitation on the non-aqueous solvent, as long as it achieves the purpose of this application; for example, it may include, but is not limited to, at least one selected from carbonate compounds, carboxylic acid ester compounds, ether compounds, or other organic solvents. The aforementioned carbonate compounds may include, but are not limited to, at least one selected from chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (EMC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). The aforementioned fluorinated carbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The aforementioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0060] The secondary battery of this application also includes a separator membrane for separating the positive and negative electrode plates, preventing internal short circuits, allowing electrolyte ions to pass freely, and not affecting the electrochemical charging and discharging process. This application does not impose any particular limitation on the separator membrane, as long as it achieves the purpose of this application. For example, the material of the separator membrane may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid; the type of separator membrane may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane, or spun membrane. For example, the separator membrane may include a substrate layer and a surface treatment layer. The substrate layer may be a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer may include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be used. Optionally, a surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic materials. For example, the inorganic layer includes inorganic particles and a binder. This application does not have any particular limitation on the inorganic particles, which may include at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. This application does not have any particular limitation on the binder in the inorganic layer, which may be at least one of the above-mentioned negative electrode binders. The polymer layer contains a polymer. This application does not have any particular limitation on the polymer, which may include at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). In this application, there is no particular limitation on the thickness of the separator, as long as it can achieve the purpose of this application. For example, the thickness of the separator can be from 5 μm to 500 μm.

[0061] The secondary battery of this application also includes a packaging bag for containing the positive electrode, separator, negative electrode, and electrolyte, as well as other components known in the art for secondary batteries. This application does not impose any particular limitation on these other components. This application also does not impose any particular limitation on the packaging bag; it can be any packaging bag known in the art, as long as it achieves the purpose of this application. For example, an aluminum-plastic film packaging bag can be used.

[0062] This application does not impose any particular limitation on the type of secondary battery, which may include any device in which an electrochemical reaction occurs. For example, secondary batteries may include, but are not limited to: lithium metal secondary batteries, lithium-ion secondary batteries (lithium-ion batteries), sodium-ion secondary batteries (sodium-ion batteries), lithium polymer secondary batteries, and lithium-ion polymer secondary batteries.

[0063] The fabrication process of secondary batteries is well known to those skilled in the art, and this application does not impose any particular limitations. For example, it may include, but is not limited to, the following steps: stacking a separator, a positive electrode, a separator, and a negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking a separator, a positive electrode, a separator, and a negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection components, conductive plates, etc., may be placed in the packaging bag as needed to prevent the internal pressure of the secondary battery from rising and overcharging / discharging.

[0064] A third aspect of this application provides an electronic device that includes the secondary battery provided in the second aspect of this application. The secondary battery provided in this application has high energy density and good cycle performance, thereby providing the electronic device with a long service life and good performance.

[0065] The electronic device described in this application is not particularly limited and can be any electronic device known in the prior art. For example, the electronic device may include, but is not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, household large-capacity batteries, and lithium-ion capacitors.

[0066] Example

[0067] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0068] Test methods and equipment:

[0069] Particle size testing

[0070] According to the national standard GB / T19077-2016 "Particle size distribution by laser diffraction", the Dv50 and Dv99 of the negative electrode active material were tested using a laser particle size analyzer (model MS3000).

[0071] Thickness test

[0072] The lithium-ion batteries of each embodiment and comparative example were discharged to 2.5V at 0.1C, and the negative electrode sheets were obtained by disassembly. The negative electrode sheets were cleaned with dimethyl carbonate (DMC), dried at 60°C, and the cross-section of the negative electrode sheet along the thickness direction was polished with argon ion polishing. Then, the cross-section of the negative electrode sheet was observed using a scanning electron microscope (SEM), and the thickness H of the negative electrode material layer was measured.

[0073] Test of the average pore size D of the through-pores in the negative electrode active material particles

[0074] The lithium-ion battery of the test embodiment was discharged to 2.5V at 0.1C, and the negative electrode sheet was obtained by disassembly. The negative electrode sheet was then subjected to brittle fracture along the line connecting the midpoints of its two long sides, and one of these fractured sections was randomly selected as the test sample. The negative electrode sheet test sample was cleaned with dimethyl carbonate (DMC) and dried at 60°C. The surface of the negative electrode sheet perpendicular to its thickness direction and the cross-section along the thickness direction were polished using argon ion polishing. The cross-section and surface of the negative electrode material layer were then observed using a scanning electron microscope (SEM). Fifty negative electrode active material particles were randomly selected from the cross-section and surface, and the pore size of their through-holes was measured. The average value of the 50 pore sizes was calculated as the average pore size D of the negative electrode active material particles.

[0075] Testing of porosity of negative electrode material layer

[0076] The lithium-ion batteries of each embodiment and comparative example were discharged to 2.5V at 0.1C, and the negative electrode sheets were obtained by disassembly. The negative electrode sheets were cleaned with dimethyl carbonate (DMC) and then dried at 60°C. The porosity of the negative electrode material layer was tested using a porosity analyzer via mercury porosimetry. The specific porosity test was conducted according to the national standard GB / T 21650.1-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Porosimetry and Gas Adsorption Methods Part 1: Mercury Porosimetry".

[0077] Testing the compaction density of the negative electrode sheet

[0078] After discharging the lithium-ion batteries of each embodiment and comparative example to 2.5V at 0.1C, the negative electrode sheets were disassembled. These negative electrode sheets were cleaned with dimethyl carbonate (DMC) and then dried at 60°C. A punch with an area of ​​1540.25 mm² was then made at any location on the negative electrode sheet coated with the negative electrode material layer. 2The test sample was weighed, and its mass was recorded as m1 mg. Its thickness was measured and recorded as h1 μm. Then, the negative electrode material layer of the test sample was peeled off, and the mass of the negative electrode current collector was weighed and recorded as m2 mg. Its thickness was measured and recorded as h2 μm. The compaction density of the negative electrode sheet was calculated using the following formula: Compaction density of the negative electrode sheet = (m1 - m2) × 10 3 / (1540.25×(h1-h2)), in g / cm³ 3 .

[0079] Testing of coating crack width of negative electrode material layer

[0080] The lithium-ion batteries of each embodiment and comparative example were discharged to 2.5V at 0.1C, and the negative electrode sheets were obtained after disassembly. The negative electrode sheets were cleaned with dimethyl carbonate (DMC) and then dried at 60°C. The surface of the negative electrode sheet perpendicular to its thickness direction was observed using an optical microscope, and the maximum width of the surface cracks was measured and recorded as the coating crack width of the negative electrode material layer. The processing performance of the negative electrode sheet was evaluated by the coating crack width of the negative electrode material layer. The larger the coating crack width of the negative electrode material layer, the worse the processing performance of the negative electrode sheet.

[0081] Cyclic performance testing

[0082] In a constant temperature environment of 25℃, the lithium-ion battery was charged to 4.2V at a constant current of 0.5C, then charged to 0.05C at a constant voltage of 4.2V, allowed to stand for 10 minutes, and then discharged to 2.8V at a constant current of 0.5C, allowed to stand for 10 minutes. This constitutes one charge-discharge cycle, which is the first cycle, and the discharge capacity of the first cycle is recorded. The lithium-ion battery was charged and discharged in the same manner, and the discharge capacity of each cycle was recorded until the discharge capacity of the lithium-ion battery decreased to 70% of the discharge capacity of the first cycle. The test was then stopped, and the number of charge-discharge cycles was recorded, which is the number of cycles at 25℃.

[0083] Storage performance testing

[0084] The lithium-ion battery was charged at a constant current of 0.2C to 3.95V, then charged at a constant voltage of 3.95V to 0.02C. After storage in a 60℃ oven for 42 days, it was removed and placed in a -10℃ oven. The battery was then charged at a constant current of 0.2C to 4.2V, then charged at a constant voltage of 4.2V to 0.05C, allowed to stand for 60 minutes, and then discharged at a constant current of 0.5C to 2.8V, allowed to stand for 120 minutes. This process was repeated 10 times. The negative electrode was then disassembled, and the surface of the negative electrode was observed for lithium deposition. The non-lithium-deposited areas on the negative electrode surface were golden yellow, while the lithium-deposited areas were grayish-white. The lithium deposition on the surface of the negative electrode in each example and comparative example was statistically analyzed in 10 lithium-ion batteries. The average value was calculated, and the percentage of lithium-deposited area was used to evaluate the lithium deposition status of the negative electrode. The percentage of lithium plating area is calculated based on the total area of ​​the single-sided negative electrode material layer. The criteria for judging the lithium plating state on the surface of the negative electrode are as follows: lithium plating area less than or equal to 1% is considered no lithium plating; lithium plating area greater than 1% and less than or equal to 3% is considered slight lithium plating; lithium plating area greater than 3% and less than or equal to 5% is considered moderate lithium plating; and lithium plating area greater than 5% is considered severe lithium plating. The storage performance of lithium-ion batteries is evaluated by the lithium plating condition of the negative electrode after storage at 60℃. The lower the degree of lithium plating, the better the storage performance of the lithium-ion battery; the higher the degree of lithium plating, the worse the storage performance of the lithium-ion battery.

[0085] Energy density ratio test

[0086] The lithium-ion batteries of each embodiment and comparative example were charged at a constant current of 0.2C to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current of 0.02C, allowed to stand for 30 minutes, and then discharged at a constant current of 0.2C to a voltage of 2.8V. The discharge energy at this point is recorded as the energy E of the lithium-ion battery, and the volume of the lithium-ion battery is measured and recorded as V. The energy density of the lithium-ion battery is calculated using the following formula: Energy density = E / V.

[0087] The energy density test value of Example 1-1 is used as a baseline and recorded as 100%. The ratio of the energy density test values ​​of the lithium-ion batteries in the other examples and comparative examples to the energy density test value of the lithium-ion battery in Example 1-1 is recorded as the energy density ratio. The energy density of lithium-ion batteries is evaluated by the energy density ratio. The higher the energy density ratio of a lithium-ion battery, the higher its energy density; the lower the energy density ratio, the lower its energy density.

[0088] Example 1-1

[0089] <Preparation of Negative Electrode Sheets>

[0090] The negative electrode active material is graphite, and the thickener is sodium carboxymethyl cellulose (CMC-Na, weight average molecular weight Mw = 7 × 10⁻⁶). 5 ), negative electrode binder styrene-butadiene rubber (SBR, Mw = 5 × 10),6 The conductive carbon black (Super P) and negative electrode conductive agent were mixed at a mass ratio of 96:1.5:1.5:1, and then deionized water was added as a solvent. The mixture was stirred evenly under vacuum to obtain a negative electrode slurry with a solid content of 50 wt%. The negative electrode slurry was then uniformly coated on one surface of a 6 μm thick copper foil negative electrode current collector, with a coating density of 15 mg / cm³. 2 The copper foil was dried at 85℃. Then, the above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of negative electrode material. After cold pressing, cutting, and welding of negative electrode tabs and nickel tabs, a negative electrode sheet with dimensions of 76mm × 867mm was obtained for use. The thickness H, compaction density, and porosity of the negative electrode material layer, as well as the Dv50 and Dv99, Dv50 / H, and Dv99 / H of the negative electrode active material, are shown in Table 1.

[0091] <Preparation of the positive electrode>

[0092] The positive electrode active material is lithium iron phosphate, the positive electrode active material is lithium manganese oxide, and the positive electrode binder is polyvinylidene fluoride (PVDF, Mw = 7 × 10⁻⁶). 6 Conductive carbon black (Super P) and carbon nanotubes (CNTs) were mixed in a mass ratio of 19:78:1.5:0.8:0.7, with N-methylpyrrolidone (NMP) added as a solvent. The mixture was stirred evenly under vacuum to obtain a positive electrode slurry with a solid content of 75 wt%. The positive electrode slurry was uniformly coated onto one surface of a 15 μm thick aluminum foil for the positive electrode current collector and dried at 100°C. The above steps were then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive electrode material. After cold pressing, cutting, and welding of positive electrode tabs, a positive electrode sheet with a size of 74 mm × 851 mm was obtained for use. The areal density of the positive electrode material layer was 38.1 mg / cm³. 2 The thickness of the positive electrode material layer is 145 μm.

[0093] <Preparation of Electrolyte>

[0094] In a dry argon atmosphere, non-aqueous solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are first mixed uniformly in a mass ratio of EC:EMC:DEC = 3:5:2 to obtain a base solvent. Then, lithium salt lithium hexafluorophosphate (LiPF6) is added to the base solvent, dissolved, and mixed uniformly to obtain an electrolyte with a lithium salt concentration of 1.15 mol / L.

[0095] <Preparation of the separating membrane>

[0096] A porous polyethylene (PE) polymer film (supplied by Celgard) with a thickness of 8 μm was used as the separator.

[0097] <Preparation of Lithium-ion Batteries>

[0098] The separator, positive electrode, separator, and negative electrode prepared above are stacked and wound in sequence to obtain a wound electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. After vacuum sealing, standing, formation (0.3C constant current charging to 3.5V, and then 1C constant current charging to 3.9V), capacity testing, degassing, and edge trimming, a lithium-ion battery is obtained.

[0099] Examples 1-2 to Examples 1-13

[0100] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Negative Electrode Sheet> section, the rest is the same as in Examples 1-1. Specifically, H is controlled by adjusting the coating surface density of the negative electrode material layer, and Dv50 and Dv99 of the negative electrode active material are controlled by adjusting the ball milling time.

[0101] Examples 1-14 to Examples 1-15

[0102] Except for adjusting the cold pressing pressure to achieve the compaction density of the negative electrode sheet as shown in Table 1 in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0103] Example 2-1

[0104] Except for the following preparation method used for the negative electrode active material in <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0105] <Preparation of Negative Electrode Active Materials>

[0106] Graphite, the negative electrode active material, and silica, the template material (cylindrical in shape with a bottom diameter of 1 μm and a height of 85 μm), were mixed at a mass ratio of 8:2 and sintered at 2500℃ in the absence of air for 12 hours to allow the template material to detach, thus preparing a graphite negative electrode active material with through-pores. The D-value and porosity of the negative electrode material layer are shown in Table 2.

[0107] Examples 2-2 to 2-6

[0108] Except for the fact that the D value and the porosity of the negative electrode material layer are adjusted by controlling the particle size of the template material (specifically the bottom diameter of the cylindrical particles) in the <Preparation of Negative Electrode Active Material> as shown in Table 2, the rest is the same as in Example 2-1.

[0109] Examples 2-7

[0110] Except for adjusting the relevant preparation parameters according to Table 2 in the <Preparation of Negative Electrode Sheet>, the rest is the same as in Example 1-1.

[0111] Examples 2-8

[0112] Except for adjusting the relevant preparation parameters according to Table 2 in <Preparation of Negative Electrode Sheet>, the rest is the same as in Examples 2-3.

[0113] Examples 2-9

[0114] In addition to the negative electrode active material graphite and the thickener sodium carboxymethyl cellulose (CMC-Na, weight average molecular weight Mw = 7 × 10⁻⁶) in the <Preparation of Negative Electrode Sheet>, the other two components are: 5 ), negative electrode binder styrene-butadiene rubber (SBR, Mw = 5 × 10), 6 Except for mixing the negative electrode conductive agent conductive carbon black (Super P) at a mass ratio of 80:5:10:5, the rest is the same as in Example 1-1.

[0115] Example 2-10

[0116] In addition to the negative electrode active material graphite and the thickener sodium carboxymethyl cellulose (CMC-Na, weight average molecular weight Mw = 7 × 10⁻⁶) in the <Preparation of Negative Electrode Sheet>, the other two components are: 5 ), negative electrode binder styrene-butadiene rubber (SBR, Mw = 5 × 10), 6 Except for mixing the negative electrode conductive agent, conductive carbon black (Super P), at a mass ratio of 98:0.5:1:0.5, the rest is the same as in Example 1-1.

[0117] Comparative Examples 1 to 7

[0118] Except for adjusting the relevant preparation parameters according to Table 1 in the <Preparation of Negative Electrode Sheet> section, the rest is the same as in Examples 1-1. Specifically, H is controlled by adjusting the coating surface density of the negative electrode material layer, and Dv50 and Dv99 of the negative electrode active material are controlled by adjusting the ball milling time.

[0119] The preparation and performance parameters of each embodiment and comparative example are shown in Tables 1 and 2.

[0120] Table 1

[0121]

[0122]

[0123] As can be seen from Examples 1-1 to 1-15 and Comparative Examples 1 to 7, the lithium-ion batteries in this application, by adjusting the H and Dv50 / H of the negative electrode sheet within the scope of this application, do not exhibit coating drying cracking problems in the negative electrode material layer, or the coating crack width is smaller. The lithium-ion batteries have more cycles at 25°C, and the lithium deposition after storage at 60°C is improved, indicating that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is high, with improved storage performance. However, in the lithium-ion batteries of Comparative Examples 1 to 7, the H and / or Dv50 / H of the negative electrode sheet are not within the scope of this application, the coating crack width of the negative electrode material layer is larger, and / or, the number of cycles at 25°C is fewer, and / or, the lithium deposition phenomenon is severe after storage at 60°C, indicating that the processing performance of the negative electrode sheet is worse, and / or, the cycle performance of the lithium-ion battery is worse, and / or, the storage performance of the lithium-ion battery is worse.

[0124] The value of H typically affects the cycle performance of lithium-ion batteries. As seen in Examples 1-1 to 1-2, Examples 1-8 to 1-11, and Comparative Examples 4 to 5, when the value of H is too large, such as in Comparative Example 5, the coating crack width of the negative electrode material layer is larger, and the number of 25°C cycles for the lithium-ion battery is fewer. When the value of H is too small, such as in Comparative Example 4, although the lithium-ion battery has a higher number of 25°C cycles, the manufacturing cost is higher, and the energy density is lower. This indicates that the processing performance of the negative electrode sheet is worse, and the cycle performance of the lithium-ion battery is worse. By adjusting the value of H within the scope of this application, the negative electrode material layer does not exhibit coating drying cracking problems or the coating crack width is smaller, and the lithium-ion battery has a higher number of 25°C cycles, indicating that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is higher.

[0125] The Dv50 / H value typically affects the cycle performance of lithium-ion batteries. As seen in Examples 1-1, 1-4 to 1-7, Comparative Examples 1 to 3, and 6 to 7, when the Dv50 / H value is too high, such as in Comparative Examples 3 and 7, the lithium-ion battery has fewer cycles at 25°C. Conversely, when the Dv50 / H value is too low, such as in Comparative Examples 1, 2, and 6, the coating crack width of the negative electrode material layer is larger, resulting in fewer cycles at 25°C. This indicates poorer processing performance of the negative electrode sheet and consequently, poorer cycle performance of the lithium-ion battery. By adjusting the Dv50 / H value within the scope of this application, the negative electrode material layer did not exhibit coating drying cracking issues, or the coating crack width was smaller. The lithium-ion battery exhibited more cycles at 25°C, indicating that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is high.

[0126] The value of Dv50 typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-12, when the Dv50 value is within the range specified in this application, the negative electrode material layer does not exhibit coating drying cracking or the crack width is small. The lithium-ion battery exhibits a high number of cycles at 25°C, indicating that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is high.

[0127] The values ​​of Dv99 and Dv99 / H typically affect the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1 to 1-13, when the values ​​of Dv99 and Dv99 / H are within the range of this application, the negative electrode material layer does not exhibit coating drying cracking problems or the coating crack width is small. The lithium-ion battery has a high number of cycles at 25°C, indicating that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is high.

[0128] The compaction density of the negative electrode sheet typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-14, and 1-15, when the compaction density of the negative electrode sheet is within the range specified in this application, the negative electrode material layer does not exhibit coating drying cracking issues, and the lithium-ion battery has a high number of cycles at 25°C. This indicates that the negative electrode sheet has good processing performance, the lithium-ion battery has good cycle performance, and the energy density is high.

[0129] Table 2

[0130]

[0131] Note: " / " in Table 2 indicates that there is no corresponding parameter.

[0132] The average pore size D of the through-holes in the negative electrode active material particles typically affects the cycle performance and storage performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-6, when the value of D is within the range specified in this application, the negative electrode material layer does not exhibit coating drying cracking problems, the lithium-ion battery has a high number of cycles at 25°C, and no lithium plating occurs after storage at 60°C, indicating that the negative electrode sheet has good processing performance, and the lithium-ion battery has good cycle performance and storage performance.

[0133] The porosity of the negative electrode material layer typically affects the cycle performance and storage performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-1 to 2-6, when the porosity of the negative electrode material layer is within the range specified in this application, no coating drying cracking problem occurs. The lithium-ion battery exhibits a high number of cycles at 25°C and does not show lithium plating after storage at 60°C, indicating that the negative electrode sheet has good processing performance and the lithium-ion battery has good cycle performance and storage performance.

[0134] The type of negative electrode active material typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-3, 2-7 to 2-8, when the negative electrode active material within the scope of this application is selected, the negative electrode material layer does not exhibit coating drying cracking problems, and the lithium-ion battery has a high number of cycles at 25°C, indicating that the negative electrode sheet has good processing performance and the lithium-ion battery has good cycle performance.

[0135] The mass percentage of the negative electrode active material typically affects the cycle performance of lithium-ion batteries. As can be seen from Examples 1-1, 2-9 to 2-10, when the mass percentage of the negative electrode active material is within the range specified in this application, the negative electrode material layer does not exhibit coating drying cracking issues, and the lithium-ion battery has a high number of cycles at 25°C. This indicates that the negative electrode sheet has good processing performance, and the lithium-ion battery exhibits good cycle performance.

[0136] It should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0137] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0138] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode material layer has a thickness of H μm, 85 ≤ H ≤ 250. The negative electrode material layer comprises a negative electrode active material, and Dv50 μm of the negative electrode active material and the H satisfy: 0.4 ≤ Dv50 / H ≤ 0.

8.

2. The negative electrode sheet according to claim 1, wherein 34 ≤ Dv50 ≤ 200.

3. The negative electrode sheet according to claim 1, wherein 0.5 ≤ Dv50 / H ≤ 0.

8.

4. The negative electrode sheet according to claim 1, wherein Dv99 μm of the negative electrode active material and the H satisfy: 0.6 ≤ Dv99 / H ≤ 0.

9.

5. The negative electrode sheet according to claim 4, wherein 0.7 ≤ Dv99 / H ≤ 0.

9.

6. The negative electrode sheet according to claim 4, wherein 51 ≤ Dv99 ≤ 225.

7. The negative electrode sheet according to claim 1, wherein The particles of the negative electrode active material have through holes, and the average pore diameter of the through holes is D μm, 1 ≤ D ≤ 10.

8. The negative electrode sheet according to claim 7, wherein 1≤D≤2。 9. The negative electrode sheet according to claim 1, wherein The negative electrode active material comprises at least one of graphite, silicon or silicon-carbon.

10. The negative electrode sheet according to claim 1, wherein The negative electrode sheet satisfies at least one of the following characteristics: (1) the mass percentage content of the negative electrode active material is 80% to 98% based on the mass of the negative electrode material layer; (2) the coating crack width of the negative electrode material layer is less than or equal to 0.3 mm; (3) the compaction density of the negative electrode plate is 1.2 g / cm 3 to 1.8 g / cm 3 ; (4) the porosity of the negative electrode material layer is 20% to 60%. 11.A secondary battery comprising the negative electrode sheet according to any one of claims 1 to 10. 12.An electronic device comprising the secondary battery according to claim 11.

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