Positive electrode sheet, secondary battery and electronic device

Abstract

The present application provides a positive electrode sheet, a secondary battery, and an electronic device, wherein the positive electrode sheet includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. A protective layer is disposed on the positive current collector surfaces on both sides of the positive active material layer along the width of the positive electrode sheet. The saturated water absorption rate of the protective layer is A, and 40% ≤ A ≤ 125%. By disposing the protective layer on the positive current collector surfaces on both sides of the positive active material layer, the present application provides the edge of the positive electrode sheet with a protective layer, and regulating the saturated water absorption rate A of the protective layer within the above range, thereby reducing side reactions caused by excess water and the possibility of side reactions between the protective layer and the positive active material, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a positive electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] To increase the energy density of lithium-ion batteries, the cathode active materials used in lithium-ion batteries are gradually shifting to high-nickel ternary (NCM) materials. However, due to the high amount of residual lithium on the surface of NCM materials, they easily absorb water when exposed to air. This water absorption increases with exposure time, causing the performance of lithium-ion batteries to deteriorate.

[0003] At present, in order to solve the problem of high moisture content in lithium-ion batteries, a high-temperature baking process is usually added (for example, a temperature of 80°C to 110°C and a time of 4h to 36h). This process consumes a lot of energy, time and manpower, and the effect is not ideal. It cannot significantly reduce the water content of lithium-ion batteries, especially it cannot avoid the irreversible phase change on the surface of positive active materials such as high-nickel ternary and LiFePO4 after absorbing water. Alternatively, the problem of water absorption by positive active materials can be avoided by adopting a dry workshop production throughout the process, but this will lead to a sharp increase in manufacturing costs. Therefore, the development of a new method to reduce the water content in positive electrode sheets has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the Invention

[0004] The purpose of this application is to provide a positive electrode sheet, a secondary battery, and an electronic device to reduce the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery. The specific technical solution is as follows:

[0005] It should be noted that in the invention content of this application, lithium-ion batteries are used as an example of secondary batteries to explain this application, but the secondary batteries of this application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] In a first aspect, the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. A protective layer is disposed on the positive electrode current collector surfaces on both sides of the positive electrode active material layer along the width direction of the positive electrode plate; the saturated water absorption rate of the protective layer is A, 40% ≤ A ≤ 125%. The present application provides protective layers on the positive electrode current collector surfaces on both sides of the positive electrode active material layer, thereby providing protective layers along the width direction edges of the positive electrode plate. The saturated water absorption rate of the protective layer is controlled within the above-mentioned range, thereby forming protective layers along the width direction edges of the positive electrode plate, i.e., on both sides of the positive electrode active material layer along the width direction. The protective layers have excellent drying and water-fixing effects, can reduce the possibility of water vapor penetrating into the interior of the positive electrode plate during storage of the positive electrode plate and during production of secondary batteries, and can reduce the water content in the positive electrode plate. Furthermore, compared to providing a protective layer on the entire surface of the positive electrode active material layer, providing a protective layer on both sides of the positive electrode active material layer along the width direction reduces the possibility of the protective layer contacting the positive electrode active material in the positive electrode active material layer, and reduces the risk of side reactions caused by the contact between the protective layer and the positive electrode active material. As a result, when the positive electrode sheet is used in a secondary battery, it can reduce side reactions caused by excessive moisture in the positive electrode sheet, as well as side reactions between desiccant or other substances and the positive electrode active material, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0007] Preferably, 60%≤A≤125%. Adjusting the saturated water absorption rate A of the protective layer within the above preferred range is beneficial to reducing the water content in the positive electrode sheet, thereby further reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0008] In some embodiments of the present application, the protective layer includes a desiccant, and the desiccant includes at least one of anhydrous sodium sulfate, anhydrous calcium chloride, anhydrous magnesium sulfate, anhydrous aluminum chloride, anhydrous aluminum oxide, silica gel desiccant, montmorillonite desiccant, or an active mineral desiccant; based on the mass of the protective layer, the mass percentage of the desiccant is W g %,70≤W g ≤99. Preferably, 90≤W g ≤95. The above desiccant is selected for the protective layer, and the mass percentage of the desiccant in the protective layer is W g % is controlled within the above range, which is beneficial to reducing the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0009] In some embodiments of the present application, the volume average particle size of the desiccant is Dv50 μm, and 0.01 ≤ Dv50 ≤ 200. Preferably, 0.05 ≤ Dv50 ≤ 50. Controlling the volume average particle size Dv50 of the desiccant within the above range helps reduce the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0010] In some embodiments of the present application, the width of the protective layer along the width direction of the positive electrode sheet is L1 mm, where 0.1 ≤ L1 ≤ 50. Preferably, 10 ≤ L1 ≤ 20. Controlling the width of the protective layer L1 mm within the above range helps reduce the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0011] In some embodiments of the present application, the thickness of the protective layer is H1 μm, the thickness of the positive electrode active material layer is H2 μm, 10 ≤ H2 ≤ 8000, and 0.8H2 ≤ H1 ≤ 1.01H2. Preferably, 0.95H2 ≤ H1 ≤ 1.0H2. This helps reduce the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0012] In some embodiments of the present application, the positive electrode active material layer and the protective layer have an overlapping region along the thickness direction of the positive electrode sheet. In this overlapping region, the positive electrode active material layer is located between the protective layer and the positive electrode current collector. Along the width direction of the positive electrode sheet, the width of the overlapping region is L2 mm, where 0 < L2 ≤ 5. By regulating the thickness of the positive electrode active material layer and the protective layer within the above range, the loss of gram capacity in the secondary battery is reduced and the storage performance of the secondary battery is improved without affecting the energy density and packaging performance of the secondary battery.

[0013] In some embodiments of the present application, the protective layer further comprises a dispersant and a binder; based on the mass of the protective layer, the mass percentage of the dispersant is W f %, 0.5≤W f ≤15, the mass percentage of the binder is W n %, 0.5≤W n ≤15. The protective layer includes the above-mentioned dispersant and binder, and the mass percentage of the dispersant and binder in the protective layer is W f % and W n % is controlled within the above range, which is beneficial to reducing the water content in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0014] The second aspect of the present application provides a secondary battery comprising the positive electrode sheet described in any one of the aforementioned embodiments. Therefore, the secondary battery has good storage performance.

[0015] The third aspect of the present application provides an electronic device, which includes the secondary battery according to any one of the aforementioned embodiments. Therefore, the electronic device has good storage performance.

[0016] Beneficial effects of the embodiments of the present application:

[0017] Generally, the exposure time of the positive electrode active material layer is mainly during the storage process of the positive electrode sheet and the production process of the secondary battery. The positive electrode sheet is usually stored in a roll. In the embodiment of the present application, a protective layer is provided on the surface of the positive current collector on both sides of the positive electrode active material layer, so that the edge of the positive electrode sheet is provided with a protective layer, and the saturated water absorption rate A of the protective layer is regulated within the above range. In this way, the edge of the positive electrode sheet has a drying and water-fixing function, which can reduce the possibility of water vapor penetrating into the interior of the positive electrode sheet during the storage of the positive electrode sheet and the production process of the secondary battery, reduce the possibility of water absorption in the positive electrode active material layer, and reduce the water content in the positive electrode sheet. In addition, compared with the protective layer being provided on the entire surface of the positive electrode active material layer, the protective layer is provided on both sides of the positive electrode active material layer along the width direction, which reduces the possibility of the protective layer contacting the positive electrode active material in the positive electrode active material layer, and reduces the risk of side reactions occurring when the protective layer contacts the positive electrode active material. This reduces side reactions caused by excess moisture and the possibility of side reactions between the desiccant and other substances in the protective layer and the positive electrode active material, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery. Furthermore, the positive electrode sheets can be produced in a non-drying room, eliminating the need for a high-temperature baking process, thereby reducing the production cost of the secondary battery.

[0018] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other embodiments can also be obtained based on these drawings.

[0020] Figure 1 This is a schematic diagram of the structure of the positive electrode sheet in some embodiments of the present application;

[0021] Figure 2 for Figure 1 Schematic diagram of the cross-section structure along the AA direction;

[0022] Figure 3 This is a schematic diagram of the positive electrode sheets being placed in a roll during storage in some embodiments of the present application. DETAILED DESCRIPTION

[0023] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field based on this application are within the scope of protection of this application.

[0024] like Figures 1 to 3 As shown, the first aspect of the present application provides a positive electrode sheet 10. For ease of understanding, a three-dimensional rectangular coordinate system is established with the length direction of the positive electrode sheet 10 itself as the X direction, the width direction of the positive electrode sheet 10 itself as the Y direction, and the thickness direction of the positive electrode sheet 10 itself as the Z direction. It can be understood that the length direction, width direction, and thickness direction of the positive electrode collector, the positive electrode active material layer, and the protective layer are the same as those of the positive electrode sheet 10. The positive electrode sheet 10 includes a positive electrode collector 13 and a positive electrode active material layer 12 disposed on at least one surface of the positive electrode collector 13. Along the width direction Y of the positive electrode sheet 10, a protective layer 11 is disposed on the surface of the positive electrode collector 13 on both sides of the positive electrode active material layer 12; the saturated water absorption rate of the protective layer 11 is A, 40%≤A≤125%. For example, A is 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, 105%, 110%, 115%, 120%, 125% or any value between any two of the above numerical ranges.

[0025] Generally, the exposure time of the positive electrode active material layer 12 is mainly during the storage process of the positive electrode sheet 10 and the production process of the secondary battery, especially the storage process of the positive electrode sheet 10 occupies most of the exposure time. The positive electrode sheet 10 is usually placed in a roll when stored (such as Figure 3As shown in the figure, when the roll is placed, the edge of the positive electrode sheet 10 along the width direction Y is closer to the air and more likely to come into contact with moisture, which increases the water content in the positive electrode active material layer 12. When the saturated water absorption rate A of the protective layer 11 is less than 40%, the protective layer 11 has poor adsorption properties for liquids (such as water vapor) and has a poor drying and water-binding effect. As a result, liquids (such as water vapor) easily infiltrate the surface of the protective layer 11, increasing the water content in the positive electrode sheet 10, thereby affecting the performance of the secondary battery, such as a decrease in specific capacity and a deterioration in storage performance. In the present application, protective layers 11 are provided on the surfaces of the positive current collector 13 on both sides of the positive active material layer 12, thereby providing protective layers 11 on the edges of the positive electrode sheet 10 along the Y direction. The saturated water absorption rate A of the protective layer 11 is controlled within the above-mentioned range, thereby forming protective layers 11 on the edges of the positive electrode sheet 10 along the Y direction, that is, on both sides of the positive active material layer 12 along the Y direction. This protective layer 11 has excellent drying and water-binding properties, reducing the possibility of water vapor penetrating into the interior of the positive electrode sheet 10 during storage and secondary battery production, thereby reducing the water content in the positive electrode sheet 10. Furthermore, compared to providing protective layers 11 on the entire surface of the positive active material layer 12, providing protective layers 11 on both sides of the positive active material layer 12 along the Y direction reduces the possibility of contact between the protective layer 11 and the positive active material in the positive active material layer 12, thereby reducing the risk of side reactions caused by contact between the protective layer and the positive active material. Thus, when the positive electrode sheet 10 is used in a secondary battery, side reactions caused by excess moisture in the positive electrode sheet 10 can be reduced, as can side reactions between desiccants or other substances and the positive electrode active material. This reduces the loss of gram capacity in the secondary battery and improves the storage performance of the secondary battery. Furthermore, the positive electrode sheet 10 can be produced in a non-drying room, eliminating the need for a high-temperature baking process, thereby reducing the production cost of the secondary battery.

[0026] Preferably, 60%≤A≤125%. For example, A is 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, 105%, 110%, 115%, 120%, 125% or any value between any two of the above numerical ranges. The saturated water absorption rate A of the protective layer is regulated within the above preferred range, and a protective layer is formed on both sides of the positive electrode active material layer along the Y direction. The drying and water-solidifying effect of the protective layer is further enhanced, which can further reduce the possibility of water vapor penetrating into the interior of the positive electrode during the storage of the positive electrode and the production of the secondary battery, and further reduce the water content in the positive electrode. Therefore, when the positive electrode is used in a secondary battery, the side reactions caused by the high moisture content of the positive electrode can be further reduced, thereby further reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0027] The present application does not specifically limit the method for controlling the saturated water absorption rate of the protective layer, as long as the objectives of the present application can be achieved. For example, the saturated water absorption rate of the protective layer can be controlled by adjusting the type of desiccant, the average volume particle size Dv50 of the desiccant, the mass percentage of the desiccant in the protective layer, the surface smoothness of the protective layer, and other factors.

[0028] The above-mentioned "positive electrode active material layer provided on at least one surface of the positive electrode current collector" refers to a positive electrode active material layer provided on one surface of the positive electrode current collector, or a positive electrode active material layer provided on both surfaces of the positive electrode current collector. Here, "surface" can be part of the surface or the entire surface of the positive electrode current collector. It can be understood that along the width direction of the positive electrode sheet, the protective layer is provided on both sides of the positive electrode active material layer. In some embodiments of the present application, the positive electrode active material layer is provided on both surfaces of the positive electrode current collector, and the protective layer is also provided on both surfaces of the positive electrode current collector.

[0029] In some embodiments of the present application, the protective layer includes a desiccant, and the desiccant includes at least one of anhydrous sodium sulfate, anhydrous calcium chloride, anhydrous magnesium sulfate, anhydrous aluminum chloride, anhydrous aluminum oxide, silica gel desiccant, montmorillonite desiccant, or an active mineral desiccant; based on the mass of the protective layer, the mass percentage of the desiccant is W g %,70≤W g ≤99. Preferably, 90≤W g ≤95. For example, W g The desiccant is 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 95, 97, 99 or any value between any two of the above numerical ranges. The above-mentioned desiccant has a good saturated water absorption rate. The above-mentioned desiccant is selected for use in the protective layer, and the mass percentage of the desiccant in the protective layer is W g When the % is controlled within the above range, the saturated water absorption rate of the protective layer is within the scope of this application, so that the protective layer has a good drying and water-fixing effect, and the protective layer has good stability, which can reduce the possibility of water vapor penetrating into the interior of the positive electrode sheet during storage and secondary battery production, thereby reducing the water content in the positive electrode sheet. As a result, when the positive electrode sheet is used in a secondary battery, it can reduce side reactions caused by excessive moisture in the positive electrode sheet, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0030] In some embodiments of the present application, the volume average particle size of the desiccant is Dv50 μm, 0.01≤Dv50≤200. Preferably, 0.05≤Dv50≤50. For example, Dv50 is 0.01, 0.03, 0.05, 0.1, 1, 10, 20, 30, 40, 50, 100, 150, 200 or any value between any two of the above numerical ranges. Regulating the volume average particle size Dv50 of the desiccant within the above range is beneficial to the uniform distribution of the desiccant in the protective layer, and is also beneficial to regulating the thickness of the protective layer, so that the protective layer has a good drying and water-fixing effect, which can reduce the possibility of water vapor penetrating into the interior of the positive electrode during the storage of the positive electrode and during the production of the secondary battery, reduce the water content in the positive electrode, and reduce the risk of damage to the energy density of the secondary battery due to increased thickness of the protective layer. Therefore, when the positive electrode plate is applied to a secondary battery, while taking into account the energy density, the side reactions caused by the high moisture content in the positive electrode plate can be reduced, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0031] In some embodiments of the present application, the protective layer further includes a dispersant and a binder, the dispersant is at least one of sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li), and the binder is at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene, polyvinylidene fluoride, styrene-butadiene rubber or polyvinyl alcohol; based on the mass of the protective layer, the mass percentage of the dispersant is W f %, 0.5≤W f ≤15, for example, W f The mass percentage of the binder is W n %, 0.5≤W n ≤15. For example, W n The protective layer includes the above-mentioned dispersant and binder, and the mass percentage of the dispersant and binder in the protective layer is W f % and W n When the percentage of desiccant is controlled within the above range, it is beneficial for the desiccant to be evenly dispersed in the protective layer slurry when preparing the protective layer slurry, and it is beneficial for the protective layer slurry to be evenly coated when coating the positive electrode sheet. In this way, the protective layer formed is more conducive to having a good drying and water-fixing effect, which can reduce the possibility of water vapor penetrating into the interior of the positive electrode sheet during storage of the positive electrode sheet and during the production of the secondary battery, and reduce the water content in the positive electrode sheet. Therefore, when the positive electrode sheet is used in a secondary battery, the side reactions caused by the high moisture content of the positive electrode sheet can be reduced, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0032] In some embodiments of the present application, Figure 1 and Figure 2 , along the width direction Y of the positive electrode sheet 10, the width of the protective layer 11 is L1 mm, 0.1≤L1≤50. Preferably, 10≤L1≤20. For example, L1 is 0.1, 1, 3, 5, 8, 10, 20, 25, 30, 35, 40, 45, 50 or any value between any two of the above numerical ranges. By regulating the width L1 mm of the protective layer within the above range, the protective layer can achieve a good drying and water-fixing effect, reduce the possibility of water vapor penetrating into the interior of the positive electrode sheet during the storage of the positive electrode sheet and the production of the secondary battery, and reduce the water content in the positive electrode sheet. Therefore, when the positive electrode sheet is applied to the secondary battery, the side reactions caused by the high moisture content of the positive electrode sheet can be reduced, thereby reducing the loss of gram capacity in the secondary battery and improving the storage performance of the secondary battery.

[0033] In some embodiments of the present application, the thickness of the protective layer is H1 μm, the thickness of the positive electrode active material layer is H2 μm, 10≤H2≤8000, 0.8H2≤H1≤1.01H2. Preferably, 0.95H2≤H1≤1.0H2. For example, H2 is 10, 100, 200, 300, 400, 500, 600, 700, 800, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, or any value between any two of the above numerical ranges. H1 is 0.8H2, 0.9H2, 0.92H2, 0.95H2, 0.97H2, 0.99H2, 1.0H2, 1.01H2, or any value between any two of the above numerical ranges. By regulating the thickness of the positive electrode active material layer and protective layer within the above range, the protective layer has a good drying and water-retaining effect without affecting the energy density and packaging performance of the secondary battery. This reduces the possibility of water vapor penetrating into the positive electrode sheet during storage and secondary battery production, reduces the water content in the positive electrode sheet, and can reduce the waste caused by excessive use of protective layer material. As a result, the loss of gram capacity in the secondary battery is reduced and the storage performance of the secondary battery is improved without affecting the energy density and packaging performance of the secondary battery.

[0034] The present application has no particular limitation on the thickness H1 of the protective layer, as long as the purpose of the present application can be achieved. For example, H1 is 8 to 8080 μm.

[0035] In some embodiments of the present application, Figure 2As shown, along the thickness direction Z of the positive electrode sheet 10, the positive electrode active material layer 12 and the protective layer 11 have an overlapping region 20. In this overlapping region 20, the positive electrode active material layer 12 is located between the protective layer 11 and the positive electrode current collector 13. Along the width direction Y of the positive electrode sheet 10, the width of the overlapping region 20 is L2 mm, where 0 < L2 ≤ 5. For example, L2 is 0.1, 0.5, 1, 2, 4, 5, or any value between any two of the aforementioned ranges. The overlapping region between the positive electrode active material layer and the protective layer, and the width of the overlapping region controlled within the aforementioned range, can reduce the risk of direct exposure of the edge of the positive electrode active material layer. While minimizing the contact area between the positive electrode active material layer and the protective layer, the protective layer can exert its excellent drying and water-binding properties, reducing the possibility of water vapor penetrating into the interior of the positive electrode sheet and reducing the water content in the positive electrode sheet. Therefore, when the positive electrode sheet is used in a secondary battery, side reactions caused by excessive moisture in the positive electrode sheet can be reduced, as can side reactions between the desiccant or other substances in the protective layer and the positive electrode active material. This reduces the loss of gram capacity in the secondary battery, improves the storage performance of the secondary battery, and can also reduce the production cost of the secondary battery.

[0036] The present application has no particular restrictions on the type of positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector may include aluminum foil, aluminum alloy foil, etc. The positive electrode active material layer of the present application includes positive electrode active materials. The present application has no particular restrictions on the type of positive electrode active materials, as long as the transition metal elements of the present application are included and the purpose of the present application can be achieved. For example, the positive electrode active material may include lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganate, lithium iron manganese phosphate or lithium titanate, etc. In the present application, the positive electrode active material may also contain non-metallic elements, for example, non-metallic elements include at least one of fluorine, phosphorus, boron, chlorine, silicon or sulfur, which can further improve the stability of the positive electrode active material. In the present application, there is no particular restriction on the thickness of the positive electrode current collector and the positive electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5μm to 20μm, preferably 6μm to 18μm. The thickness of the positive electrode active material layer is 30μm to 120μm.

[0037] Optionally, the positive electrode active material layer may further include a positive electrode conductor and a positive electrode binder. The present application has no particular restrictions on the types of positive electrode conductors and positive electrode binders in the positive electrode active material layer, as long as the purpose of the present application can be achieved. The present application has no particular restrictions on the mass ratio of the positive electrode active material, the positive electrode conductor, and the positive electrode binder in the positive electrode active material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved. For example, the mass ratio of the positive electrode active material, the positive electrode conductor, and the positive electrode binder in the positive electrode active material layer is (95-98): (0.5-2.5): (1.5-3.4).

[0038] The present application has no particular restrictions on the method for preparing the positive electrode sheet, as long as the purpose of the present application can be achieved. For example, the method for preparing the positive electrode sheet includes the following steps: (1) a positive electrode slurry is coated on the middle area of one surface of the positive electrode collector along the width direction of the positive electrode collector, and then a protective layer slurry is coated on the positive electrode collector surface on both sides of the positive electrode slurry, and after drying and cold pressing, a positive electrode sheet with a positive electrode active material layer and a protective layer set on one side is formed; (2) optionally, step (1) is repeated on the other surface of the positive electrode collector to obtain a positive electrode sheet with a positive electrode active material layer and a protective layer set on both sides. The protective layer slurry includes the desiccant, dispersant and binder of the present application, and the mass ratio of the desiccant, dispersant and binder is (80-99): (0.5-15): (0.5-15) based on the mass of the protective layer. The present application has no particular restrictions on the solid content of the positive electrode slurry and the protective layer slurry, as long as the purpose of the present application can be achieved. For example, the solid content of the positive electrode slurry is 50 wt % to 80 wt %, and the solid content of the protective layer slurry is 80 wt % to 80 wt %.

[0039] The second aspect of the present application provides a secondary battery comprising the positive electrode sheet described in any one of the aforementioned embodiments. Therefore, the secondary battery has good storage performance.

[0040] In some embodiments of the present application, a secondary battery includes a packaging bag, an electrode assembly, and an electrolyte, wherein the electrode assembly and the electrolyte are contained in the packaging bag. The present application does not particularly limit the structure of the electrode assembly, as long as it can achieve the purpose of the present application. For example, the structure of the electrode assembly is a laminated structure or a wound structure. The electrode assembly includes a negative electrode sheet, a separator, and a positive electrode sheet as described in any of the aforementioned embodiments of the present application, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet.

[0041] This application places no particular restrictions on the negative electrode sheet, as long as the objectives of this application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. This application places no particular restrictions on the negative electrode current collector, as long as the objectives of this application can be achieved. For example, the negative electrode current collector can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, etc. The negative electrode active material layer of this application contains negative electrode active material. This application places no particular restrictions on the type of negative electrode active material, as long as the objectives of this application can be achieved. For example, the negative electrode active material can include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0 < x < 2), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithium titanate with a spinel structure Li4Ti5O 12 , Li-Al alloy or at least one of metallic lithium. In this application, there are no particular restrictions on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the objectives of this application can be achieved. For example, the thickness of the negative electrode current collector is 6 μm to 10 μm, and the thickness of the negative electrode active material layer is 30 μm to 130 μm. Optionally, the negative electrode active material layer can further include at least one of a conductive agent, a stabilizer, and a binder. This application places no particular restrictions on the types of the negative electrode conductive agent, stabilizer, and negative electrode binder in the negative electrode active material layer, as long as the objectives of this application can be achieved. This application places no particular restrictions on the mass ratio of the negative electrode active material, negative electrode conductive agent, thickener, and negative electrode binder in the negative electrode active material layer, as long as the objectives of this application can be achieved. For example, the mass ratio of the negative electrode active material, negative electrode conductive agent, thickener, and negative electrode binder in the negative electrode active material layer is (96 - 98):(0.5 - 2):(0 - 1.5):(1.0 - 1.9).

[0042] This application places no particular restrictions on the separator, as long as the objectives of this application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator can include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a calendared membrane or a spun membrane.

[0043] This application places no particular restrictions on the packaging bag and the electrolyte, and they can be the packaging bag and electrolyte well-known in the art, as long as the objectives of this application can be achieved.

[0044] The present application does not particularly limit the type of secondary battery, which may include any device that undergoes an electrochemical reaction. 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.

[0045] This application does not particularly limit the preparation method of the secondary battery, and any preparation method known in the art may be selected as long as the purpose of this application can be achieved. For example, the preparation method of the secondary battery includes but is not limited to the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, and other operations as needed to obtain a wound electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the packaging bag to obtain an electrochemical device; or stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fixing the four corners of the entire stacked structure to obtain a stacked electrode assembly, placing the electrode assembly in a packaging bag, injecting an electrolyte into the packaging bag, and sealing the packaging bag to obtain an electrochemical device.

[0046] The third aspect of the present application provides an electronic device, which includes the secondary battery according to any one of the aforementioned embodiments. Therefore, the electronic device has good storage performance.

[0047] The electronic device of the present application is not particularly limited and can be any electronic device known in the art. For example, the electronic device can include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0048] Example

[0049] Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

[0050] Test methods and equipment:

[0051] Test of saturated water absorption A:

[0052] The lithium-ion batteries of each embodiment and comparative example were fully discharged and disassembled to obtain the positive electrode sheets. Ten samples of the same size (length × width = 20 mm × 5 mm) were cut from the part of the positive electrode sheet including the positive current collector and the protective layer. In order to ensure that the 10 samples were initially free of water, they needed to be pretreated in a drying room, that is, the 10 samples were placed in an oven at 180°C and baked for 30 minutes, and then weighed, recorded as m1; thereafter, the 10 samples were placed in a humid environment with a humidity (RH) of 75%, left to stand for 24 hours, and weighed, recorded as m2; the saturated water absorption rate (RSA) = (m2-m1) / m1×100%.

[0053] Test of water content of positive electrode:

[0054] The water content of the positive electrode was obtained by Karl Fischer coulometric method and Karl Fischer moisture meter.

[0055] Gram Capacity Loss Test:

[0056] In an environment of 25°C, the lithium-ion batteries of each embodiment and comparative example were charged and discharged for the first time. Constant current charging was performed at a charging current of 1A to an upper cut-off voltage of 4.3V, and then constant voltage charging was performed to a current of ≤0.05A, and the charging capacity of the first cycle was recorded; after standing for 60 minutes, constant current discharge was performed at a discharge current of 1A to a lower cut-off voltage of 3V, and the discharge capacity of the first cycle was recorded.

[0057] Gram capacity (mAh / g) = discharge capacity (mAh) / mass of positive electrode active material.

[0058] The mass of the positive electrode active material = the coating weight per unit area × the amount of positive electrode active material loaded per unit area × the area of the positive electrode sheet.

[0059] Based on the gram capacity of Comparative Example 1, the loss in gram capacity=(gram capacity of each example or comparative example−gram capacity of Comparative Example 1) / gram capacity of Comparative Example 1×100%.

[0060] Storage performance test:

[0061] The lithium-ion batteries of the embodiments and comparative examples were charged at a constant current of 1 A at 25° C. to an upper cutoff voltage of 4.3 V, and then charged at a constant voltage to a current of ≤ 0.05 A. The thickness of the lithium-ion batteries was measured and recorded as the initial thickness d0. The batteries were placed in an 85° C. oven for 4 h, and the thickness at this time was monitored and recorded as d.

[0062] The thickness expansion ratio (%) of the lithium-ion battery after storage at 85°C for 4 hours = (d-d0) / d0×100%.

[0063] Example 1-1

[0064] <Preparation of positive electrode sheet>

[0065] The positive electrode active material lithium nickel cobalt manganese oxide (LiNi 0.8 Co 0.1 Mn 0.1 O2), positive electrode conductive agent conductive carbon nanotubes (CNT), positive electrode binder polyvinylidene fluoride (abbreviated as PVDF, weight average molecular weight 70×10 5 ) were mixed in a mass ratio of 95:2.5:2.5, N-methylpyrrolidone (NMP) was added as a solvent, and stirred under a vacuum stirrer until a positive electrode slurry with a solid content of 75 wt% and a uniform system was obtained.

[0066] The desiccant anhydrous calcium chloride (Dv50μm=0.5μm), dispersant sodium carboxymethyl cellulose, binder PVDF (weight average molecular weight 70×10 5 ) were mixed in a mass ratio of 95:2.5:2.5, N-methylpyrrolidone (NMP) was added as a solvent, and the mixture was stirred under a vacuum stirrer until a protective layer slurry with a solid content of 72 wt% and a uniform system was obtained.

[0067] The positive electrode slurry and the protective layer slurry were uniformly coated on one surface of the positive electrode current collector aluminum foil with a thickness of 10 μm, and dried at 90°C to obtain a positive electrode sheet (such as Figure 1 The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated on both sides with a positive electrode active material layer and a protective layer. After drying at 90°C and cold pressing, the sheet is cut and the tabs are welded to obtain a 74mm x 867mm positive electrode sheet ready for use.

[0068] The thickness of the positive electrode active material layer is H2 μm=110 μm, the thickness of the protective layer is H1 μm=0.8 μm and H2 μm=88 μm, the width of the protective layer is L1 mm=10 mm, and the width of the overlapping region is L2 mm=0.1 mm.

[0069] <Preparation of negative electrode sheet>

[0070] The negative electrode active material hard carbon, the negative electrode conductive agent acetylene black, the negative electrode binder styrene butadiene rubber (abbreviated as SBR, with a weight average molecular weight of 50×10 5), thickener carboxymethyl cellulose (CMC) are mixed in a mass ratio of 96:2:1:1, and then deionized water is added as a solvent. The mixture is stirred under a vacuum mixer until the solid content is 70wt% and the system is uniform. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 8μm and dried at 90°C to obtain a negative electrode sheet with a single-sided coating of a negative electrode active material layer (thickness 130μm). Thereafter, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided coating of a negative electrode active material layer. After drying at 90°C, the sheet is cold pressed, cut into pieces, and the tabs are welded to obtain a negative electrode sheet with a specification of 76mm×851mm for standby use.

[0071] <Preparation of Separator>

[0072] A porous polyethylene film with a thickness of 7 μm (manufacturer: Hunan Zhongli New Materials Co., Ltd.) was used.

[0073] <Preparation of Electrolyte>

[0074] In an environment with a water content of less than 10 ppm, non-aqueous organic solvents ethylene carbonate (EC), diethyl carbonate (DEC), and propylene carbonate (PC) are mixed in a mass ratio of 30:40:30, and then lithium hexafluorophosphate (LiPF6) is added to the non-aqueous organic solvent to dissolve and mix evenly to obtain an electrolyte, wherein the concentration of LiPF6 is 1 mol / L.

[0075] <Preparation of lithium-ion batteries>

[0076] The negative electrode sheet, separator, and positive electrode sheet prepared above were stacked and wound in sequence to form a wound electrode assembly. The electrode assembly was placed in an aluminum-plastic film packaging bag, dried, and then injected with electrolyte. The lithium-ion battery was produced through vacuum packaging, resting, forming, degassing, and trimming. The upper formation voltage was 4.15V, the formation temperature was 70°C, and the formation rest time was 2 hours.

[0077] Example 1-2 to Example 1-18

[0078] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0079] Example 2-1 to Example 2-20

[0080] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0081] Comparative Example 1

[0082] Except that the protective layer is not provided, the rest is the same as Example 1-1.

[0083] Comparative Example 2

[0084] <Preparation of positive electrode sheet>

[0085] The positive electrode slurry is evenly coated on one surface of the positive electrode current collector aluminum foil and dried at 90°C to obtain a positive electrode active material layer. The protective layer slurry is then evenly coated on the surface of the positive electrode active material layer, dried at 90°C, and cold pressed to obtain a positive electrode sheet coated on one side with the positive electrode active material layer and the protective layer. The positive electrode active material layer is located between the positive electrode current collector and the protective layer. The above steps are then repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated on both sides with the positive electrode active material layer and the protective layer.

[0086] The rest is the same as Example 1-1.

[0087] Comparative Example 3

[0088] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Comparative Example 2.

[0089] Comparative Example 4

[0090] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0091] The preparation parameters and performance parameters of each embodiment and comparative example are shown in Table 1 and Table 2.

[0092] Table 1

[0093]

[0094]

[0095] Note: “\” in Table 1 indicates no corresponding preparation parameters; the difference between Comparative Example 2 and Example 1-1 is that in Comparative Example 2, the protective layer is arranged on the surface of the positive electrode active material layer away from the positive electrode current collector; the difference between Comparative Example 3 and Comparative Example 4 is that in Comparative Example 3, the protective layer is arranged on the surface of the positive electrode active material layer away from the positive electrode current collector.

[0096] From Examples 1-1 to 1-18 and Comparative Examples 1 to 4, it can be seen that the embodiments of the present application use a positive electrode sheet having a protective layer on the surface of the positive current collector on both sides of the positive active material layer, and the saturated water absorption rate A of the protective layer is within the scope of the present application, which has a low water content. When applied to a secondary battery, the secondary battery has a low gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved. In Comparative Example 1, a positive electrode sheet without a protective layer is selected, which has a high water content. The secondary battery using this positive electrode sheet has a high gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is not reduced and the storage performance is not improved. In Comparative Examples 2 to 4, the protective layer of the positive electrode plate is not arranged on the surface of the positive electrode collector on both sides of the positive electrode active material layer, and / or the saturated water absorption rate A of the protective layer is not within the scope of this application. The positive electrode plate has a high water content, and the secondary battery using the positive electrode plate has a high gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery has not been reduced and the storage performance has not been improved.

[0097] The type of desiccant typically affects the saturated water absorption rate A of the protective layer, which in turn affects the gram capacity and storage performance of the secondary battery. As can be seen from Examples 1-1 to 1-5, positive electrode sheets using desiccant types within the scope of this application have lower water content, and secondary batteries using such positive electrode sheets have lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved.

[0098] The volume average particle size Dv50 of the desiccant typically affects the saturated water absorption rate A of the protective layer, which in turn affects the gram capacity and storage performance of the secondary battery. As can be seen from Examples 1-1, 1-6, and 1-11, positive electrode sheets using desiccant with a volume average particle size Dv50 within the range of this application have lower water content, and secondary batteries using such positive electrode sheets have lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved.

[0099] The mass percentage of desiccant in the protective layer W g %, mass percentage of dispersant W f % and the mass percentage of the binder W n % usually also affects the gram capacity and storage performance of the secondary battery. From Examples 1-1, 1-12 to 1-18, it can be seen that the mass percentage of the desiccant selected is W g %, mass percentage of dispersant W f % and the mass percentage of the binder W nThe positive electrode sheet within the scope of this application has a lower water content. The secondary battery using the positive electrode sheet has a lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved. Among them, it can be seen from Examples 1-1, 1-16 and 1-18 that when the mass percentage of the desiccant is W g When the water content of the positive electrode increases, the gram capacity loss and thickness expansion rate of the secondary battery increase. This is because the mass percentage of the desiccant W g When the desiccant content is greater than 95%, as the mass percentage of the desiccant increases, the processing performance of the protective layer decreases, and the water content of the positive electrode sheet and the gram capacity loss and thickness expansion rate of the secondary battery will be affected.

[0100] Table 2

[0101]

[0102] The width of the protective layer will usually also affect the gram capacity and storage performance of the secondary battery. From Example 1-1, Example 2-1 to Example 2-9, it can be seen that the positive electrode sheet with a protective layer width within the scope of this application has a lower water content, and the secondary battery using this positive electrode sheet has a lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved. Among them, compared with Example 2-9, Example 2-3 has smaller advantages in water content of the positive electrode sheet, gram capacity loss of the secondary battery and thickness expansion rate, but the protective layer width L1 of Example 2-3 is 55mm less than the protective layer width L1 of Example 2-9, which has obvious advantages in terms of production cost. Compared with Example 2-5, Examples 2-6, 2-7 and 2-9 have smaller differences in the water content of the positive electrode sheets, the gram capacity loss of the secondary battery and the thickness expansion rate. However, the width L1 of the protective layer of Examples 2-6, 2-7 and 2-9 is 20 mm to 40 mm larger than the width L1 of the protective layer of Example 2-5, which will increase the production cost of the positive electrode sheets and is therefore not preferred.

[0103] The thickness of the protective layer and the thickness of the positive electrode active material layer also generally affect the gram capacity and storage performance of the secondary battery. As can be seen from Examples 1-1, 2-10, and 2-18, positive electrode sheets with protective layer thicknesses and positive electrode active material layer thicknesses within the ranges of this application have lower water content, and secondary batteries using such positive electrode sheets have lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved.

[0104] The width of the overlap region generally also affects the gram capacity and storage performance of the secondary battery. As can be seen from Examples 2-11, 2-19, and 2-20, positive electrode sheets with overlap region widths within the range of this application have lower water content, and secondary batteries using these positive electrode sheets have lower gram capacity loss and thickness expansion rate, indicating that the gram capacity loss of the secondary battery is reduced and the storage performance is improved.

[0105] It should be noted that, in this article, the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements that are inherent to such process, method, article or apparatus.

[0106] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0107] The above description is only a preferred embodiment of the present application and is not intended to limit the scope of protection of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application are included in the scope of protection of the present application.

Claims

1. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein a protective layer is disposed on the positive electrode current collector surface on both sides of the positive electrode active material layer along the width direction of the positive electrode sheet; the saturated water absorption rate of the protective layer is A, 40%≤A≤125%; and in, The protective layer includes a desiccant, and the volume average particle size of the desiccant is Dv50 μm, and 0.01≤Dv50≤200.

2. The positive electrode sheet according to claim 1, wherein: 60％≤A≤125％。 3. The positive electrode sheet according to claim 1, wherein: The desiccant includes at least one of anhydrous sodium sulfate, anhydrous calcium chloride, anhydrous magnesium sulfate, anhydrous aluminum chloride, anhydrous aluminum oxide, silica gel desiccant, montmorillonite desiccant or active mineral desiccant; Based on the mass of the protective layer, the mass percentage of the desiccant is W g %,70≤W g ≤99.

4. The positive electrode sheet according to claim 3, wherein: 90≤W g ≤95。 5. The positive electrode sheet according to claim 1, wherein: 0.05≤Dv50≤50.

6. The positive electrode sheet according to claim 1, wherein: Along the width direction of the positive electrode sheet, the width of the protective layer is L1 mm, 0.1≤L1≤50.

7. The positive electrode sheet according to claim 6, wherein: 10≤L1≤20。 8. The positive electrode sheet according to claim 1, wherein: The thickness of the protective layer is H1 μm, the thickness of the positive electrode active material layer is H2 μm, 10≤H2≤8000, 0.8H2≤H1≤1.01H2.

9. The positive electrode sheet according to claim 8, wherein: 0.95H2≤H1≤1.0H2.

10. The positive electrode sheet according to claim 1, wherein: Along the thickness direction of the positive electrode sheet, the positive electrode active material layer and the protective layer have an overlapping area, and in the overlapping area, the positive electrode active material layer is located between the protective layer and the positive electrode current collector; Along the width direction of the positive electrode sheet, the width of the overlapping area is L2 mm, 0<L2≤5.

11. The positive electrode sheet according to claim 1, wherein: The protective layer further comprises a dispersant and a binder; Based on the mass of the protective layer, the mass percentage of the dispersant is W f %, 0.5≤W f ≤15, the mass percentage of the binder is W n %, 0.5≤W n ≤15. 12 . A secondary battery comprising the positive electrode sheet according to claim 1 . 13 . An electronic device comprising the secondary battery according to claim 12 .

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