WO2023197946A1 - Positive electrode plate, secondary battery, electronic device, and mobile equipment - Google Patents

Abstract

Embodiments of the present application provide a positive electrode plate, a secondary battery, an electronic device, and mobile equipment. The positive electrode plate comprises a positive electrode current collector, and a first positive electrode coating and a second positive electrode coating which are sequentially stacked on at least one side surface of the positive electrode current collector; the first positive electrode coating comprises a first positive electrode active material, a first binder and a first conductive agent; the second positive electrode coating comprises a second positive electrode active material, a second binder and a second conductive agent; the thickness of the first positive electrode coating is less than or equal to the second positive electrode coating, the tensile stress or shear stress of the first positive electrode coating is greater than that of the second positive electrode coating, and the energy density of the first positive electrode active material is greater than or equal to the second positive electrode active material. The secondary battery prepared by using the positive electrode plate can effectively solve the problem of thermal runaway caused by the positive electrode current collector of the battery being in contact with the negative electrode, without affecting the energy density of the battery.

Description

Cathode plates, secondary batteries, electronic equipment and mobile devices

This application claims priority to the Chinese patent application submitted to the China Patent Office on April 14, 2022, with application number 202210390904.4 and the application name "Cathode Plate, Secondary Battery, Electronic Equipment and Mobile Device", and its entire contents incorporated herein by reference.

Technical field

Embodiments of the present application relate to the technical field of secondary batteries, and in particular to a positive electrode plate, secondary batteries, electronic equipment and mobile devices.

Background technique

Secondary batteries have been widely used in consumer electronics (such as mobile phones, tablets) and electric vehicles. However, secondary batteries can easily cause internal damage when they are continuously overcharged or damaged by external forces (such as punctures and collisions). Short circuit, and then thermal runaway occurs. Among them, the short circuit caused by the contact between the positive electrode current collector (such as aluminum foil) and the negative electrode is the most dangerous short circuit method of the battery. Therefore, reducing the risk of thermal runaway of secondary batteries is particularly important to promote their further widespread application.

In order to avoid thermal runaway caused by direct contact between the positive electrode current collector and the negative electrode, the industry generally installs a safety coating between the positive electrode current collector and the positive electrode active material layer. The safety coating mainly contains active materials with high safety performance (such as lithium iron phosphate). ), conductive agent and binder. Although this coating can avoid direct contact between the positive electrode current collector and the negative electrode when the battery is damaged by external force, because the coating occupies a certain thickness space of the battery, and the unit mass it can contribute or The energy per unit volume is generally much lower than that of the cathode active material layer, so its introduction will reduce the energy density of the battery.

Contents of the invention

In view of this, embodiments of the present application provide a positive electrode plate and a secondary battery to effectively solve the problem of thermal runaway caused by the positive electrode current collector of the battery contacting the negative electrode without affecting the energy density of the battery.

A first aspect of the embodiment of the present application provides a positive electrode sheet, including a positive electrode current collector and a first positive electrode coating and a second positive electrode coating sequentially laminated on at least one side surface of the positive electrode current collector; A positive electrode coating includes a first positive electrode active material, a first binder and a first conductive agent, and the second positive electrode coating includes a second positive electrode active material, a second binder and a second conductive agent; wherein, the The thickness of the first cathode coating is less than or equal to the second cathode coating, the tensile force or shear force of the first cathode coating is greater than the second cathode coating, and the first cathode active material The energy density is greater than or equal to the energy density of the second cathode active material.

The positive electrode sheet provided in the embodiment of the present application controls the tensile force and/or shearing force of the first positive electrode coating close to the positive electrode current collector to be greater than the second positive electrode coating far away from the positive electrode current collector, and the first positive electrode coating The thickness is less than or equal to the second cathode coating, so that the difficulty of the first cathode coating falling off from the cathode current collector is greater than the difficulty of the second cathode coating falling off from the cathode plate. The first cathode coating can play a safety protection role. , when the second positive electrode coating peels off under abnormal conditions, the first positive electrode coating can still wrap and protect the positive electrode current collector to a certain extent, preventing it from direct contact with the negative electrode piece of the battery and causing thermal runaway. In addition, this application also controls that the energy density of the first positive electrode active material is not less than that of the second positive electrode active material, so that the first positive electrode coating can efficiently deintercalate lithium under normal operation of the battery, and its existence will not reduce the use of the positive electrode. Battery energy density of pole pieces.

In the embodiment of the present application, the thickness of the first cathode coating is greater than 0 and less than or equal to 50 μm. At this time, the first positive electrode coating is not easy to fall off from the positive electrode current collector, and has better safety performance.

In the embodiment of the present application, the mass proportion of the first binder in the first cathode coating is greater than the mass proportion of the second binder in the second cathode coating. This helps the first cathode coating have better bonding properties than the second cathode coating.

In the embodiment of the present application, the adhesive force of the first adhesive is greater than the adhesive force of the second adhesive. This also helps the first cathode coating have better bonding properties than the second cathode coating.

In the embodiment of the present application, the first adhesive and the second adhesive are made of the same material, and the molecular weight of the first adhesive is greater than that of the second adhesive.

In the embodiment of the present application, the mass proportion of the first binder in the first cathode coating is equal to the mass proportion of the second binder in the second cathode coating, and the The bonding force of the first adhesive is greater than the bonding force of the second adhesive. In this case, it can be ensured that the tensile force or shearing force of the first positive electrode coating is greater than that of the second positive electrode coating, which is conducive to its better performance of safety functions.

In an embodiment of the present application, the volume resistivity of the first cathode coating is greater than the volume resistivity of the second cathode coating. At this time, the first positive electrode coating can increase the ohmic polarization and electrochemical polarization of the positive electrode piece during the short circuit process, avoid the risk of thermal runaway caused by rapid heat accumulation, and can better passivate the positive electrode current collector and reduce its Thermal risk from contact with the negative battery tab.

In the embodiment of the present application, the first cathode coating includes the following mass percentages of each component: 85%-96.5% of the first cathode active material, 2.5%-10% of the first binder, 1% -5% first conductive agent. The sum of the mass proportions of the first binder and the first conductive agent does not exceed 15%, which can ensure that the capacity of the first positive electrode coating will not be too low, which will help it contribute energy and will not affect its safety function. .

In the embodiment of the present application, the mass proportion of the first binder in the first cathode coating is greater than the mass proportion of the first conductive agent in the first cathode coating. In this way, the first positive electrode coating itself has strong bonding performance, is not easy to fall off, and can effectively perform its safety function under abnormal conditions of the battery.

In the embodiment of the present application, the charging upper limit voltage of the first cathode active material is above 4.25V, the specific capacity is greater than or equal to 170mAh/g, and the compacted density of the first cathode coating is greater than or equal to 3.4g/cm ³ . At this time, the energy density of the first positive electrode active material contained in the first positive electrode coating is relatively large, so that the introduction of the first positive electrode coating will not reduce the energy density of the battery produced using the above-mentioned positive electrode sheet.

In the embodiment of the present application, the first cathode active material includes a material represented by the general formula LiCo _1-x M _x O ₂ , where 0≤x≤1, M is selected from Ni, Mn, Al, Ca, Mg, One or more of Sr, Ti, V, Cr, Fe, Cu, Zn, Mo, W, Y, La, Zr, Sn, Se, Te and Bi. The energy density of the first cathode active material conforming to this general formula is generally larger.

The second aspect of the embodiments of the present application provides a secondary battery, including the positive electrode plate described in the first aspect of the present application.

The secondary battery provided by the embodiment of the present application can take into account both good safety performance and high battery energy density.

The third aspect of the embodiments of the present application provides an electronic device, including the secondary battery described in the second aspect of the present application.

The fourth aspect of the embodiment of the present application also provides a mobile device, the mobile device includes the electrochemical device described in the fourth aspect of the embodiment of the present application.

The above-mentioned electronic equipment or mobile device can enhance product competitiveness by using the electrochemical device provided by the embodiment of the present application for power supply.

Description of the drawings

Figure 1 is a schematic diagram of a common structure of a secondary battery.

Figure 2 is a schematic structural diagram of a positive electrode plate provided by an embodiment of the present application.

Figure 3 is a schematic structural diagram of a secondary battery provided by an embodiment of the present application.

FIG. 4 is a schematic structural diagram of an electronic device provided by an embodiment of the present application.

FIG. 5 is another schematic structural diagram of a mobile device provided by an embodiment of the present application.

Detailed ways

The technical solutions of the embodiments of the present application will be described below with reference to the accompanying drawings.

Please refer to Figure 1, which is a schematic structural diagram of an exemplary secondary battery. The secondary battery can be used in 3C consumer products, such as mobile phones, tablets, laptops, smart watches and other wearable or mobile electronic devices, as well as electric vehicles. The secondary battery may be a lithium secondary battery, a sodium secondary battery, a potassium secondary battery, a magnesium secondary battery, an aluminum secondary battery, a zinc secondary battery, or the like.

The secondary battery 100 in Figure 1 includes a positive electrode plate 10, a negative electrode plate 20, a separator 30 and an electrolyte (shown in the figure), as well as corresponding communication accessories and circuits. Among them, the positive electrode sheet 10 generally includes a positive electrode current collector 11 (usually aluminum foil) and a positive electrode active material layer 12 disposed on at least one side surface of the positive electrode current collector 11. The negative electrode sheet 20 has a similar structure. The positive electrode piece 10 and the negative electrode piece 20 are both housed in the battery case, and adjacent positive and negative electrodes are separated by a separator 30 in the case to maintain insulation between them and avoid Contact causes short circuit. However, when the secondary battery is damaged by external force (such as puncture, collision), for example, the positive electrode piece 10 is damaged and the positive active material layer 12 falls off from the positive current collector, the positive current collector aluminum foil leaks out and comes into contact with the negative electrode, which will This causes the most serious and dangerous short circuit in the battery, leading to thermal runaway. In order to solve the thermal runaway problem caused by the positive electrode current collector of the battery contacting the negative electrode without affecting the energy density of the battery, embodiments of the present application provide a positive electrode plate.

Referring to Figure 2, the positive electrode sheet 10' provided by the embodiment of the present application includes a positive current collector 11 and a first positive electrode coating 121 and a second positive electrode coating 122 sequentially laminated on at least one side surface of the positive electrode current collector 11; The first positive electrode coating 121 includes a first positive electrode active material, a first binder and a first conductive agent, and the second positive electrode coating 122 includes a second positive electrode active material, a second binder and a second conductive agent; wherein, The thickness of the first cathode coating 121 is less than or equal to the second cathode coating 122 , the tensile force and/or shear force of the first cathode coating 121 is greater than the second cathode coating 122 , and the mass energy of the first cathode active material The density is greater than or equal to the mass energy density of the second cathode active material.

The positive electrode sheet 10' provided in the embodiment of the present application adopts a double-layer positive electrode coating and controls the tensile force and/or shearing force of the first positive electrode coating 121 close to the positive electrode current collector 11 to be greater than that far away from the positive electrode current collector 11. 11 of the second positive electrode coating 122, the thickness of the first positive electrode coating 121 is less than or equal to the second positive electrode coating 122, so that the first positive electrode coating 121 can play a safety protection role, and it is difficult to fall off from the positive electrode current collector 11 It is more difficult for the second positive electrode coating 122 to fall off from the positive electrode piece 10', so that even if the second positive electrode coating 122 falls off when the battery is squeezed, collided, punctured, overcharged and other abnormal conditions, the first positive electrode coating 122 will fall off. The layer 121 can still protect the positive electrode current collector 11 and can wrap the positive electrode current collector 11 to a certain extent to prevent it from being exposed, thereby preventing it from contacting the negative electrode plate of the battery and causing thermal runaway, thereby improving the safety performance of the battery. In addition, the first cathode coating 121 can also participate in the charging and discharging process of the battery under normal conditions, and efficiently remove/intercalate lithium, and the energy density of the first cathode active material contained in the first cathode coating 121 is not less than that of the second cathode active material. The positive electrode coating 122 contains the second positive electrode active material. Therefore, compared with the conventional positive electrode sheet that only uses the second positive electrode coating 122 of the present application as the positive electrode active material layer, the positive electrode sheet 10 of the embodiment of the present application has the same thickness. Has a relatively high energy density. Therefore, a battery using the positive electrode plate 10' can have both good safety performance and high battery energy density.

The stacked structure of the first positive electrode coating 121 and the second positive electrode coating 122 constitutes the positive active material layer 12' of the positive electrode sheet 10' of the present application, and the positive active material layer 12' is disposed on at least one side of the positive current collector 11 Superficially, in FIG. 2 , there are cathode active material layers 12 ′ on both sides of the cathode current collector 11 .

In this application, the thickness of the first positive electrode coating 121 is less than or equal to the thickness of the second positive electrode coating 122 . In this way, relatively speaking, the first positive electrode coating 121 has better bonding performance and mainly plays a safety protection role. The second positive electrode coating 122 mainly contributes energy and controls the first positive electrode coating 121 which mainly plays a safety protection role. The thickness is no larger than the second positive electrode coating, which mainly contributes to the capacity, so that the safety performance of the battery is better.

In order to ensure that the first positive electrode coating has excellent safety performance, in some embodiments of the present application, the thickness of the first positive electrode coating 121 is controlled to be greater than 0 and less than or equal to 50 μm. For example, the thickness of the first positive electrode coating 121 may be 2 μm, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, etc. In some embodiments, the thickness of the first cathode coating 121 is 10 μm-40 μm. At this time, the thickness of the first positive electrode coating 121 is more appropriate, which can not only ensure that it can effectively improve the safety performance of the battery when it is damaged by external forces, but also avoid its excessive thickness, which increases the amount of binder and reduces the strength of the first active material. The mass proportion further affects the specific capacity of the first positive electrode coating.

In the embodiment of the present application, the first cathode coating includes the following mass percentages of each component: 85%-96.5% of the first cathode active material, 2.5%-10% of the first binder, 1% -5% first conductive agent. The second cathode coating includes the following mass percentages of each component: 50%-98% of the second cathode active material, 1%-35% of the second binder, 0.5%-15% of the second cathode active material Conductive agent. Among them, the mass proportions of corresponding components in the two positive electrode coatings can be the same or different. Controlling the mass proportion of the cathode active material in each cathode coating to be high can prevent the specific capacity of the two cathode coatings from being too low; controlling the mass proportion of the binder in each cathode coating within the above range can prevent the two cathode coatings from being too low. The bonding performance of the positive electrode coating is high without reducing the proportion of active material too much.

In some embodiments of the present application, the mass proportion of the first binder in the first cathode coating 121 is greater than the mass proportion of the second binder in the second cathode coating 122 . In this way, even when the bonding properties of the first binder and the second binder are equivalent, the aforementioned condition "the tensile force and/or shear force of the first positive electrode coating 121 is greater than that of the second positive electrode coating 122" can still be satisfied. , which helps the first positive electrode coating 121 to effectively play a safety protection role in abnormal situations where the positive electrode piece is damaged by external force.

When the mass proportion of the first binder in the first cathode coating 121 is greater than the mass proportion of the second binder in the second cathode coating 122 , the first conductive agent in the first cathode coating 121 The mass proportion of may be smaller than the mass proportion of the second conductive agent in the second positive electrode coating 122 . In this way, the volume resistivity of the first positive electrode coating 121 is greater than that of the second positive electrode coating 122, and the first positive electrode coating 121 with slightly poor conductivity can better survive abnormal conditions such as battery overcharging, acupuncture, and collision. The positive electrode current collector 11 is wrapped and passivated, thereby reducing the risk of thermal runaway caused by contact with the negative electrode piece of the battery.

In some embodiments of the present application, the mass proportion of the first binder in the first cathode coating 121 is greater than the mass proportion of the first conductive agent in the first cathode coating 121 . In this way, the first positive electrode coating 121 itself has strong adhesive force and is not easy to fall off from the positive electrode current collector 11. When the second positive electrode coating 122 falls off under abnormal conditions of the battery, the first positive electrode coating 121 can still adhere to the positive electrode current collector 11. The fluid 11 plays a certain protective role to prevent it from directly contacting the negative electrode piece of the battery and causing thermal runaway. In some embodiments, the mass proportion of the second binder in the second cathode coating 122 is smaller than the mass proportion of the second conductive agent in the second cathode coating 122 .

In some embodiments of the present application, the adhesive force of the first adhesive is greater than the adhesive force of the second adhesive. In this way, even if the mass proportion of the first binder in the first cathode coating 121 is equal to the mass proportion of the second binder in the second cathode coating 122, the stretching of the first cathode coating 121 can still be ensured. The force and/or shearing force is greater than that of the second positive electrode coating 122, which helps the first positive electrode coating 121 improve the battery safety performance when the positive electrode piece is damaged by external force.

The above-mentioned "bonding force of the first adhesive agent" can be understood as the shear force or tensile force of the first adhesive layer formed by the simple first adhesive agent, and the "adhesive force of the second adhesive agent" can be understood as It is understood as the shear force or stretching of the second adhesive layer formed by the simple second adhesive. When the first adhesive layer and the second adhesive layer have the same size, the shear force of the first adhesive layer is greater than the shear force of the second adhesive layer, and the tensile force of the first adhesive layer is greater than that of the second adhesive layer. The tensile force of the second bonding layer. It can also be understood that when the mass proportion of the first binder in the first positive electrode coating 121 is equal to the mass proportion of the second binder in the second positive electrode coating 122 , the first positive electrode coating 121 The shearing force or tensile force is greater than the shearing force or tensile force of the second positive electrode coating 122 with the same size.

In some embodiments, the mass proportion of the first binder in the first cathode coating 121 is equal to the mass proportion of the second binder in the second cathode coating 122; the bonding of the first binder The force is greater than the bonding force of the second adhesive. In this way, the bonding performance of the first positive electrode coating 121 is better than that of the second positive electrode coating 122 .

In other embodiments, the mass proportion of the first binder in the first cathode coating 121 is greater than the mass proportion of the second binder in the second cathode coating 122; The bonding force is greater than the bonding force of the second adhesive. In this case, the bonding performance of the first positive electrode coating 121 is better than that of the second positive electrode coating 122 . Furthermore, in this case, if the mass proportions of the first and second active materials in each cathode coating are equal, the mass proportion of the first conductive agent in the first cathode coating 121 is smaller than that of the second conductive agent. As for the mass proportion in the second positive electrode coating 122, as mentioned above, the volume resistivity of the first positive electrode coating 121 is generally greater than the volume resistivity of the second positive electrode coating 122. In this way, the first cathode coating 121 with slightly poor conductivity can better wrap the cathode current collector 11 and passivate its conductivity under abnormal conditions of the battery, thereby reducing the risk of contact with the negative electrode plate of the battery. Risk of thermal runaway.

As mentioned above, in some embodiments of the present application, the volume resistivity of the first positive electrode coating 121 is greater than the volume resistivity of the second positive electrode coating 122 . In this way, the first positive electrode coating 121 with slightly poor conductivity can better wrap and passivate the positive electrode current collector 11 under abnormal conditions such as battery overcharging, acupuncture, collision, etc., thereby reducing its contact with the negative electrode plate of the battery. , and also increases the ohmic polarization and electrochemical polarization of the positive electrode during the short circuit process, avoiding the risk of thermal runaway caused by rapid heat accumulation.

Of course, in other embodiments of the present application, the volume resistivity of the first positive electrode coating 121 may also be less than or equal to the volume resistivity of the second positive electrode coating 122 . As long as the volume resistivity of the first positive electrode coating 121 is greater than the volume resistivity of the positive electrode current collector 11 , it can block the electrical contact between the positive electrode current collector 11 and the negative electrode under abnormal conditions of the battery.

As mentioned previously in this application, the mass proportion of the first cathode active material in the first cathode coating 121 and the mass proportion of the second cathode active material in the second cathode coating 122 may be equal or different. In some embodiments of the present application, the mass proportion of the first cathode active material in the first cathode coating 121 is equal to the mass proportion of the second cathode active material in the second cathode coating 122 . At this time, it is helpful to narrow the energy gap provided by the two positive electrode coatings under normal operating conditions of the battery.

In an embodiment of the present application, the energy density of the first cathode active material is greater than or equal to the energy density of the second cathode active material. "Energy density" here can specifically refer to "mass energy density" or "volume energy density". The relationship between the energy density of the first cathode active material and the second cathode active material is mainly determined by the characteristics of each cathode active material. Specifically, the energy density of the cathode active material is its charging specific capacity, charging voltage, It is the result of comprehensive effects such as the compaction density of the positive active material layer (since the charging voltage, compaction density, etc. are also related to the positive active material, essentially it can be said that the energy density is related to the positive active material). The energy density of cathode active materials of different materials is generally different. For example, the energy density of a battery that simply uses lithium cobalt oxide as the cathode active material is generally greater than the energy density of a battery that simply uses lithium nickel cobalt manganate as the cathode active material (for comparison). Energy density can be both mass energy density or volume energy density); the energy density of batteries that simply use nickel-containing ternary materials as positive active materials is generally greater than those that simply use phosphate materials (such as lithium iron phosphate, lithium manganese iron phosphate , lithium vanadium phosphate, lithium vanadium oxyphosphate) as the positive electrode active material battery energy density.

In this application, the energy density of a certain cathode active material can be measured by the product of U×M×P, where U represents the charging upper limit voltage of the cathode active material, in volts (V), and Q represents the charging specific capacity, in unit mAh/g, P represents the compacted density of the cathode active material layer, the unit is g/cm ³ , the product of the three gives the energy of the cathode active material in Wh/L quantity density. In the embodiment of the present application, the charging upper limit voltage of the first cathode active material in the first cathode coating 121 is above 4.25V, the charge specific capacity is greater than or equal to 170mAh/g, and the compacted density of the first cathode coating 121 is greater than or equal to 3.4g/cm ³ . At this time, the energy density of the first positive electrode active material defined according to the product of the aforementioned parameters is relatively large, which can be above 2456.5Wh/L. This means that the introduction of the first positive electrode coating will not reduce the energy density of the battery made with the above-mentioned positive electrode sheet. Energy Density.

In the embodiment of the present application, the first cathode active material includes a material represented by the general formula LiCo _1-x M _x O ₂ , where 0≤x≤1, M is selected from Ni, Mn, Al, Ca, Mg, One or more of Sr, Ti, V, Cr, Fe, Cu, Zn, Mo, W, Y, La, Zr, Sn, Se, Te and Bi. In some embodiments, the first positive active material includes lithium cobalt oxide (corresponding to x=0), lithium nickel cobalt manganate (corresponding to 0<x<1, M is Ni and Mn), lithium nickel cobalt manganate (corresponding to 0<x<1, M is Ni, Mn and Al), lithium nickel cobalt aluminate (corresponding to 0<x<1, M is Ni and Al), lithium nickelate (corresponding to x=1, M is Ni). Among them, materials whose general formula conforms to LiCo _1-x M _x O ₂ generally have larger energy densities. Similarly, the second cathode active material may also include a material represented by the general formula LiCo _1-x M _x O ₂ . The first and second cathode active materials may be the same or different materials, but the energy density of the first cathode active material needs to be greater than or equal to the second cathode active material.

In the embodiment of the present application, the positive electrode current collector 11 may include aluminum foil, aluminum alloy foil, aluminum-plated polymer film, carbon-coated copper foil, carbon-coated aluminum alloy foil, or carbon-coated aluminum-plated polymer film. The aluminum-plated polymer film refers to a polymer film with an aluminum layer plated on the surface. The polymer film can be, for example, polyethylene terephthalate (PET) or aluminum/polyimide (PI). The positive electrode current collector 11 containing metallic aluminum has high electrical conductivity. When the battery is damaged by external force, it may cause thermal runaway due to the short distance between it and the negative electrode. Therefore, it is necessary to provide a first positive electrode coating 121 and a second positive electrode on its surface. Stack of coatings 122.

In the embodiment of the present application, the first binder contained in the first positive electrode coating 121 and the second binder contained in the second positive electrode coating 122 are independently selected from polytetrafluoroethylene (PTFE), polyvinylidene fluoride Ethylene (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polypropylene oxide (PPO), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN) ), one or more of polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyimide (PI), styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC), sodium alginate, etc. . The materials of the first adhesive and the second adhesive may be the same or different.

In the embodiment of the present application, the first conductive agent contained in the first positive electrode coating 121 and the second conductive agent contained in the second positive electrode coating 122 may be the same or different. Wherein, the first conductive agent and the second conductive agent can be independently selected from at least one of conductive carbon black (such as acetylene black, Ketjen black), carbon nanotubes (CNT), graphene, carbon fiber, graphite, furnace black, etc. species, but not limited to this.

The embodiments of the present application also provide a method for preparing the above-mentioned positive electrode sheet. The preparation method may specifically include:

Coating a first positive electrode slurry containing a first positive electrode active material, a first binder and a first conductive agent on at least one side surface of the positive electrode current collector, and forming a first positive electrode coating after drying;

A second positive electrode slurry containing a second positive electrode active material, a second binder and a second conductive agent is coated on the surface of the first positive electrode coating. After drying, a second positive electrode coating is formed, and after pressing, a second positive electrode slurry is obtained. Positive electrode sheet; wherein the mass energy density of the first cathode active material is greater than or equal to the mass energy density of the second cathode active material, and the thickness of the first cathode coating is less than or equal to the second cathode coating. layer, the tensile force or shearing force of the first cathode coating is greater than that of the second cathode coating.

The solvents contained in the first cathode slurry and the second cathode slurry may be the same or different, and may be independently selected from N-methylpyrrolidone (NMP), dimethylformamide (DMF), water, and alcohols. One or more solvents (such as ethanol, etc.). The pressing method may specifically be roller pressing. The coating method may be one or a combination of spin coating, brush coating, spray coating, dip coating, blade coating, etc. The above-mentioned positive electrode current collector may be coated on one side or on both sides. In other words, one side surface of the cathode current collector may have a cathode active material layer composed of a first cathode coating layer and a second cathode coating layer, or the cathode current collector may have a first cathode layer on opposite sides of the surface. coating and a second positive electrode coating.

The preparation method of the positive electrode piece is simple and the structure of the prepared positive electrode piece is novel and stable, which can effectively improve the safety performance of the battery without reducing the energy density.

The above-mentioned positive electrode plate can be assembled to obtain a secondary battery by the following method: stacking the positive electrode plate, separator and negative electrode plate in sequence to form a battery core; placing the battery core in a battery case and injecting electrolyte, The battery case is then sealed to produce a lithium ion battery.

Among them, the cells of the secondary battery can be of a wound type or a laminated type. The battery case can be aluminum-plastic film, copper-plastic film, steel-plastic film or other packaging films, or it can be aluminum shell, steel shell, other metal shell, etc., which can be selected according to the type of secondary battery required. When sealing the battery case, an air bag is usually reserved so that the gas can be discharged through the air bag after the battery is formed. After formation, secondary packaging can be carried out, and then the capacity dividing process can be carried out to obtain a secondary battery that can be shipped out of the factory.

An embodiment of the present application also provides a secondary battery, which includes the above-mentioned positive electrode plate as in the embodiment of the present application.

Referring to Figure 3, Figure 3 is a schematic structural diagram of a secondary battery provided by an embodiment of the present application. The secondary battery 100' shown in Figure 3 is different from the secondary battery 100 shown in Figure 1 only in that the positive electrode plates used in the two are different, and the structures of other components are similar. Please refer to the previous description of this application. Since this secondary battery adopts the above-mentioned positive electrode plate 10' of the embodiment of the present application, it can achieve both high battery energy density and good safety performance.

The negative electrode sheet 20 includes a negative current collector 21 and a negative active material layer 22 disposed on at least one side surface of the negative current collector 21 . The negative active material layer 22 includes a negative active material, a binder and an optional conductive agent. The range of the binder and conductive agent contained in the negative electrode piece 20 can be found in the above description of the positive electrode piece. The negative electrode current collector 21 includes but is not limited to metal foil, alloy foil, metal-plated film or metal foil with carbon coating on the surface, alloy foil, metal-plated film, etc. In some embodiments, the negative electrode current collector 21 includes copper foil, copper alloy foil, stainless steel foil, copper-coated polymer film, carbon-coated copper foil, carbon-coated copper alloy foil, or carbon-coated copper-coated polymer film. The surface of the fluid 21 may be etched or roughened to form secondary structures to facilitate effective contact with the negative active material layer.

When the secondary battery in Figure 3 is specifically a lithium secondary battery, the negative active material may include but is not limited to lithium titanate, metal lithium, lithium alloy, carbon-based materials, silicon-based materials, tin-based materials, and phosphorus-based materials. of one or more. Among them, carbon-based materials can include graphite (such as natural graphite, artificial graphite), non-graphitized carbon (soft carbon, hard carbon, etc.); silicon-based materials can include elemental silicon, silicon-based alloys, silicon oxides and silicon-carbon composite materials etc.; tin-based materials may include one or more of elemental tin, tin alloys, etc.; phosphorus-based materials may include elemental phosphorus (such as black phosphorus), phosphorus-carbon composite materials, etc. In addition, the separator 30 can be a polymer separator, non-woven fabric, etc., including but not limited to single-layer PP (polypropylene), single-layer PE (polyethylene), double-layer PP/PE, double-layer PP/PP, and three-layer PP. /PE/PP and other separators.

The secondary battery provided by the embodiment of the present application can be used in terminal consumer products, such as mobile phones, tablet computers, mobile power supplies, portable machines, notebook computers, digital cameras and other wearable or mobile electronic devices, as well as drones and automobiles. and other products to improve product performance.

An embodiment of the present application also provides an electronic device including the above-mentioned secondary battery. Since the above-mentioned secondary battery provided by the embodiment of the present application has good safety performance and high energy density, electronic devices equipped with the secondary battery can have good safety performance, good product use experience, and outstanding market competitiveness.

Specifically, the electronic device may include various consumer electronic products, such as mobile phones, tablet computers, laptops, mobile power supplies, portable machines, smart watches and other wearable or removable electronic devices, televisions, DVD players, Video recorders, camcorders, radios, cassette players, combo speakers, record players, compact disc players, home office equipment, home electronic health care equipment, and automobiles and other electronic products.

In some embodiments, referring to Figure 4, this embodiment of the present application provides an electronic device 300, which includes a housing 301, electronic components (not shown in Figure 4) accommodated in the housing 301, and a battery 302. The battery 302 supplies power to the electronic device 300, and the battery 302 includes the secondary battery described in the embodiment of the present application. The housing 301 may include a front cover assembled on the front side of the terminal and The back shell is assembled on the rear side, and the battery 302 can be fixed on the inside of the back shell. The electronic device 300 shown in FIG. 4 is usually a small portable electronic device, such as a mobile phone.

Referring to Figure 5, an embodiment of the present application also provides a mobile device 400, which includes the above-mentioned secondary battery provided by an embodiment of the present application. The mobile device 400 can be various movable devices used for loading, transportation, assembly, disassembly, security, etc., and can be various forms of vehicles. Specifically, the mobile device 400 may include a vehicle body 401, a mobile component 402 (such as a wheel), and a driving component. The driving component includes a motor 403 and a battery system 404 for powering the motor 403. The battery system 404 includes the components provided by the embodiments of the present application. of the above-mentioned secondary batteries. The battery system 404 may be a battery pack of the above-mentioned secondary battery, which is housed at the bottom of the vehicle body and is electrically connected to the motor 403 . In this way, the battery system 404 can power the motor 403, and the motor 403 provides power to drive the moving component 402 of the mobile device 400 to move.

Mobile devices using the secondary battery provided by embodiments of the present application have high battery life and good safety performance.

The embodiments of the present application will be further described below in multiple embodiments.

Example 1

A method for preparing a positive electrode sheet, including the following steps:

Weigh the positive electrode active material (specifically lithium cobalt oxide LiCoO ₂ ), binder PVDF (molecular weight: 700,000), and conductive carbon black at a mass ratio of 95%:3%:2%, mix them with NMP, and stir thoroughly , obtain the first positive electrode slurry; combine the positive active material lithium cobalt oxide (the same as the lithium cobalt oxide in the first positive electrode slurry), the binder PVDF (the same molecular weight as the PVDF in the first positive electrode slurry, the binding force Same), conductive carbon black is weighed according to the mass ratio of 95%: 2%: 3%, mixed with NMP, and stirred thoroughly to obtain the second positive electrode slurry;

Select aluminum foil as the positive electrode current collector, apply the first positive electrode slurry on the opposite sides of the surface, and dry to form the first positive electrode coating; then apply the above-mentioned second positive electrode slurry on the surface of the first positive electrode coating, and dry Finally, a second positive electrode coating is formed, and the positive electrode sheet is obtained by rolling.

The schematic structural diagram of the positive electrode piece of Example 1 is shown in Figure 2. The cathode plate 10' includes a cathode current collector 11 and cathode active material layers 12' disposed on opposite side surfaces thereof. The cathode active material layer 12' includes a first cathode coating 121 close to the cathode current collector 11 and away from the cathode current collector 11. The second positive electrode coating 122 of the positive electrode current collector 11; wherein, the thickness of each first positive electrode coating 121 is 10 μm, the thickness of each second positive electrode coating 122 is 50 μm, and the tensile force of the first positive electrode coating 121 Or the shear force is greater than the second positive electrode coating, and the energy density of lithium cobalt oxide in the first positive electrode coating 121 is equal to the mass energy density of lithium cobalt oxide in the second positive electrode coating 122 . In addition, the volume resistivity of the first positive electrode coating 121 is greater than that of the second positive electrode coating 122 . Among them, the compacted density of the first positive electrode coating 121 is 4.15g/cm ³ , the charging specific capacity of 121 lithium cobalt oxide in the first positive electrode coating is about 180mAh/g, and the upper charging voltage is 4.45V. Through these three The energy density of lithium cobalt oxide calculated by product is approximately 3324.15Wh/L.

Preparation of lithium secondary batteries, including:

Preparing the negative electrode sheet: Mix the negative active material (specifically artificial graphite) with conductive carbon black, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) in a mass ratio of 96%:1%:2%:1% Disperse in deionized water, stir evenly to obtain negative electrode slurry, apply the negative electrode slurry on copper foil, dry, compact, and slice to prepare negative electrode sheets;

Assemble the battery: Use 1 mol/L LiPF ₆ EC (ethylene carbonate) + DEC (diethyl carbonate) mixture (the volume ratio of EC and DEC is 1:1) as the electrolyte, and the ceramic-treated PE film as the separator , the positive electrode sheet, separator and negative electrode sheet of Example 1 are stacked in order to form a bare battery core. The bare battery core is packaged in an aluminum plastic film case, and then the electrolyte is injected into the battery case, and the electrolyte is formed and pumped. After gasification, it is packaged again and divided into volumes to prepare a lithium-ion battery.

Example 2

A positive electrode piece, which is different from Embodiment 1 in that: in the positive electrode piece of Embodiment 2, the positive active materials in the first positive electrode coating 121 and the second positive electrode coating 122 are both nickel cobalt manganese oxide. Lithium, the general structural formula is LiCo _0.8 Ni _0.1 Mn _0.1 O ₂ .

Among them, in the positive electrode sheet of Example 2, the energy density of lithium nickel cobalt manganate in the first positive electrode coating 121 is equal to the mass energy density of lithium nickel cobalt manganate in the second positive electrode coating 122 . Among them, the compacted density of the first positive electrode coating 121 is 3.6g/cm ³ , the charging specific capacity of the lithium nickel cobalt manganate in the first positive electrode coating 121 is about 200mAh/g, and the charging upper limit voltage is 4.25V. The energy density of lithium nickel cobalt manganate calculated by the product of these three is approximately 3060Wh/L.

According to the method described in Example 1, the positive electrode sheet of Example 2 was assembled into a lithium secondary battery.

Example 3

A positive electrode sheet, which is different from Embodiment 1 in that: in the positive electrode sheet of Embodiment 3, the positive active material in the first positive electrode coating 121 is lithium cobalt oxide, and the positive electrode active material in the second positive electrode coating 122 is lithium cobalt oxide. The positive active material is lithium nickel cobalt manganate, and the energy density of lithium cobalt oxide in the first positive electrode coating 121 is greater than the energy density of lithium nickel cobalt manganate in the second positive electrode coating 122 . For the general formula and energy density characteristics of lithium cobalt oxide and lithium nickel cobalt manganate used in Example 3, please refer to the descriptions in Examples 1 and 2.

According to the method described in Example 1, the positive electrode sheet of Example 3 was assembled into a lithium secondary battery.

Example 4

A positive electrode piece, which is different from Embodiment 1 in that: in the positive electrode piece of Embodiment 4, the first binder in the first positive electrode coating 121 is PVDF with a molecular weight of about 1 million, and the second The second binder in the positive electrode coating 122 is PVDF with a molecular weight of about 700,000 (its binding force is smaller than that of the first binder), and the first positive electrode coating 121 and the second positive electrode coating 122 , the mass ratio of lithium cobalt oxide to the corresponding binder and conductive agent is 95%: 2%: 3%. In addition, the volume resistivity of the first positive electrode coating 121 is equal to the second positive electrode coating 122 .

According to the method described in Example 1, the positive electrode sheet of Example 4 was assembled into a lithium secondary battery.

In order to highlight the beneficial effects of the embodiments of the present application, the following Comparative Examples 1-2 are provided.

Comparative example 1

A lithium secondary battery is different from Example 1 in that: the structure of the positive electrode sheet used in Comparative Example 1 is as shown in Figure 1, and the positive active material layer on one side of the positive electrode current collector 11 is only one layer. Indicated by reference numeral 12, the thickness of the positive electrode active material layer 12 is 60 μm, and its composition is the same as the second positive electrode coating in Example 1 of the present application, that is, it includes lithium cobalt oxide in a mass ratio of 95%:2%:3%. , PVDF and conductive carbon black.

Comparative example 2

A conventional lithium secondary battery differs from Example 2 in that: the structure of the positive electrode sheet used in Comparative Example 1 is shown in Figure 1. The positive active material layer on one side of the aluminum foil is only one layer, and its thickness is 60 μm, and its composition is the same as the second positive electrode coating in Example 2 of the present application, that is, it includes lithium nickel cobalt manganate, PVDF and conductive carbon black in a mass ratio of 95%: 2%: 3%.

Comparative example 3

A lithium secondary battery differs from Example 1 in that: in the positive electrode sheet of Comparative Example 3, the positive active material in the first positive electrode coating 121 is lithium nickel cobalt manganate, and the second positive electrode coating 122 The positive active material is lithium cobalt oxide.

In order to strongly support the beneficial effects of the embodiments of the present application, the lithium secondary batteries of the above embodiments and comparative examples are Perform normal charge and discharge tests to measure energy density, as well as perform pinprick tests.

Among them, the battery energy density test method is: charge each lithium secondary battery at a constant current rate of 0.2C to the corresponding full charge voltage for the first time, then charge at a constant voltage until the cut-off current is 0.025C, leave it aside for 10 minutes; discharge at a rate of 0.2C When the rated lower limit voltage is 3.0V, record the energy released by the battery. The ratio of this energy to the battery volume or mass is the volume or mass energy density of the battery.

Battery acupuncture test: After each secondary battery is fully charged at 0.2C, perform a nail penetration test. First, place each secondary battery on a flat surface, use a steel needle with a diameter of 2mm, and perform a nail penetration test at a speed of 100mm/s. Penetrate the battery in the direction of the battery poles. The steel needle pierces the battery and remains in the battery for 5 minutes or the battery indicates that the temperature drops to 50°C. The test will be stopped. If the battery does not catch fire or explode, it means the battery has passed the test. Each test is 5 Parallel samples are taken, and the ratio of the number of samples that pass the test to the total number of samples is regarded as the passing rate of the steel needle test.

In addition, the shear force and tensile force of the two positive electrode coatings of the positive electrode sheet in the embodiment of the present application were also tested. Specifically, each positive electrode piece is cut into a sample of a certain size, the tape is adhered to the surface of the positive electrode piece sample (that is, adhered to the second positive electrode coating) and pressed tightly, and the top of the tape and the electrode are Leave an unadhered area at the bottom of the sample for stretching purposes. Use a tensile machine to test as follows:

1) Perpendicular to the cross-sectional direction of the pole piece sample (i.e., parallel to the thickness direction of the pole piece sample), stretch in two opposite directions (the 2 F directions shown in Figure 2), and the tape adheres The corresponding force when the second positive electrode coating is peeled off from the electrode sample is recorded as the tensile force of the second positive electrode coating;

2) Parallel to the cross-sectional direction of the pole piece sample, stretch in two opposite directions (the two F' directions shown in Figure 2). The tape adheres to the second positive electrode coating and is peeled off from the pole piece sample. The corresponding force is recorded as the shear force of the second positive electrode coating.

Similarly, after the second positive electrode coating on each positive electrode piece sample is peeled off, the tape is adhered to the first positive electrode coating and pressed tightly in a similar manner as above, and the top of the tape and the electrode piece are Leave an unadhered area at the bottom of the sample, and use a tensile machine to perform the test as above. In the direction perpendicular to the cross-section of the positive electrode sheet sample, the tape adheres to the first positive electrode coating when it is peeled off from the positive electrode current collector. The force is recorded as the tensile force of the first positive electrode coating; along the cross-sectional direction parallel to the pole piece sample, the force when the tape adheres to the first positive electrode coating is peeled off from the positive electrode current collector, is recorded as the first Shear force of positive electrode coating.

It should be noted that when Comparative Example 1-2 is provided with a single-layer positive electrode active material layer on one side of the positive electrode current collector (composing a second positive electrode coating similar to the example of this application), the tension of the active material layer is Tensile force and shear force both refer to the force peeled off from the positive electrode current collector.

The test results of each Example and Comparative Example are summarized in Table 1 below.

Table 1

It can be known from Table 1 that the battery produced by using the double-layer positive electrode coating cathode sheet provided in the embodiment of the present application has better safety performance when encountering external force damage (in Table 1, the acupuncture pass rate is higher ) without reducing the energy density of the battery under normal operating conditions. This can be clearly seen from the comparison between Examples 1 and 4 and Comparative Example 1, and the comparison between Examples 2-3 and Comparative Example 2. In addition, the comparison between Example 3 and Comparative Example 2 illustrates that Example 3 controls the positive electrode active material in the positive electrode coating close to the positive electrode current collector to have a higher energy density than the positive electrode active material in the positive electrode coating far away from the positive electrode current collector. At the same time, increasing the tensile force or shear force of the first positive electrode coating close to the positive electrode current collector side can effectively increase the battery energy density and improve the battery safety effect. The comparison between Comparative Example 3 and Comparative Example 1 shows that when the energy density of the positive active material in the positive electrode coating close to the positive electrode current collector side of the double-layer positive electrode coated pole piece is small (Comparative Example 3), this positive electrode is used. The energy density of a battery made from a sheet will be lower than that of a battery made from a cathode sheet that only uses a one-layer cathode active material layer that is the same composition as the second cathode coating that mainly contributes energy.

The above description only expresses several exemplary embodiments of the present application. The descriptions are relatively specific and detailed, but should not be construed as limiting the patent scope of the present application. It should be noted that, for those of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all fall within the protection scope of the present application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims (14)

1. A positive electrode sheet, characterized in that the positive electrode sheet includes a positive electrode current collector and a first positive electrode coating and a second positive electrode coating sequentially laminated on at least one side surface of the positive electrode current collector; the third A positive electrode coating includes a first positive electrode active material, a first binder and a first conductive agent, and the second positive electrode coating includes a second positive electrode active material, a second binder and a second conductive agent; wherein, the The thickness of the first cathode coating is less than or equal to the second cathode coating, the tensile force or shear force of the first cathode coating is greater than the second cathode coating, and the first cathode active material The energy density is greater than or equal to the energy density of the second cathode active material.
2. The positive electrode sheet according to claim 1, wherein the thickness of the first positive electrode coating is greater than 0 and less than or equal to 50 μm.
3. The positive electrode sheet according to claim 1 or 2, characterized in that the mass proportion of the first binder in the first positive electrode coating is greater than that of the second binder in the second The mass proportion of the positive electrode coating.
4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that the adhesive force of the first adhesive agent is greater than the adhesive force of the second adhesive agent.
5. The positive electrode sheet according to claim 4, wherein the first adhesive and the second adhesive are made of the same material, and the molecular weight of the first adhesive is greater than that of the second adhesive. agent.
6. The positive electrode sheet according to claim 1 or 2, characterized in that the mass proportion of the first binder in the first positive electrode coating is equal to the mass proportion of the second binder in the second The mass proportion of the positive electrode coating, the bonding force of the first binder is greater than the bonding force of the second binder.
7. The positive electrode sheet according to any one of claims 1 to 6, wherein the volume resistivity of the first positive electrode coating is greater than the volume resistivity of the second positive electrode coating.
8. The positive electrode sheet according to any one of claims 1 to 7, characterized in that the first positive electrode coating includes the following mass percentages of each component: 85%-96.5% of the first positive electrode active material, 2.5%-10% first binder, 1%-5% first conductive agent.
9. The positive electrode sheet according to claim 8, wherein the mass proportion of the first binder in the first positive electrode coating is greater than the mass proportion of the first conductive agent in the first positive electrode coating. The quality ratio in .
10. The positive electrode sheet according to any one of claims 1 to 9, characterized in that the charging upper limit voltage of the first positive active material is above 4.25V, and the specific capacity is greater than or equal to 170mAh/g. The compacted density of the coating is greater than or equal to 3.4g/cm ³ .
11. The cathode plate according to claim 10, wherein the first cathode active material includes a material represented by the general formula LiCo _1-x M _x O ₂ , where 0≤x≤1 and M is selected from Ni , one or more of Mn, Al, Ca, Mg, Sr, Ti, V, Cr, Fe, Cu, Zn, Mo, W, Y, La, Zr, Sn, Se, Te and Bi.
12. A secondary battery, characterized by comprising the positive electrode plate according to any one of claims 1-11.
13. An electronic device, characterized by including the secondary battery according to claim 12.
14. A mobile device, characterized by comprising the secondary battery according to claim 12.