Positive electrode sheet and preparation method thereof, battery, battery pack and electrical equipment

Abstract

The present invention discloses a positive electrode sheet and a preparation method thereof, a battery, a battery pack and an electrical device. The positive electrode sheet includes a current collector and multiple positive electrode active layers. The multiple positive electrode active layers are sequentially arranged on at least one side surface of the current collector in the thickness direction. The positive electrode active layer includes a polymer and a plasticizer. In the direction away from the current collector, the weight-average molecular weight of the polymer of the multiple positive electrode active layers increases sequentially, and the molar molecular weight of the plasticizer of the multiple positive electrode active layers increases sequentially. The positive electrode sheet according to the present invention is conducive to exerting the synergistic effect of the polymer and the plasticizer, optimizing the ion path inside the positive electrode sheet, and improving the ion conductivity of the positive electrode sheet. At the same time, the activity of the polymer molecular chain is suitable for the needs of the corresponding positive electrode active layer, which can effectively reduce the polarization of the battery and improve the electrochemical performance of the positive electrode sheet. As a result, the gradient ion conductivity of the positive electrode sheet is improved, the ion conductivity of the positive electrode sheet is improved and the polarization is reduced, the ion transmission path is optimized, and the ion conductivity and rate performance are improved.

Description

Technical Field

[0001] The present invention relates to the field of electrode technology, and in particular to a positive electrode and a preparation method thereof, a battery, a battery pack and an electrical device. Background Art

[0002] Cathode materials are one of the core components that determine battery performance. They not only affect the battery's energy density, but also have a direct impact on power density, cycle stability, and safety. Ideal cathode materials must possess high theoretical specific capacity, good structural stability, and high electronic and ionic conductivity.

[0003] In related technologies, nanotechnology, doping modification, surface coating and other treatment methods are usually used to improve the ionic conductivity of the positive electrode material and enhance the rapid charging and discharging capabilities and overall performance of the battery. However, the above treatment methods have limited effect on improving the ionic conductivity of the positive electrode sheet and are prone to cause battery polarization (polarization is the phenomenon that the charge distribution inside the dielectric changes under the action of an electric field, forming an electric dipole or changing the orientation of the original electric dipole). Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the first object of the present invention is to provide a positive electrode sheet that improves the gradient ion conductance of the positive electrode sheet, improves the ionic conductivity of the positive electrode sheet while reducing polarization, optimizes the ion transport path, and improves ionic conductivity and rate performance.

[0005] The second object of the present invention is to provide a method for preparing a positive electrode sheet.

[0006] The third object of the present invention is to provide a battery.

[0007] A fourth objective of the present invention is to provide a battery pack.

[0008] A fifth object of the present invention is to provide an electrical device.

[0009] According to an embodiment of the first aspect of the present invention, the positive electrode sheet includes: a current collector; a plurality of positive electrode active layers, wherein the plurality of positive electrode active layers are sequentially arranged on at least one side surface of the current collector in the thickness direction, the positive electrode active layers include a polymer and a plasticizer, and along the direction away from the current collector, the weight-average molecular weight of the polymer of the plurality of positive electrode active layers increases sequentially, and the molar molecular weight of the plasticizer of the plurality of positive electrode active layers increases sequentially.

[0010] According to the positive electrode sheet of the embodiment of the present invention, by regulating the weight-average molecular weight of the polymer and the molar molecular weight of the plasticizer in multiple positive electrode active layers, it helps to exert the synergistic effect of the polymer and plasticizer, optimize the ion path within the positive electrode sheet, and improve the ionic conductivity of the positive electrode sheet. The gradient design of the plasticizer helps to adjust the interaction force between the corresponding polymer molecular chains, so that the mobility of the polymer molecular chains is suitable for the needs of the corresponding positive electrode active layer, which can effectively reduce battery polarization and improve the electrochemical performance of the positive electrode sheet. As a result, the gradient ionic conductivity of the positive electrode sheet is improved, the ionic conductivity of the positive electrode sheet is improved, the polarization is reduced, the ion transmission path is optimized, and the ionic conductivity and rate performance are improved.

[0011] According to some embodiments of the present invention, the multiple positive electrode active layers include at least a first positive electrode active layer and a second positive electrode active layer, wherein the weight average molecular weight of the polymer in the first positive electrode active layer is 10,000 g / mol to 300,000 g / mol, preferably 50,000 g / mol to 150,000 g / mol; and / or the weight average molecular weight of the polymer in the second positive electrode active layer is 400,000 g / mol to 1,000,000 g / mol, preferably 600,000 g / mol to 800,000 g / mol.

[0012] According to some embodiments of the present invention, the polymers of the plurality of positive electrode active layers are independently selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyacrylonitrile, polypropylene carbonate, polyethylene oxide, and ethylene oxide-propylene oxide copolymer.

[0013] According to some embodiments of the present invention, the multiple positive electrode active layers include at least a first positive electrode active layer and a second positive electrode active layer, wherein the molar molecular weight of the plasticizer in the first positive electrode active layer is 40g / mol to 250g / mol, preferably 80g / mol to 150g / mol; and / or the molar molecular weight of the plasticizer in the second positive electrode active layer is 250g / mol to 2000g / mol, preferably 500g / mol to 1000g / mol.

[0014] According to some embodiments of the present invention, the plasticizers in the plurality of positive electrode active layers are independently selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, 1,4-dioxane, succinonitrile and polyethylene glycol dimethyl ether.

[0015] According to some embodiments of the present invention, the positive electrode active layer further includes at least one of an active material, a conductive agent and an electrolyte salt, wherein the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt and the plasticizer is (50-90): (1-10): (6-20): (0-10): (0-10).

[0016] According to some embodiments of the present invention, the active materials of the plurality of positive electrode active layers are independently selected from lithium cobalt oxide, lithium nickel oxide, lithium iron phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium nickel phosphate, lithium manganese iron phosphate, lithium cobalt iron phosphate, lithium nickel iron phosphate, lithium manganese oxide, xLi2MnO3·(1-x)LiMO2, LiNi y A (1-y) O2 and LiNimBnC (1-m-n) At least one of O2, wherein 0<x<1, M is selected from transition metals, A is selected from Co or Mn, 0<y<1, B and C are independently selected from Co, Al and Mn, and B and C are different, 0<m<1, 0<n<1; and / or, the conductive agent of the plurality of said positive electrode active layers is independently selected from at least one of conductive graphite, acetylene black, Super P, graphene, carbon fiber, carbon nanotubes, Ketjen black, metal nanowires and metal nanotubes; and / or, the electrolyte salt of the plurality of said positive electrode active layers is at least one of potassium salt, sodium salt and lithium salt.

[0017] According to some embodiments of the present invention, when the electrolyte salts of the plurality of positive electrode active layers are all lithium salts, the electrolyte salts of the plurality of positive electrode active layers are independently selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium difluorooxalatoborate and lithium tetrafluorooxalatophosphate.

[0018] According to some embodiments of the present invention, the thickness of the current collector is 8um to 25um, preferably 10um to 15um; and / or the multiple positive electrode active layers include at least a first positive electrode active layer and a second positive electrode active layer, and the thickness of the first positive electrode active layer is 100um to 200um, preferably 120um to 150um; and / or the thickness of the second positive electrode active layer is 100um to 250um, preferably 150um to 200um.

[0019] A method for preparing a positive electrode sheet according to an embodiment of the second aspect of the present invention comprises the following steps:

[0020] Providing a plurality of composite slurries, each of the composite slurries comprising a polymer and a plasticizer, wherein the polymers in at least two of the composite slurries have different weight-average molecular weights and the plasticizers have different molar molecular weights;

[0021] The multiple composite slurries are coated on the surface of the current collector by co-extrusion to form multiple positive electrode active layers, wherein the weight-average molecular weight of the polymer in the multiple positive electrode active layers increases successively along the direction away from the current collector, and the molar molecular weight of the plasticizer decreases successively. After drying, the positive electrode sheet is obtained.

[0022] According to an embodiment of the third aspect of the present invention, a battery comprises: a positive electrode sheet, a negative electrode sheet and a separator; wherein the positive electrode sheet is the positive electrode sheet according to the embodiment of the first aspect of the present invention or the positive electrode sheet prepared by the preparation method of the embodiment of the first aspect of the present invention.

[0023] A battery pack according to an embodiment of the fourth aspect of the present invention comprises: a positive electrode sheet according to an embodiment of the first aspect of the present invention or a positive electrode sheet prepared by the preparation method of the embodiment of the first aspect of the present invention; or a battery according to an embodiment of the third aspect of the present invention.

[0024] An electrical device according to an embodiment of a fifth aspect of the present invention includes: a battery according to an embodiment of the third aspect of the present invention or a battery pack according to an embodiment of the fourth aspect of the present invention.

[0025] Additional aspects and advantages of the present invention will be set forth in part in the description which follows and, in part, will be obvious from the description which follows, or may be learned by practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the following description of the embodiments with reference to the accompanying drawings, in which:

[0027] Figure 1 is a schematic diagram of a positive electrode sheet according to an embodiment of the present invention;

[0028] Figure 2 Schematic diagram of the preparation of a positive electrode sheet according to an embodiment of the present invention.

[0029] Reference numerals:

[0030] 100: positive electrode;

[0031] 1: current collector; 2: first positive electrode active layer; 3: second positive electrode active layer; 4: composite slurry. DETAILED DESCRIPTION

[0032] Reference below Figure 1-Figure 2 A positive electrode sheet 100 according to an embodiment of the first aspect of the present invention will be described.

[0033] like Figure 1-Figure 2 As shown, the positive electrode sheet 100 according to the first embodiment of the present invention includes a current collector 1 and a plurality of positive electrode active layers.

[0034] Specifically, multiple positive electrode active layers are sequentially disposed on at least one surface of the current collector 1 in the thickness direction. The positive electrode active layers include a polymer and a plasticizer. The weight-average molecular weight of the polymer in the multiple positive electrode active layers increases sequentially as the layer moves away from the current collector 1, and the molar molecular weight of the plasticizer in the multiple positive electrode active layers increases sequentially. In the description of the present invention, "multiple" means two or more.

[0035] For example, in Figure 1 and Figure 2 In the example, multiple positive electrode active layers may be sequentially provided on only one side of the current collector 1 in the thickness direction; or, multiple positive electrode active layers may be sequentially provided on both sides of the current collector 1 in the thickness direction.

[0036] Polymers are introduced into multiple cathode active layers, forming a three-dimensional network structure through cross-linking reactions. This not only improves the mechanical properties of the polymer but also promotes ion conduction to a certain extent. The cross-linking structure restricts excessive movement of the polymer chains while providing pathways for ion transport. Ion conduction in polymers primarily relies on chain segment motion to provide migration pathways for ions. The more mobile the chain segments, the easier it is for ions to move through them. For example, polymers with more amorphous regions have freer chain segment motion and better ion conduction performance.

[0037] Among them, the weight-average molecular weight of the polymers in the multiple positive electrode active layers increases in sequence along the direction away from the current collector 1. The weight-average molecular weight of the polymer in the positive electrode active layer closer to the current collector 1 is smaller, and accordingly, the chain length of the polymer is shorter, which has better fluidity, can provide higher ion conductivity, and is beneficial to the transmission of ions in the positive electrode active layer. The weight-average molecular weight of the polymer in the positive electrode active layer farther from the current collector 1 is larger, the chain length of the polymer is longer, and the fluidity of the polymer is smaller, which is beneficial to improving the structural stability of the positive electrode active layer, reducing the risk of deformation and cracking of the above-mentioned positive electrode active layer and the positive electrode sheet 100 using the same during the process, and further beneficial to improving the stability of the positive electrode sheet 100 in use.

[0038] Therefore, by adjusting and controlling the weight-average molecular weight of the polymers of the multiple positive electrode active layers, the chain lengths of the polymers of the multiple positive electrode active layers increase successively in the direction away from the current collector 1, which is conducive to achieving gradient ion conductivity of the multiple positive electrode active layers. The design of gradient ion conductivity can improve the utilization rate of the active material on the current collector 1 side of the positive electrode sheet 100 and increase the capacity, balance the charge in the depth direction of the positive electrode sheet 100 and reduce polarization, optimize the ion transmission path, and improve the ion conductivity and the rate performance of the battery compared with the existing positive electrode sheet design.

[0039] At the same time, the molar molecular weight of the plasticizer in the multiple positive electrode active layers is controlled so that the molar molecular weight of the plasticizer in the multiple positive electrode active layers increases in sequence as it moves away from the current collector 1. Among them, plasticizer molecules with low molar molecular weights are smaller and can more easily insert between polymer molecular chains, weakening the interaction forces between polymer molecular chains, such as van der Waals forces and hydrogen bonds, thereby enhancing the mobility of the polymer molecular chains. Thus, at a lower addition amount, the glass transition temperature of the polymer can be significantly reduced, the flexibility and plasticity of the polymer can be increased, and a good plasticizing effect can be exhibited. Plasticizer molecules with high molar molecular weights are larger, have a slower diffusion rate in the polymer matrix, and interact more strongly with the polymer molecular chains. Therefore, they have better migration resistance and can remain stable in the polymer for a long time. They are not easily volatilized or migrated to the surface of the positive electrode sheet 100 or other substances. This makes the plasticizing effect more durable, allowing the positive electrode sheet 100 to maintain good performance for a long time, which is conducive to ensuring reliable contact between the positive electrode sheet 100 and the electrolyte.

[0040] Thus, by adjusting the polymer chain length to achieve gradient ion conductivity, and adding plasticizers of varying molecular weights, the synergistic effect of the polymer and plasticizer can be fully utilized, allowing the construction of a positive electrode sheet 100 with gradient ion conductivity using different electrolyte formulations. This positive electrode sheet 100 utilizes the synergistic effect of polymer segments and plasticizers. The positive electrode active layer near the current collector 1 utilizes a combination of short-chain polymers (polymers with low weight-average molecular weight) and strong plasticizers (plasticizers with low molar molecular weight). This promotes the migration of polymer segments and lithium ion transport, providing high ion conductivity. The positive electrode active layer away from the current collector 1 utilizes a combination of long-chain polymers (polymers with high weight-average molecular weight) and weak plasticizers (plasticizers with high molar molecular weight). This ensures the viscosity and flexibility of the components in the positive electrode active layer, as well as the contact between the positive electrode sheet 100 and the electrolyte. The positive electrode sheet 100 designed with the above-mentioned gradient ion conductivity strategy can effectively improve the utilization rate of the active material on the current collector 1 side of the positive electrode sheet 100 and increase the capacity, balance the charge in the depth direction of the electrode sheet and reduce polarization, optimize the ion transmission path and improve the battery's rate performance.

[0041] According to the positive electrode sheet 100 of the embodiment of the present invention, by regulating the weight-average molecular weight of the polymer and the molar molecular weight of the plasticizer in the multiple positive electrode active layers, it is helpful to leverage the synergistic effect of the polymer and plasticizer, optimize the ion pathway within the positive electrode sheet 100, and improve the ionic conductivity of the positive electrode sheet 100. The gradient design of the plasticizer helps to adjust the interaction force between the corresponding polymer molecular chains, so that the mobility of the polymer molecular chains is suitable for the needs of the corresponding positive electrode active layer, which can effectively reduce battery polarization and improve the electrochemical performance of the positive electrode sheet 100. As a result, the gradient ionic conductivity of the positive electrode sheet 100 is improved, the ionic conductivity of the positive electrode sheet 100 is improved, the polarization is reduced, the ion transmission path is optimized, and the ionic conductivity and rate performance are improved.

[0042] According to some embodiments of the present invention, referring to Figure 1 and Figure 2 The multiple positive electrode active layers include at least a first positive electrode active layer 2 and a second positive electrode active layer 3. The weight-average molecular weight of the polymer in the first positive electrode active layer 2 is 10,000 g / mol to 300,000 g / mol, preferably 50,000 g / mol to 150,000 g / mol. The polymer has a reasonable weight-average molecular weight, reasonable viscosity, and good fluidity, which helps improve the ionic conductivity of the first positive electrode active layer 2.

[0043] The weight-average molecular weight of the polymer in the second positive electrode active layer 3 is 400,000 g / mol to 1,000,000 g / mol, preferably 600,000 g / mol to 800,000 g / mol. The weight-average molecular weight of the above polymer is relatively reasonable, which enhances the interaction between the molecular chains of the polymer and the ability to resist external damage. It also reasonably increases the strength, hardness and toughness of the polymer, ensures the viscosity of each component in the second positive electrode active layer 3, thereby facilitating the improvement of the connection stability between the second positive electrode active layer 3 and the first positive electrode active layer 2, and ensuring the contact between the positive electrode sheet 100 and the electrolyte.

[0044] According to some embodiments of the present invention, the polymers of the multiple positive electrode active layers are independently selected from at least one of polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), polytetrafluoroethylene (PTFE), polyacrylonitrile (PAN), polypropylene carbonate (PPC), polyethylene oxide (PEO), and ethylene oxide-propylene oxide copolymer (PEO-PO). Polyvinylidene fluoride is a homopolymer of vinylidene fluoride (VDF) or a copolymer of vinylidene fluoride and a small amount of fluorine-containing vinyl monomer. It exhibits excellent chemical resistance, high impact strength, wear resistance, creep resistance, high mechanical strength, and toughness. Polyvinylidene fluoride-hexafluoropropylene copolymer is a copolymer of vinylidene fluoride and hexafluoropropylene. It has low rigidity, good flexibility, high flowability, and high adhesion. Polytetrafluoroethylene has excellent corrosion and weather resistance, good electrical insulation, a low dielectric constant, and high light transmittance. Polyacrylonitrile is obtained by free radical polymerization of acrylonitrile. Polypropylene carbonate is synthesized from carbon dioxide and propylene oxide. Polyethylene oxide contains flexible segments, giving it excellent flexibility. Ethylene oxide-propylene oxide copolymers combine the properties of both ethylene oxide and propylene oxide. Their performance can be controlled by adjusting the ratio of ethylene oxide to propylene oxide, and they generally exhibit good solubility, surface activity, and flexibility.

[0045] The polymers of multiple positive electrode active layers are independently selected from the above polymers, which facilitates gradient control of multiple positive electrode active layers, and at the same time helps to ensure the flexibility of multiple positive electrode active layers, reduce the risk of deformation and cracking of the positive electrode sheet 100 during use, improve the reliability of the positive electrode sheet 100, and extend the service life of the positive electrode sheet 100.

[0046] According to other embodiments of the present invention, the plurality of positive electrode active layers include at least a first positive electrode active layer 2 and a second positive electrode active layer 3, wherein the molar molecular weight of the plasticizer in the first positive electrode active layer 2 is 40 g / mol to 250 g / mol, preferably 80 g / mol to 150 g / mol. The molar molecular weight of the plasticizer is relatively reasonable, which helps promote the migration of polymer chain segments, thereby promoting ion transport and improving the ionic conductivity of the first positive electrode active layer 2.

[0047] The molar molecular weight of the plasticizer in the second positive electrode active layer 3 is 250 g / mol to 2000 g / mol, preferably 500 g / mol to 1000 g / mol. This plasticizer has a reasonable molar molecular weight, which helps ensure adhesion between the components in the positive electrode sheet 100 and contact between the positive electrode sheet 100 and the electrolyte.

[0048] According to some further embodiments of the present invention, the plasticizers in the multiple positive electrode active layers are independently selected from at least one of N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMAC), ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), ethylene glycol dimethyl ether (DME), ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, 1,4-dioxane, succinonitrile and polyethylene glycol dimethyl ether (PEGDM).

[0049] The plasticizers in the multiple positive electrode active layers are independently selected from the aforementioned organic substances. Each of the aforementioned plasticizers has good compatibility with the polymers and can plasticize the polymers in the multiple positive electrode active layers to a certain extent, thereby helping to improve the ion conductivity of the corresponding positive electrode active layer and enhance the flexibility and stability of the positive electrode active layer. Multiple plasticizers can be selected for each positive electrode active layer to increase the plasticizing effect and enhance the gradient conductivity performance.

[0050] At the same time, during the preparation process of the positive electrode sheet 100, the plasticizer will coordinate with the polymer body and the electrolyte salt, and will not be completely volatilized in the later preparation process, and can be measured at the finished end of the positive electrode sheet 100. For example, the plasticizer can be measured by Fourier-transform infrared spectroscopy (FIIR). By measuring the degree of absorption of infrared light of different wavelengths by the sample, the vibration and rotation information of the molecule can be obtained, and then the chemical composition and functional group structure of the sample can be inferred. The slurry for preparing the above-mentioned multiple positive electrode active layers can be extruded from the screw and collected on a PET (polyethylene terephthalate) film; the multiple positive electrode active layers after film formation can be peeled off from the PET film and placed on a clean sample table for testing, and the wave number collected is 4000nm. -1 ~500nm -1 The data can be used to distinguish the types of plasticizers based on the characteristic peaks.

[0051] At the same time, increasing molecular weight typically increases thermal performance parameters such as the polymer's glass transition temperature (Tg) and melting point (Tm). This is because longer molecular chains increase intermolecular interactions, requiring higher energies to initiate molecular chain motion or melt the crystal structure. Therefore, differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) can be used in conjunction to distinguish polymers in multiple cathode active layers.

[0052] A differential scanning calorimeter measures the heat flow difference between a sample and a reference by simultaneously programming the temperature to rise or fall. When a sample undergoes physical or chemical changes, such as phase transitions, melting, crystallization, or chemical reactions, it absorbs or releases heat, resulting in a heat flow difference between the sample and the reference. The instrument measures this heat flow difference and converts it into a temperature-heat curve, thereby obtaining information about the thermal properties of the sample. Specifically, when a polymer has a low molecular weight, its glass transition temperature is low. Therefore, by comparing the endothermic and exothermic peaks, it is possible to distinguish the use of polymers in multiple positive electrode active layers.

[0053] By accurately measuring the change in sample mass over temperature or time, a thermogravimetric analyzer can provide information on the sample's thermal stability, thermal decomposition process, and reaction kinetics. Polymers tend to lose weight first when the analyzed mass is small. Therefore, combining a differential scanning calorimeter with a thermogravimetric analyzer can increase the accuracy of confirming the polymer in the positive electrode active layer.

[0054] According to some embodiments of the present invention, the positive electrode active layer further comprises at least one of an active material, a conductive agent, and an electrolyte salt, wherein the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt, and the plasticizer is (50-90):(1-10):(6-20):(0-10):(0-10). Thus, the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt, and the plasticizer is reasonable, which is conducive to fully exerting the effects of the active material, the conductive agent, the polymer, the electrolyte salt, and the plasticizer, and is suitable for improving the uniformity, stability, and structural integrity of the positive electrode active layer.

[0055] Among them, the active material, as the main substance of the redox reaction, realizes the battery's charge and discharge process by gaining and losing electrons, which determines important parameters such as the battery's energy density, voltage platform, and cycle performance. Conductive agents help improve the electronic conductivity of the positive electrode material and ensure the rapid transmission of electrons in the electrode material, thereby improving the battery's charge and discharge efficiency and rate performance, and reducing the polarization phenomenon of the battery during the charge and discharge process. Electrolyte salts are used to provide mobile ions. These ions migrate from the positive electrode to the negative electrode (during charging) or vice versa (during discharge) through the electrolyte, realizing the flow of current.

[0056] Furthermore, the active materials of the plurality of positive electrode active layers are independently selected from lithium cobalt oxide, lithium nickel oxide, lithium iron phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium nickel phosphate, lithium manganese iron phosphate, lithium cobalt iron phosphate, lithium nickel iron phosphate, lithium manganese oxide, xLi2MnO3·(1-x)LiMO2, LiNi y A (1-y) O2 and LiNimBnC (1-m-n)At least one of O2, wherein 0<x<1, M is selected from transition metals, A is selected from Co or Mn, 0<y<1, B and C are independently selected from Co, Al and Mn, and B and C are different, 0<m<1, 0<n<1. Among them, lithium cobalt oxide (LiCoO2) has high energy density, good cycle stability and high working voltage. Lithium nickel oxide (LiNiO2) has high specific capacity. Lithium iron phosphate (LiFePO4) has excellent safety, long cycle life, stable chemical structure, low cost and environmental friendliness. Lithium cobalt phosphate (LiCoPO4) has good chemical stability and high working voltage. Lithium manganese phosphate (LiMnPO4) has high specific capacity. Lithium nickel phosphate (LiNiPO4) is similar to lithium cobalt phosphate, but has higher theoretical energy density. Lithium manganese oxide (LiMn2O4) has low cost and good thermal stability. Lithium manganese iron phosphate, lithium cobalt iron phosphate and lithium nickel iron phosphate, xLi2MnO3·(1-x)LiMO2 and LiNi y A (1-y) O2 has a high energy density, which helps reduce the volume of the positive electrode active layer while meeting the energy density requirements of the positive electrode active layer, thereby improving the applicability of the positive electrode active layer. Therefore, the active materials of multiple positive electrode active layers independently use the above-mentioned active materials, which helps improve the applicability of the positive electrode sheet 100 using the above-mentioned active materials, improves the energy density and volume efficiency of the battery using the above-mentioned positive electrode sheet 100, improves the battery's operating safety, and extends the battery's service life.

[0057] The conductive agents of the multiple positive electrode active layers are independently selected from at least one of conductive graphite, acetylene black, Super P, graphene, carbon fiber, carbon nanotubes, Ketjen black, metal nanowires, and metal nanotubes. Conductive graphite has high electronic conductivity. Flake graphite or granular graphite can form a continuous conductive network through physical contact, closely connecting the active material particles and reducing interruptions in the electron transport path. Conductive graphite is also low in cost and has good chemical stability. Acetylene black has good conductivity, a large specific surface area, and excellent dispersibility, effectively increasing the conductive network of the positive electrode sheet 100. Super P can provide a good conductive path, helping to improve the overall conductivity of the first positive electrode active layer 2 and the second positive electrode active layer 3. Super S has high conductivity and good dispersibility and is suitable for enhancing the electron conduction capability of the positive electrode sheet 100. Graphene is a single-layer or multi-layer two-dimensional carbon nanomaterial with extremely high conductivity and mechanical strength. Due to its large specific surface area and excellent conductivity, it can improve the conductivity and stability of the electrode while also increasing its flexibility and mechanical strength. Carbon fiber is a fibrous material composed of carbon atoms with high strength and modulus. In addition to excellent conductivity, it also possesses good mechanical properties and can be used to enhance the structural stability and conductive network of electrode materials. Carbon nanotubes (CNTs) are tubular one-dimensional nanomaterials, divided into single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). They have an extremely high aspect ratio and excellent conductivity, forming a highly efficient three-dimensional conductive network, significantly improving the conductivity and cycling stability of the positive electrode sheet 100. Ketjen black is a special type of high-structure carbon black with a very large specific surface area and a highly branched structure. Its unique structure provides excellent conductivity and dispersibility, enabling it to form an efficient conductive network within the electrode, thereby improving battery performance. Metal nanowires and metal nanotubes have high conductivity, good conductive network formation capabilities, low contact resistance, and efficient charge transfer. Therefore, the conductive agent in each of the multiple positive electrode active layers is independently selected from at least one of the above conductive agents, which helps improve the conductivity and structural stability of the positive electrode sheet 100 using the first positive electrode active layer 2 and the second positive electrode active layer 3 containing such conductive agents.

[0058] The electrolyte salt of the multiple positive electrode active layers is at least one of a potassium salt, a sodium salt, and a lithium salt. In other words, the electrolyte salts of the multiple positive electrode active layers may all be potassium salts; or, the electrolyte salts of the multiple positive electrode active layers may all be sodium salts; or, the electrolyte salts of the multiple positive electrode active layers may all be lithium salts. In addition, a variety of composite salts of potassium salts, sodium salts, and lithium salts may be selected as the electrolyte salts of the multiple positive electrode active layers. In this way, the uniformity of the ions in the positive electrode sheet 100 is ensured, and the electrolyte salts of the multiple positive electrode active layers can provide mobile ions, increase the ion concentration, and thus improve the ionic conductivity of the positive electrode sheet 100.

[0059] Furthermore, when the electrolyte salts of the multiple positive electrode active layers are lithium salts, the electrolyte salts of the multiple positive electrode active layers are independently selected from at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(oxalatoborate) (LiBOB), lithium difluorooxalatoborate (LiDFOB), and lithium tetrafluorooxalatophosphate (LiTFOP). These electrolyte salts all have good thermal stability and electrochemical windows. Using at least one of these electrolyte salts helps improve the ionic conductivity of the positive electrode sheet 100, while also helping to improve the safety of batteries using these positive electrode sheets 100 and extend the battery life.

[0060] According to some embodiments of the present invention, the thickness of the current collector 1 is 8um to 25um, preferably 10um to 15um. The current collector 1 plays the role of carrying active materials, collecting and conducting current in the battery. The selection of its thickness has an important impact on the overall performance of the battery, including energy density, power density, cycle life and cost. The thickness of the above-mentioned current collector 1 is relatively reasonable, which helps to provide a better conductive path, reduce resistance loss, and effectively control the total weight and volume of the positive electrode sheet 100 and the battery using the above-mentioned positive electrode sheet 100, thereby ensuring the energy density of the battery. In addition, it can also ensure the mechanical strength of the positive electrode sheet 100 to prevent rupture or damage during the manufacturing process or use.

[0061] The thickness of the first positive electrode active layer 2 is 100 μm to 200 μm, preferably 120 μm to 150 μm; and / or the thickness of the second positive electrode active layer 3 is 100 μm to 250 μm, preferably 150 μm to 200 μm. The thickness of the first positive electrode active layer 2 and / or the second positive electrode active layer 3 is relatively reasonable, which helps achieve high power output, reduce internal resistance, and improve power density. It also helps prevent the first positive electrode active layer 2 and / or the second positive electrode active layer 3 from shedding or cracking due to increased mechanical stress caused by volume changes during charging and discharging, thereby extending the cycle life of the positive electrode sheet 100 and the battery using the positive electrode sheet 100. In addition, the molecular chain of the polymer in the first positive electrode active layer 2 is relatively short, which prevents the positive electrode sheet 100 from cracking during manufacturing and use due to its high thickness.

[0062] The thickness of the first positive electrode active layer 2 and the thickness of the second positive electrode active layer 3 may be equal, or the thickness of the first positive electrode active layer 2 may be greater than the thickness of the second positive electrode active layer 3; or the thickness of the first positive electrode active layer 2 may be less than the thickness of the second positive electrode active layer 3. This is not specifically limited here.

[0063] According to some specific embodiments of the present invention, the current collector 1 is aluminum foil, foamed aluminum, carbon-coated aluminum foil, carbon mesh or carbon cloth.

[0064] The method for preparing the positive electrode sheet 100 according to the second embodiment of the present invention includes the following steps:

[0065] Providing a plurality of composite slurries 4, each composite slurry 4 comprising a polymer and a plasticizer, wherein at least two of the composite slurries 4 have polymers with different weight average molecular weights and plasticizers with different molar molecular weights;

[0066] A plurality of composite slurries 4 are coated on the surface of the current collector 1 by co-extrusion to form a plurality of positive electrode active layers, wherein the weight average molecular weight of the polymer in the plurality of positive electrode active layers increases successively as it moves away from the current collector 1, and the molar molecular weight of the plasticizer decreases successively. After drying, the positive electrode sheet 100 is obtained.

[0067] Using a co-extrusion coating method, multiple composite slurries 4 are simultaneously applied to the surface of the current collector 1. After drying and roll pressing, a positive electrode sheet 100 with gradient ionic conductivity is obtained. Co-extrusion coating simplifies the coating process, eliminating the need for multiple coating steps. The composite slurry 4 containing a plasticizer has high fluidity, and applying the two composite slurries 4 simultaneously can prevent cracking during drying (e.g., baking).

[0068] After coating, the solvent may be removed by vacuum drying at 60° C. to 120° C. for 4-16 hours, and then roller pressing may be performed to obtain the positive electrode sheet 100 having gradient ion conductivity.

[0069] The above preparation method has low cost, simple process and is suitable for large-scale mass production and commercialization.

[0070] According to the method for preparing the positive electrode sheet 100 of the embodiment of the present invention, the positive electrode sheet 100 with gradient ion conductivity can be produced with a simple preparation process at low cost. The produced positive electrode sheet 100 has high structural strength and long service life.

[0071] A battery according to an embodiment of the third aspect of the present invention includes: a positive electrode sheet 100, a negative electrode sheet and a separator; wherein the positive electrode sheet 100 is the positive electrode sheet 100 according to the embodiment of the first aspect of the present invention or the positive electrode sheet 100 prepared by the preparation method of the embodiment of the first aspect of the present invention.

[0072] According to the battery of the embodiment of the present invention, the use of the above-mentioned positive electrode sheet 100 helps to improve the charging efficiency and discharging efficiency of the battery, so that the battery can output a larger current to meet the needs of high-power equipment; at the same time, the conductivity of the positive electrode sheet 100 is improved, which helps to reduce the energy loss of the battery and improve the energy utilization rate of the battery; in addition, by improving the conductivity, the reaction inside the battery is made more uniform, reducing the irreversible changes of the electrode material, thereby extending the cycle life of the battery.

[0073] A battery pack according to an embodiment of the fourth aspect of the present invention includes: a positive electrode sheet 100 according to the embodiment of the first aspect of the present invention or a positive electrode sheet 100 prepared by the preparation method of the embodiment of the first aspect of the present invention; or a battery according to an embodiment of the third aspect of the present invention.

[0074] The battery pack according to the embodiment of the fourth aspect of the present invention helps to improve the conductivity of the battery pack and extend the service life of the battery pack.

[0075] An electrical device according to an embodiment of a fifth aspect of the present invention includes: a battery according to an embodiment of the third aspect of the present invention or a battery pack according to an embodiment of the fourth aspect of the present invention.

[0076] The electrical equipment according to the embodiments of the present invention helps to improve the energy supply stability and reliability of the electrical equipment, thereby helping to improve the operational stability of the electrical equipment and further helping to improve the market competitiveness of the electrical equipment.

[0077] The following embodiments of the present invention are described in detail. It should be noted that the following embodiments are illustrative and are intended only to explain the present invention and are not to be construed as limiting the present invention. In addition, unless otherwise expressly stated, all reagents used in the following embodiments are commercially available or can be synthesized according to methods described herein or known methods. Reaction conditions not listed are also readily available to those skilled in the art.

[0078] Example 1

[0079] Lithium iron phosphate is selected as the active material, SuperP is used as the conductive agent, and lithium bis(trifluoromethylsulfonyl)imide is used as the electrolyte salt. The above materials, polymer and plasticizer are dissolved in N,N-dimethylformamide to prepare the first composite slurry and the second composite slurry.

[0080] (1) The polymer in the first composite slurry is polyethylene oxide with a weight-average molecular weight of 100,000 g / mol, and the polymer in the second composite slurry is polyethylene oxide with a weight-average molecular weight of 600,000 g / mol; the plasticizer in the first composite slurry is succinonitrile (SN, Mw = 80 g / mol), and the plasticizer in the second composite slurry is polyethylene glycol dimethyl ether (Mw = 1000 g / mol, i.e., the plasticizer is PEGDM-1000).

[0081] (2) In the first composite slurry and the second composite slurry, the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt and the plasticizer is 79:3:9:4.5:4.5. In terms of mass, the number of active materials in the first composite slurry is 13 parts, and the number of active materials in the second composite slurry is 17 parts.

[0082] (3) The first composite slurry and the second composite slurry are coated together on the surface of the current collector 1 by co-extrusion coating, wherein the first composite slurry is close to the side of the current collector 1, and the second composite slurry is close to the side of the solid electrolyte, that is, away from the current collector 1. The prepared solid-state battery positive electrode precursor is placed under vacuum drying at 80°C for 8 hours, and after roller pressing, a positive electrode sheet 100 with gradient ion conductivity performance is obtained.

[0083] (4) The compaction density of the positive electrode sheet 100 of the prepared solid-state battery is 2.35 g / cm 3 The thickness of the first positive electrode active layer 2 made from the first composite slurry is 130 μm, and the thickness of the second positive electrode active layer 3 made from the second composite slurry is 170 μm.

[0084] Example 2

[0085] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (2), the plasticizer in the first composite slurry is SN, and the plasticizer in the second composite slurry is PEGDM-2000.

[0086] Example 3

[0087] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (2), the plasticizer in the first composite slurry is acetonitrile (Mw=41 g / mol), and the plasticizer in the second composite slurry is PEGDM-1000.

[0088] Example 4

[0089] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (2), the plasticizer in the first composite slurry is SN, and the plasticizer in the second composite slurry is PEGDM-250.

[0090] Example 5

[0091] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (2), the plasticizer in the first composite slurry is SN, and the plasticizer in the second composite slurry is PEGDM-500 and PEGDM-1000. The mass ratio of PEGDM-500 to PEGDM-1000 is 2:8.

[0092] Example 6

[0093] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (2), the plasticizer in the first composite slurry is acetonitrile, and the plasticizer in the second composite slurry is PEGDM-3000.

[0094] Example 7

[0095] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (1), the polymer in the first composite slurry is polyethylene oxide with a weight-average molecular weight of 100,000 g / mol, and the polymer in the second composite slurry is polyethylene oxide with a weight-average molecular weight of 700,000 g / mol.

[0096] Example 8

[0097] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (1), the polymer in the first composite slurry is polyethylene oxide with a weight-average molecular weight of 10,000 g / mol, and the polymer in the second composite slurry is polyethylene oxide with a weight-average molecular weight of 700,000 g / mol.

[0098] Example 9

[0099] The preparation method of this embodiment is substantially the same as that of Example 1, except that in step (1), the polymer in the first composite slurry is polyethylene oxide with a weight-average molecular weight of 100,000 g / mol, and the polymer in the second composite slurry is polyethylene oxide with a weight-average molecular weight of 1,000,000 g / mol.

[0100] Example 10

[0101] (1) Lithium iron phosphate is selected as the active material, SuperP as the conductive agent, polyethylene oxide as the polymer, lithium bis(trifluoromethylsulfonyl)imide as the electrolyte salt, and the plasticizer is dissolved in N,N-dimethylformamide to prepare the first composite slurry, the second composite slurry and the third composite slurry.

[0102] (2) The polymer in the first composite slurry is polyethylene oxide with a weight average molecular weight of 100,000 g / mol, the polymer in the second composite slurry is polyethylene oxide with a weight average molecular weight of 300,000 g / mol, and the polymer in the third composite slurry is polyethylene oxide with a weight average molecular weight of 700,000 g / mol.

[0103] (3) The plasticizer in the first composite slurry is acetonitrile, the plasticizer in the second composite slurry is PEGDM-250, and the third plasticizer in the third composite slurry is PEGDM-1000.

[0104] (4) In the first composite slurry, the second composite slurry, and the third composite slurry, the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt, and the plasticizer is 79:3:9:4.5:4.5. In terms of mass, the number of active materials in the first composite slurry is 10 parts, the number of active materials in the second composite slurry is 6 parts, and the number of the third active material in the third composite slurry is 14 parts.

[0105] (5) The first composite slurry, the second composite slurry, and the third composite slurry are applied to the surface of the current collector 1 by co-extrusion coating. The first composite slurry, the second composite slurry, and the third composite slurry are applied in the direction from the current collector 1 to the solid electrolyte. The prepared solid-state battery positive electrode precursor is vacuum dried at 80°C for 8 hours and rolled to obtain a positive electrode sheet 100 with gradient ion conductivity.

[0106] (6) The compaction density of the prepared solid-state battery positive electrode is 2.35 g / cm 3 The thickness of the first positive electrode active layer 2 made from the first composite slurry is 100 μm, the thickness of the second positive electrode active layer 3 made from the second composite slurry is 60 μm, and the thickness of the third positive electrode active layer made from the third composite slurry is 140 μm.

[0107] Example 11

[0108] The preparation method of this embodiment is substantially the same as that of embodiment 10, except that in step (3), the plasticizer in the first composite slurry is SN, the plasticizer in the second composite slurry is SN, and the third plasticizer in the third composite slurry is PEGDM-500.

[0109] Comparative Example 1

[0110] The preparation method of this embodiment is substantially the same as that of embodiment 1, except that the plasticizer in the first composite slurry in step (2) is SN, and the plasticizer in the second composite slurry is also SN.

[0111] Comparative Example 2

[0112] The preparation method of this embodiment is substantially the same as that of Example 1, except that the plasticizer in the first composite slurry in step (2) is PEGDM-1000, and the plasticizer in the second composite slurry is PEGDM-1000.

[0113] Comparative Example 3

[0114] The preparation method of this embodiment is substantially the same as that of embodiment 10, except that the plasticizer in the first composite slurry in step (3) is SN, the plasticizer in the second composite slurry is SN, and the third plasticizer in the third composite slurry is SN.

[0115] Performance Testing

[0116] Ionic conductivity testing was performed on the positive electrode sheets 100 prepared in Examples 1-7 and Comparative Examples 1-3 at 60°C. Soft-pack batteries were assembled using the positive electrode sheets 100 prepared in Examples 1-7 and Comparative Examples 1-3, polyethylene oxide as the polymer electrolyte, lithium bis(trifluoromethanesulfonylimide) as the electrolyte salt, acetonitrile as the plasticizer, and metallic lithium as the negative electrode. Overpotential testing was performed at 60°C. The mass ratio of polyethylene oxide, lithium bis(trifluoromethanesulfonylimide), and acetonitrile was 2:1:1. The results are shown in Table 1.

[0117] Table 1 Electrochemical properties of the positive electrode sheets 100 prepared in Examples 1-11 and Comparative Examples 1-3 and the soft-pack batteries assembled therefrom

[0118]

[0119] Test result analysis

[0120] Comparison of Comparative Examples 1-3 and Examples 1-6 shows that Examples 1-6 control the plasticizing strength of the plasticizer in the composite slurry to regulate the fluidity of the polymer electrolyte inside the positive electrode sheet 100, thereby achieving the goal of preparing a positive electrode sheet 100 for a solid-state battery with gradient ionic conductivity. Comparison of Comparative Examples 1-3 and Examples 7-9 shows that the gradient design of the polymer chain segments can effectively optimize the ion pathways inside the positive electrode sheet 100 and improve the ionic conductivity of the positive electrode sheet 100. Comparison of Comparative Examples 1-3 and Examples 10-11 shows that the design of increasing the molecular weight of the plasticizer on the basis of the gradient design of the polymer chain segments can further improve the ionic conductivity of the positive electrode sheet 100, reduce polarization, and thus improve the rate performance of the battery.

[0121] It can be seen from Examples 1-4 that the plasticizing strength of the plasticizer has an important influence on the performance of the positive electrode sheet 100 of the solid-state battery. Compared with Example 1, the plasticizing strength of the plasticizer in Example 2 is relatively low, the degree of movement of the polymer chain segments in the electrolyte decreases, and the lithium ion transmission speed slows down; the plasticizing strength of the plasticizer in Example 3 is relatively high, the adhesion of the polymer chain segments decreases, and the plasticizer easily flows out of the target active sublayer, which in turn leads to a decrease in the plasticizing effect of the active sublayer and a decrease in ionic conductivity; the plasticizing strength of the plasticizer in Example 4 is relatively high, the overall adhesion of the positive electrode sheet 100 is poor, and cracking is prone to occur, resulting in a decrease in ionic conductivity. Therefore, in order to make the positive electrode sheet 100 have better electrochemical performance, the present application controls the molar molecular weight of the plasticizer within an appropriate range.

[0122] It can be seen from Example 1 and Example 5 that the composite plasticizer can combine the advantages of two plasticizers and, to a certain extent, optimize the plasticizing effect of the plasticizer, thereby improving the effect of gradient ion conductivity.

[0123] It can be seen from Examples 1 and 6 that the molecular weight of the plasticizer has an important influence on the performance of the positive electrode sheet 100 of the solid-state battery. Selecting a suitable molecular weight of the plasticizer can effectively improve the ion transmission rate inside the positive electrode sheet 100.

[0124] As can be seen from Examples 1 and 7-9, the gradient design of the polymer chain segments can effectively optimize the ion pathway within the positive electrode sheet 100. If the polymer chain segments are too short, the positive electrode sheet 100 is prone to cracking during manufacturing and use; if the polymer chain segments are too long, the resistance to lithium ion migration is high, which tends to reduce the ion transmission rate.

[0125] Compared with Comparative Example 3, Examples 10-11 changed the plasticizing strength of the plasticizers in different positive electrode active layers. Through the synergistic effect of the plasticizers, the shortcomings of a single plasticizer in the ion transport path were improved. At the same time, by adjusting the plasticizing strength of the plasticizers between different active sublayers, the battery polarization can be effectively reduced.

[0126] Other structures and operations of the battery and electrical equipment according to the embodiments of the present invention are well known to those skilled in the art and will not be described in detail here.

[0127] In the description of the present invention, it should be understood that the terms "center", "width", "thickness", "up", "down", "front", "back", "left", "right", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention.

[0128] Throughout this specification, reference to terms such as "one embodiment," "some embodiments," "illustrative embodiments," "example," "specific example," or "some examples" means that a specific feature, structure, material, or characteristic described in conjunction with the embodiment or example is included in at least one embodiment or example of the present invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example.

[0129] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to the embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the claims and their equivalents.

Claims

1. A positive electrode sheet, characterized in that: include: current collector; A plurality of positive electrode active layers, wherein the plurality of positive electrode active layers are sequentially arranged on at least one side surface of the current collector in the thickness direction, the positive electrode active layers include a polymer and a plasticizer, and along a direction away from the current collector, the weight average molecular weight of the polymer of the plurality of positive electrode active layers increases sequentially, and the molar molecular weight of the plasticizer of the plurality of positive electrode active layers increases sequentially, The plurality of positive electrode active layers include at least a first positive electrode active layer and a second positive electrode active layer, wherein the weight average molecular weight of the polymer in the first positive electrode active layer is 10,000 g / mol to 300,000 g / mol, and / or, The weight average molecular weight of the polymer in the second positive electrode active layer is 400,000 g / mol to 1,000,000 g / mol; and / or, The molar molecular weight of the plasticizer in the first positive electrode active layer is 40 g / mol to 250 g / mol; and / or, The molar molecular weight of the plasticizer in the second positive electrode active layer is 250 g / mol to 2000 g / mol.

2. The positive electrode sheet according to claim 1, characterized in that The weight average molecular weight of the polymer in the first positive electrode active layer is 50,000 g / mol to 150,000 g / mol; and / or, The weight average molecular weight of the polymer in the second positive electrode active layer is 600,000 g / mol to 800,000 g / mol.

3. The positive electrode sheet according to claim 1, characterized in that The polymers of the plurality of positive electrode active layers are independently selected from at least one of polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polytetrafluoroethylene, polyacrylonitrile, polypropylene carbonate, polyethylene oxide, and ethylene oxide-propylene oxide copolymer.

4. The positive electrode sheet according to claim 1, characterized in that The molar molecular weight of the plasticizer in the first positive electrode active layer is 80 g / mol to 150 g / mol; and / or, The molar molecular weight of the plasticizer in the second positive electrode active layer is 500 g / mol to 1000 g / mol.

5. The positive electrode sheet according to claim 1, characterized in that: The plasticizers in the plurality of positive electrode active layers are independently selected from at least one of N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, ethylene glycol methyl ethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, 1,3-dioxolane, 1,4-dioxane, succinonitrile and polyethylene glycol dimethyl ether.

6. The positive electrode sheet according to claim 1, characterized in that The positive electrode active layer further includes at least one of an active material, a conductive agent, and an electrolyte salt, wherein the mass ratio of the active material, the conductive agent, the polymer, the electrolyte salt, and the plasticizer is (50-90): (1-10): (6-20): (0-10): (0-10).

7. The positive electrode sheet according to claim 6, characterized in that: The active materials of the plurality of positive electrode active layers are independently selected from lithium cobalt oxide, lithium nickel oxide, lithium iron phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium nickel phosphate, lithium manganese iron phosphate, lithium cobalt iron phosphate, lithium nickel iron phosphate, lithium manganese oxide, xLi2MnO3·(1-x)LiMO2, LiNi y A (1-y) O2 and LiNimBnC (1-m-n) At least one of O2, wherein 0<x<1, M is selected from a transition metal, A is selected from Co or Mn, 0<y<1, B and C are independently selected from Co, Al and Mn, and B and C are different, 0<m<1, 0<n<1; and / or, The conductive agents of the plurality of positive electrode active layers are independently selected from at least one of conductive graphite, acetylene black, Super P, graphene, carbon fiber, carbon nanotube, Ketjen black, metal nanowire and metal nanotube; and / or, The electrolyte salt of the plurality of positive electrode active layers is at least one of potassium salt, sodium salt and lithium salt.

8. The positive electrode sheet according to claim 7, characterized in that: When the electrolyte salts of the plurality of positive electrode active layers are all lithium salts, the electrolyte salts of the plurality of positive electrode active layers are independently selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bisoxalatoborate, lithium difluorooxalatoborate and lithium tetrafluorooxalatophosphate.

9. The positive electrode sheet according to any one of claims 1 to 8, characterized in that: The thickness of the current collector is 8um to 25um; and / or, The thickness of the first positive electrode active layer is 100 um to 200 um; and / or the thickness of the second positive electrode active layer is 100 um to 250 um.

10. The positive electrode sheet according to claim 9, characterized in that: The thickness of the current collector is 10um to 15um; and / or, The thickness of the first positive electrode active layer is 120 um to 150 um; and / or the thickness of the second positive electrode active layer is 150 um to 200 um.

11. The method for preparing a positive electrode sheet according to any one of claims 1 to 10, characterized in that: The steps include: Providing a plurality of composite slurries, each of the composite slurries comprising a polymer and a plasticizer, wherein the polymers in at least two of the composite slurries have different weight-average molecular weights and the plasticizers have different molar molecular weights; The multiple composite slurries are coated on the surface of the current collector by co-extrusion to form multiple positive electrode active layers, wherein the weight-average molecular weight of the polymer in the multiple positive electrode active layers increases successively along the direction away from the current collector, and the molar molecular weight of the plasticizer decreases successively. After drying, the positive electrode sheet is obtained.

12. A battery, characterized in that: include: A positive electrode sheet, a negative electrode sheet and a separator; wherein the positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 10 or the positive electrode sheet prepared by the preparation method according to claim 11.

13. A battery pack, characterized in that: include: The positive electrode sheet according to any one of claims 1 to 10, or the positive electrode sheet prepared by the preparation method according to claim 11; or the battery according to claim 12.

14. An electrical device, characterized in that: include: The battery according to claim 12 or the battery pack according to claim 13.

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