Positive electrode sheet and its design and performance evaluation method, electrode assembly, secondary battery

Abstract

This invention provides a novel and feasible method for designing cathode sheets, improving the rationality of cathode sheet design and material utilization, providing reliable support for material selection, saving resources required for cathode sheet verification, improving development efficiency, and being suitable for application to the actual manufacturing design of secondary batteries. [Solution] The present invention belongs to the field of power battery technology and specifically discloses a positive electrode sheet and its design and performance evaluation method, an electrode assembly, and a secondary battery. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, and the positive electrode active material layer satisfies the following relational expression. JPEG2026111529000014.jpg16169

Description

[Technical Field]

[0001] This invention relates to the technology of power batteries, and more particularly to positive electrode sheets, their design and performance evaluation methods, electrode assemblies, and secondary batteries. [Background technology]

[0002] In the manufacturing process of lithium-ion batteries, the pressure density of the positive electrode sheet significantly affects battery performance. Generally, pressure density is closely related to the specific capacity, efficiency, internal resistance, and battery cycle characteristics of the electrode sheet, making it crucial for battery design to find the optimal pressure density. Under normal circumstances, within the compression range that the material can tolerate, a higher pressure density of the electrode sheet allows for a higher battery capacity, and therefore, pressure density is also considered one of the reference indicators of the material's energy density. However, simply pursuing high density is not the optimal strategy. This is because it not only fails to guarantee an improvement in the battery's specific capacity, but can also severely degrade both the specific capacity and cycle characteristics of the battery. As pressure density increases, the compressive force between material particles strengthens, reducing the porosity of the electrode sheet, increasing processing difficulty, and decreasing the yield rate. Under these conditions, the electrode sheet's ability to absorb electrolyte decreases, resulting in insufficient electrolyte penetration and difficulty in adequate electrolyte infiltration. This insufficient penetration directly affects the material's specific capacity and reduces the battery's electrolyte retention capacity. At the same time, during the battery's circulation process, insufficient electrolyte penetration leads to more severe polarization, accelerating battery degradation and resulting in a more pronounced increase in internal resistance. Therefore, the rationality of the positive electrode sheet's pressure density has a significant impact on the processing and electrochemical performance of the finished secondary battery.

[0003] At present, numerous parameters influence the pressure density of the positive electrode sheet, such as the true density, morphology, and particle size distribution of the positive electrode material, as well as process parameters for preparing the positive electrode sheet, such as the surface density of the electrode sheet and the amount of binder and conductive agent used. However, because there are so many parameters influencing the pressure density of the electrode sheet, product design can only be achieved through numerous experiments to obtain a suitable pressure density value, leading to resource waste and low efficiency. [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] In view of the shortcomings of the prior art described above, the object of the present invention is to provide a positive electrode sheet, a method for designing and evaluating its performance, an electrode assembly, and a secondary battery. By analyzing the relationship between parameters such as material-level compacted powder density, powder particle size, electrode sheet elongation rate, and slurry viscosity and the electrode sheet pressure density, a correspondence formula is obtained that allows for direct conversion from compacted powder density to electrode sheet pressure density. This provides a positive electrode sheet with a rational positive electrode active material layer pressure density, solves the technical problem of battery electrochemical performance degradation caused by an unreasonable positive electrode sheet pressure density design, provides a rational, simple, and effective design method for the positive electrode active material layer pressure density of the positive electrode sheet, saves resources required for electrode sheet verification, and improves development efficiency. [Means for solving the problem]

[0005] To achieve the above-mentioned objectives and other related objectives, the present invention provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, wherein the positive electrode active material layer satisfies the following relational expression. JPEG2026111529000001.jpg8169In formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D v50 This indicates that the powder particle size of 50% of the active material in the positive electrode active material layer is less than or equal to this value, expressed in μm. LT is the longitudinal elongation of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s.

[0006] Furthermore, the FD is 2.4 g / cc to 2.6 g / cc.

[0007] Furthermore, D v50 The size is between 0.6 μm and 1.2 μm.

[0008] Furthermore, LT ≤ 1%.

[0009] Furthermore, μ is between 3 kPa·s and 10 kPa·s.

[0010] The present invention also provides a method for designing a positive electrode sheet. The design method for the positive electrode sheet is: The process involves preparing an active material into a slurry, applying it to at least one side of a positive electrode current collector, drying it, rolling it to form a positive electrode sheet, and measuring and calculating the current pressure density of the positive electrode active material layer on the positive electrode sheet. The steps include: calculating the target pressure density of the positive electrode active material layer on the positive electrode sheet according to the following relational expression, based on the compacted powder density and powder particle size of the active material, the viscosity of the slurry, and the longitudinal elongation rate of the positive electrode sheet under a pressure of 15 tons; Based on the result of comparing the current pressure density of the positive electrode active material layer with the target pressure density, the preparation process parameters of the positive electrode sheet are controlled until the current pressure density of the positive electrode active material layer reaches 97% to 100% of the target pressure density. It includes at least [this]. JPEG2026111529000002.jpg7169 (in the formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D v50It indicates that 50% of the powder particle size of the active material in the positive electrode active material layer is below this value, expressed in μm, LT is the longitudinal elongation rate of the positive electrode sheet under a pressure of 15 tons, expressed as a percentage, μ is the viscosity of the slurry used for preparing the positive electrode active material layer, expressed in kPa·s.)

[0011] Based on the result of comparing the current tap density and the target tap density of the positive electrode active material layer, the preparation process parameters of the positive electrode sheet are controlled until the current tap density of the positive electrode active material layer reaches 97% - 100% of the target tap density.

[0012] Furthermore, the design method of the positive electrode sheet further includes the step of selecting an active material according to preset requirements of tap powder density and / or preset powder particle size.

[0013] Furthermore, the design method of the positive electrode sheet further includes the step of preparing the active material into a slurry according to preset viscosity requirements.

[0014] Furthermore, the design method of the positive electrode sheet further includes the steps of preparing the active material into the slurry, coating it on at least one side of the positive electrode current collector, drying it, and performing roll pressing to obtain the positive electrode sheet according to preset requirements of longitudinal elongation rate.

[0015] Furthermore, the design method of the positive electrode sheet compares the current longitudinal elongation rate of the positive electrode sheet under a pressure of 15 tons with a preset longitudinal elongation rate, and based on the comparison result, controls the preparation process parameters of the positive electrode sheet until the current tap density of the positive electrode active material layer reaches 97% - 100% of the target tap density.

[0016] Furthermore, the preparation process parameters of the positive electrode sheet include at least one of roll pressing pressure, roll pressing thickness, and coating surface density.

[0017] The present invention also provides an electrode assembly, which includes the positive electrode sheet or a positive electrode sheet designed by the method described above.

[0018] The present invention also provides a secondary battery, which includes the electrode assembly described above.

[0019] The present invention further provides a method for evaluating the performance of a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector. The method for evaluating the performance of a positive electrode sheet comprises the steps of: calculating the theoretical pressure density (abbreviated as theoretical PD) of the positive electrode active material layer based on the following relational expression; measuring and calculating the current pressure density of the positive electrode active material layer on the positive electrode sheet; determining that the performance of the positive electrode sheet is excellent if the current pressure density reaches 97% to 100% of the theoretical pressure density, and determining that the performance of the positive electrode sheet is poor if the current pressure density does not reach or exceeds 97% to 100% of the theoretical pressure density. It includes at least [this]. JPEG2026111529000003.jpg8169In formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D v50 This indicates that the particle size of 50% of the active material in the positive electrode active material layer is less than or equal to the value of μm, and is expressed in μm. LT is the longitudinal elongation of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s. [Effects of the Invention]

[0020] As described above, the positive electrode sheet, its design and performance evaluation method, electrode assembly, and secondary battery of the present invention have the following advantageous effects. This invention provides a positive electrode sheet that, for any active material and positive electrode active material slurry, exhibits balanced performance in terms of flexibility, fracture and peel strength, etc., has high processability and high safety, improves permeability to the electrolyte, and enables secondary batteries to have good electrochemical performance. This invention analyzes the relationship between the material-level compacted powder density and the electrode sheet pressure density, thereby providing FD, D v50 , we obtain a correspondence formula that can directly convert LT and μ to PD, that is, JPEG2026111529000004.jpg7169

[0021] Based on this formula, the optimal PD value for a positive electrode sheet exhibiting balanced performance in terms of flexibility, fracture and peel strength can be predicted in advance. This allows for rapid adjustment of the electrode sheet preparation process, effectively improving development, verification, and mass production efficiency. [Modes for carrying out the invention]

[0022] The embodiments of the present invention will be described below with specific examples, and those skilled in the art will readily understand the other advantages and effects of the present invention from the contents disclosed herein. The present invention can also be implemented or applied in other different specific embodiments, and the details of each item herein can also be modified or changed in various ways based on different perspectives and applications without departing from the spirit of the invention.

[0023] In this invention, unless otherwise stated, the term "plural" means two or more.

[0024] The character " / " indicates that the preceding and following elements are in a kind of "or" relationship. For example, A / B means A or B.

[0025] The term "and / or" describes the relationship between two things, indicating that there can be three types of relationships. For example, A and / or B represents these three types of relationships: A or B, or A and B.

[0026] In the process of exploring rational design methods for lithium-ion battery cathode sheet compaction, the inventors of this application discovered that the density of the cathode active material powder compaction has a relatively large influence on the electrode sheet pressure density, and that a certain correspondence exists between the two. However, because there are too many parameters that affect the electrode sheet pressure density, a direct correspondence from the compaction density to the electrode sheet pressure density is not possible, and even at this stage, there is no simple and effective technical method for directly converting from the compaction density to the electrode sheet pressure density. To solve this problem, the inventors of this application conducted an analytical study on related factors that affect the electrode sheet pressure density and discovered that, in addition to the compaction density, the parameters that have a relatively large influence on the cathode sheet pressure density are the material particle size, the electrode sheet elongation rate, and the slurry viscosity. Based on these research results, the inventors of this application provide the following technical method.

[0027] One embodiment of the present invention provides a positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, wherein the positive electrode active material layer satisfies the following relation (1). JPEG2026111529000005.jpg16169In formula (1), PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D v50 This indicates that the powder particle size of 50% of the active material in the positive electrode active material layer is less than or equal to this value, expressed in μm. LT is the longitudinal elongation of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s.

[0028] A positive electrode sheet that satisfies this relationship can exhibit a balanced expression in terms of flexibility, fracture and peel strength, and also possesses high processability and high safety, as well as improved permeability to the electrolyte. As a result, the secondary battery exhibits good cycle characteristics and improves battery performance.

[0029] It is necessary to explain that the compaction density of an electrode sheet refers to the ratio of surface density to material thickness, and its calculation formula is compaction density = surface density / material thickness. In lithium-ion battery positive electrode sheet design, the positive electrode current collector generally uses pure metal foil material or polymer composite metal foil material, and considering the roll rolling of the electrode sheet, stretching occurs in the metal foil material. Therefore, the PD in this invention, i.e., the compaction density of the positive electrode active material layer, can be calculated according to the following formula.

[0030]

number

[0031] Compaction density refers to the ratio of the mass of a powder sample after compression under constant pressure to its occupied volume, and is the volume ratio of the powder mass to the powder volume after pressure. In this invention, the method for measuring the compaction density of FD, i.e., the compaction density of the active material in the positive electrode active material layer, can be carried out in accordance with GB / T 44330-2024 "Measurement of Compaction Density of Lithium-ion Battery Positive Electrode Material".

[0032] In the preparation process of the positive electrode sheet, a certain degree of ductility is given through a stretching method such as roll rolling to adapt to the expansion and contraction during the charge-discharge process of the positive electrode material. The stretch rate refers to the ability of the electrode sheet to deform when subjected to external force without bursting. The electrode sheet stretch rate can be calculated by measuring the change in electrode sheet length before and after roll rolling, or by testing the change in electrode sheet mass before and after roll rolling. In this invention, LT, that is, the longitudinal stretch rate of the positive electrode sheet under a 15-ton pressure, is the ratio of the change in electrode sheet length after roll rolling to its original length, and can be specifically calculated according to the following formula.

[0033]

number

[0034] In some embodiments, FD is 2.4 g / cc to 2.6 g / cc, D v50 is 0.6 μm to 1.2 μm, LT ≤ 1%, and μ is 3 kPa·s to 10 kPa·s. In the process of designing the positive electrode sheet, by controlling FD, D v50 , LT and μ within the above-mentioned value ranges, the tap density PD of the obtained electrode sheet can be maintained within a reasonable range, and the improvement effects of the performance such as the flexibility, breaking and peeling force of the electrode sheet are optimized.

[0035] Another embodiment of the present invention provides a method for designing a positive electrode sheet. The method for designing a positive electrode sheet includes: preparing an active material into a slurry, coating it on at least one side of a positive electrode current collector, drying it, and performing roll pressing to form a positive electrode sheet, and measuring and calculating the current tap density of the positive electrode active material layer on the positive electrode sheet; calculating the target tap density of the positive electrode active material layer on the positive electrode sheet according to the following relational expression (2) based on the tap powder density and powder particle size of the active material, the viscosity of the slurry, and the longitudinal elongation rate of the positive electrode sheet under a pressure of 15 tons; JPEG2026111529000008.jpg9169 including at least the following steps.

[0036] Based on the result of comparing the current tap density and the target tap density of the positive electrode active material layer, controlling the preparation process parameters of the positive electrode sheet until the current tap density of the positive electrode active material layer reaches 97% to 100% of the target tap density, that is, making the current tap density of the positive electrode active material layer equal to the target tap density, or making the difference value between the current tap density and the target tap density of the positive electrode active material layer not less than 3% of the target tap density.

[0037] In the process of preparing the positive electrode sheet, FD, D <头 v50By collecting LT and μ data and predicting the theoretical maximum PD value in advance based on the above-mentioned relational equation (2), the electrode sheet preparation process is adjusted to make the actual PD equivalent to the theoretical PD. As a result, the resulting cathode sheet exhibits balanced performance in terms of flexibility, fracture and peeling strength, has high processability and high safety, improves permeability to the electrolyte, and ensures that the secondary battery has good electrochemical performance.

[0038] In some embodiments, the design method for the positive electrode sheet further includes the step of selecting an active material according to a predetermined compaction density and / or predetermined powder particle size requirement. For example, the predetermined compaction density requirement is 2.4 g / cc ≤ FD ≤ 2.6 g / cc, and the predetermined powder particle size requirement is 0.6 μm ≤ D v50 The size is ≤ 1.2 μm.

[0039] In some embodiments, the design method for the positive electrode sheet further includes the step of preparing the active material into a slurry according to a predetermined viscosity requirement. Exemplarily, the predetermined viscosity requirement is 3 kPa·s ≤ μ ≤ 10 kPa·s.

[0040] In some embodiments, the design method for the positive electrode sheet further includes the steps of preparing an active material into a slurry, applying it to at least one side of a positive electrode current collector, drying it according to a predetermined longitudinal elongation requirement, and rolling it into a positive electrode sheet by roll rolling. Furthermore, in some embodiments, the design method for the positive electrode sheet further includes the step of controlling the preparation process parameters of the positive electrode sheet, based on a comparison of the current longitudinal elongation of the positive electrode sheet under a 15-ton pressure with a predetermined longitudinal elongation, until the current pressure density of the positive electrode active material layer reaches 97% to 100% of the target pressure density. Exemplarily, the predetermined longitudinal elongation requirement is LT ≤ 1%.

[0041] In some embodiments, the preparation process parameters for the positive electrode sheet are: roll rolling pressure, This includes at least one of the roll rolling thickness and the coating surface density. These parameters have a relatively important influence on the cathode sheet preparation process and the performance of the cathode sheet obtained therefrom, and therefore require careful attention during the cathode sheet design process and can be controlled according to actual demands. Another embodiment of the present invention provides an electrode assembly comprising a positive electrode sheet described above, or a positive electrode sheet designed by the method described above.

[0042] Another embodiment of the present invention provides a secondary battery including the electrode assembly described above.

[0043] Another embodiment of the present invention provides a method for evaluating the performance of a positive electrode sheet including a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, the method for evaluating the performance of a positive electrode sheet comprising at least the steps of: calculating the theoretical pressure density of the positive electrode active material layer according to the above-described relation (2); measuring and calculating the current pressure density of the positive electrode active material layer on the positive electrode sheet; determining the performance of the positive electrode sheet as excellent if the current pressure density reaches 97% to 100% of the theoretical pressure density, and determining the performance of the positive electrode sheet as poor if the current pressure density does not reach or exceeds 97% to 100% of the theoretical pressure density.

[0044] In some embodiments, the positive electrode sheet is processed by a coating method, for example, by uniformly coating a slurry onto a positive electrode current collector and then preparing it as a positive electrode sheet through processes such as firing, roll rolling, and slitting. In other embodiments, the positive electrode sheet can also be obtained by selecting any other method for forming the positive electrode sheet.

[0045] In some embodiments, the slurry used to prepare the positive electrode sheet described above contains a positive electrode active material, a dispersant, a binder, a conductive agent, and a solvent, and the mass ratio of the positive electrode active material, dispersant, binder, and conductive agent can be, for example, 85-98:0.25-5:0.25-5:0.5-5, and the slurry solid content concentration can be, for example, 65% ± 2%, and can be specifically adjusted according to the differences in the selected materials. Among these, the positive electrode active material can be selected from lithium iron phosphate-based materials, ternary materials, lithium cobalt oxide, etc., for example lithium nickel manganese oxide, lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide. Furthermore, in some embodiments, the positive electrode active material is selected from lithium iron phosphate-based materials, and its chemical formula is Li x Fe y M 1-yIt is PO4, where M is selected from at least one of Mn, Ni, Cr, Zr, Ca, Mg, Cu, Ti, Al, Mo, W, Zn, etc., 0.9 < x < 1.1, 0.65 ≤ y ≤ 1.0, which improves the energy density and cycle life of the lithium-ion secondary battery. The dispersant is selected from, for example, polyvinylpyrrolidone (PVP), polyethylene glycol (PEG), polymer block copolymers containing acidic groups, sodium salt polymers, etc. Among them, the polymer block copolymer containing the acidic group can be, for example, styrene-maleic anhydride copolymer, acrylic acid polymer, etc. The binder is selected from, for example, polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexafluoropropylene, or polymerized styrene butadiene rubber (SBR), etc., any one or more of them. The conductive agent is selected from, for example, any one or more of conductive carbon black (Super P), acetylene black, carbon nanotubes, and graphene, etc., or other compounds that can enhance conductivity. The solvent is selected from, for example, N-methyl-2-pyrrolidone (NMP), etc.

[0046] In some embodiments, the positive electrode current collector is, for example, a metal foil material such as nickel, titanium, aluminum, silver, stainless steel, etc., a metal foil material coated with a functional layer such as a carbon layer on the surface, or a polymer composite metal foil material. Among them, the thickness of the positive electrode current collector is, for example, 8 μm to 15 μm, and the single-sided coating thickness of the positive electrode active material layer is, for example, 20 μm to 100 μm. Further, in a specific embodiment, the positive electrode current collector is, for example, a carbon-coated aluminum foil.

[0047] In some embodiments, the secondary battery is a lithium-ion battery and includes an electrolyte, an outer casing, and bare cores installed inside the outer casing, the bare cores including a positive electrode sheet as described above, and further including a separator and a negative electrode sheet.

[0048] In one specific embodiment, the method for preparing the lithium-ion battery includes the following steps: A positive electrode sheet, a separator, and a negative electrode sheet are sequentially stacked, ensuring that separators are evenly distributed between any positive and negative electrode sheets; a multilayer laminate is obtained through winding, and the bare core is inserted into the battery casing; and finally, electrolyte is injected into the casing in one or more steps, completely immersing the bare core in the electrolyte. In other words, the electrolyte is injected to fill the entire internal space of the battery, and the positive electrode sheet, separator, and negative electrode sheet are completely immersed in the electrolyte.

[0049] In some embodiments, the above-described embodiment and / or the negative electrode sheet in the embodiment includes a negative electrode current collector and a negative electrode active material layer on at least one side of the negative electrode current collector. Among these, the negative electrode current collector is selected from any one of the following: for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector, the thickness of the negative electrode current collector is, for example, 8 μm to 15 μm, and the coating thickness on one side of the negative electrode active material layer is, for example, 10 μm to 80 μm. In one specific embodiment, for example, copper foil is selected as the negative electrode current collector, the thickness of the copper foil is, for example, 13 μm, and the coating thickness on one side of the negative electrode active material layer is 50 μm.

[0050] In some embodiments, the negative electrode active material layer includes a negative electrode active material, a conductive agent, a binder, and a thickener, and the mass ratio of the negative electrode active material, conductive agent, binder, and thickener can be, for example, 90-96:1-2:1-3:2-5. Among these, the negative electrode active material is a compound capable of lithium ion insertion and removal, such as graphite, soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide (SiO₂). x, (0 < x < 2), and contains at least one of silicon carbon compounds or lithium titanate. The conductive agent is selected from any one or more of, for example, conductive carbon black, acetylene black, ketjen black, carbon nanotubes, graphene, etc. The binder is selected from any one or more of, for example, polyvinylidene fluoride, polyethylene oxide, polyamide, polypropylene, polyacrylate, polyvinyl ether, polymethyl methacrylate, polyhexafluoropropylene, or styrene butadiene rubber, etc. The thickener is selected, for example, as sodium carboxymethyl cellulose (Carboxymethyl Cellulose Sodium, CMC-Na), etc.

[0051] In one specific embodiment, the negative electrode active material is selected from, for example, graphite, the conductive agent is selected from, for example, acetylene black, the binder is selected from, for example, styrene butadiene rubber, and the thickener is selected from, for example, sodium carboxymethyl cellulose. The negative electrode active material, conductive agent, binder, and thickener are mixed, for example, according to a mass ratio of 96:2:1:1, deionized water is added, and they are uniformly mixed under the action of a vacuum stirrer to obtain a negative electrode slurry. The negative electrode slurry is applied onto a copper foil, then dried at room temperature and then transferred to an oven for drying, and after passing through processes such as cold rolling and slitting, a negative electrode sheet is obtained. In other embodiments, the negative electrode sheet can also be obtained by selecting any other method for forming the negative electrode sheet.

[0052] In some embodiments, the electrolyte contains an electrolyte lithium salt, a solvent, and additives. In secondary batteries, the electrolyte plays roles such as ion conduction, provision of ion channels, and maintenance of chemical stability. The various components in the electrolyte can be classified into electrolyte lithium salt, solvent, and additives according to their function and amount used. Lithium salts are mainly used to provide lithium ions and form ion channels. In the overall electrochemical system of the battery, the orientation transfer of lithium ions and electrons generates electricity, and lithium salts have a relatively large influence on aspects such as the energy density, power density, wide electrochemical window, cycle life, and safety performance of lithium batteries. Solvents are used to dissolve the lithium salt and additives therein. Additives are substances added in small amounts to the electrolyte, and there are many types, each performing different functions. For example, they can provide different improvement effects on the high and low temperature performance, cycle characteristics, and film formation performance of the battery.

[0053] In some embodiments, the electrolyte lithium salt is selected from at least one of lithium hexafluoride phosphate, bis(fluorosulfonyl)imide lithium, bis(trifluoromethyl)sulfonylimide lithium, lithium acetate, lithium methylsulfonate, or lithium trifluoromethylsulfonate, and the mass percentage of the electrolyte lithium salt is 12% to 16% based on the total mass of the electrolyte. The present invention does not limit the type of lithium salt, and a single lithium salt or a mixture of lithium salts can be used.

[0054] In some embodiments, the solvent is selected from at least one of a carbonate ester solvent, a carboxylic acid ester solvent, an ether solvent, or a nitrile solvent, and the mass percentage of the solvent is 70% to 80% based on the total mass of the electrolyte. Among these, carbonate ester solvents are selected from, for example, ethylene carbonate, vinylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, fluorinated ethylene carbonate, polycarbonate, etc., carboxylic acid ester solvents are selected from, for example, ethyl formate, ethyl acetate, propyl acetate, ethyl propionate, propyl propionate, etc., ether solvents are selected from, for example, ethylene glycol dimethyl ether, diethylene glycol diethyl ether, etc., and nitrile solvents are selected from, for example, acetonitrile, propionitrile, butyronitrile, valeronitrile, etc. The present invention does not limit the type of solvent, and a single solvent or a mixture of solvents can be used.

[0055] In some examples, the additive is selected from at least one of vinylylene carbonate (VC), methylene methanedisulfonate (MMDS), propylene sulfite (PS), ethylene sulfate (1,3,2-dioxathiolane 2,2-dioxide, DTD), and ethylene sulfite (ES), and the mass percentage of the additive is 0.1% to 5% based on the total mass of the electrolyte. The present invention does not limit the type of additive, and a single additive or a mixture of additives can be used.

[0056] In some embodiments, the separator is, for example, a polyethylene (PE) membrane, a polypropylene (PP) membrane, a glass fiber membrane, or a composite membrane, and the thickness of the separator is, for example, 9 μm to 15 μm.

[0057] The present invention will be described in detail below with reference to specific embodiments. It should also be understood that the following embodiments are merely for illustrative purposes and should not be considered as limitations on the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above-described aspects of the present invention are all within the scope of protection. The specific process parameters, etc., in the following examples are merely examples within an appropriate range; that is, those skilled in the art can select within an appropriate range as described herein, and are not limited to the specific numerical values ​​in the following examples.

[0058] Example 1 The method for preparing the positive electrode sheet in this embodiment includes the following steps: S1. Preparation of positive electrode slurry: The positive electrode active material is FD at 2.53 g / cc, D v50 A 0.81 μm lithium iron phosphate (LFP) was selected as the binder, PVDF as the binder, conductive carbon black as the conductive agent, and a styrene-maleic anhydride copolymer solution as the dispersant. First, the LFP, PVDF, and conductive agent were added and dry-mixed for 35 minutes to uniformly disperse them. Next, the solvent NMP was added and mixed and stirred for 40 minutes. Then the dispersant was added and stirred for a further 200 minutes to uniformly disperse the slurry. The amount of NMP used was adjusted to maintain the final viscosity of the slurry at 6 kPa·s and the slurry solid content concentration at 65%. S2. Coated surface density 241.9mg / 1540.25mm 2 Select a suitable material and set the design capacity to 80 Ah. The slurry prepared in step S1 is applied onto 13 μm carbon-coated aluminum foil, and then subjected to firing, drying, and roll rolling to form a positive electrode sheet. The cathode sheet prepared in this embodiment has an LT of 0.8%, a theoretical PD of 2.59 g / cc, and an actual PD of 2.59 g / cc.

[0059] According to the method of Example 1, FD, Dv based on Table 1 50 The solid content concentration, μ, LT, and aluminum foil thickness were adjusted to prepare positive electrode sheets corresponding to Examples 2-9 and Comparative Examples 1-9, respectively.

[0060] The positive electrode sheets from Examples 1-9 and Comparative Examples 1-9 were assembled into a lithium-ion battery, the negative electrode was a combination of graphite, the separator was a PE separator, the electrolyte was a carbonate ester-based electrolyte, the lithium salt was LiPF6, and the rated capacity was 80Ah. Performance tests of the positive electrode sheets and lithium-ion batteries were conducted according to the following method, and the test results are shown in Table 2.

[0061] Test method: 1. Flexibility test: One positive electrode sheet after roll rolling, with an area of ​​20 cm². 2 Cut the material into circular pieces, fold them once, and then fold them again in the opposite direction along the crease. Repeat this process multiple times until the circular pieces exhibit a phenomenon of intermediate light transmission. 2. Tests for capacity and presence of lithium deposition: First, the batteries are discharged to 2.0V using a constant current discharge at 0.33C, then charged to 3.65V using a constant current charge at 0.33C, followed by constant voltage charging to 0.05C, and finally discharged again to 2.0V using a constant current discharge at 0.33C. The discharge capacity is recorded. The initial charge / discharge temperature is set to 25°C, and 25 battery cells are tested in each group to calculate the average value. After the test is complete, five battery cells are selected, disassembled, and observed for the presence or absence of lithium deposition. 3. Peeling force test: One end of a double-sided tape measuring approximately 10 cm x 2 cm is attached to a stainless steel plate along the marked line. The prepared sample is then attached to the other end of the double-sided tape in the order indicated, and the roller is rolled once along the direction of the electrode sheet. The roll rolling parameters are a pressure of 50 N and a time of 3 s. The sample to be measured is placed on a measurement platform (universal tensile testing machine), fixed in place, and the test is started. Relatively stable data is extracted and the average value is displayed. The peeling force is calculated as the selected average value (mN) / sampling width (mm), and the average value is obtained after 10 tests.

[0062] [Table 1]

[0063] [Table 2]

[0064] Based on the data in Tables 1 and 2, the following conclusions can be drawn:

[0065] In Examples 1-9, the actual PD of the positive electrode sheets was slightly smaller than the theoretical PD and more than 97% of the theoretical PD, or equal to the theoretical PD. However, the actual PD of the positive electrode sheet in Comparative Example 8 was larger than the theoretical PD and exceeded the reasonable range, leading to an increase in the number of electrode sheet fractures, a decrease in battery capacity, and furthermore, lithium deposition, causing deterioration of battery performance.

[0066] Further analysis of Examples 1-9 and Comparative Examples 1-7 and 9 revealed that the positive electrode sheet PD was FD, D v50 , affected by μ, LT, and FD, D v50 μ and LT and PD were found to have a positive correlation, and FD and D v50 Controlling μ and LT within a certain range is advantageous for improving the performance of electrode sheets and batteries.

[0067] Comparing Examples 1-3 with Comparative Examples 1-2, it can be seen that as the FD of the LFP material increases, the PD of the positive electrode sheet also improves. However, if the FD is too low, the peeling force of the electrode sheet decreases, and if the FD is too high, the flexibility is poor and it breaks frequently, leading to increased processing difficulty and a higher waste rate. Furthermore, it leads to a decrease in battery capacity and the occurrence of lithium deposition, resulting in deterioration of battery performance. Therefore, the optimal range for FD is 2.4-2.6 g / cc.

[0068] Comparing Examples 1, 4, and 5 with Comparative Examples 3-4, the D of the LFP material v50 As the increase in D increases, it can be seen that the PD of the positive electrode sheet also improves, v50 If it is too small, the peeling force of the electrode sheet will be low, D v50 If it is too high, the battery capacity will decrease. Therefore, D v50 The optimal range is 0.6 to 1.2 μm.

[0069] Comparing Examples 1, 6, and 7 with Comparative Examples 5-6, it can be seen that there is a positive correlation between the solid content concentration and viscosity of the slurry, and that the PD of the positive electrode sheet also improves with increasing slurry viscosity. However, if the viscosity is too low, the peeling force of the electrode sheet decreases, and if the viscosity is too high, the dispersion of the slurry becomes uneven, increasing the difficulty of processing. Therefore, the optimal viscosity range is 3-10 kPa·s.

[0070] Comparing Examples 1, 8, and 9 with Comparative Example 7, it can be seen that there is a negative correlation between LT and aluminum foil thickness, and that the PD of the positive electrode sheet also improves as LT increases. However, when LT exceeds 1%, the flexibility of the electrode sheet deteriorates, and it breaks frequently, leading to increased processing difficulty and a higher waste rate. Therefore, the optimal range for LT is ≤1%.

[0071] The embodiments described above are illustrative in illustrating the principles and effects of the present invention and do not limit the invention. Anyone familiar with this art may modify or change the embodiments described above without departing from the spirit and scope of the invention. Accordingly, all equivalent modifications or changes completed by a person with ordinary skill in the art without departing from the spirit and technical idea disclosed by the invention are still encompassed by the claims of the invention. [Industrial applicability]

[0072] The present invention relates to the technology of power batteries, and more particularly to positive electrode sheets, their design and performance evaluation methods, electrode assemblies, and secondary batteries.

Claims

1. A positive electrode sheet comprising a positive electrode current collector and a positive electrode active material layer on at least one side of the positive electrode current collector, wherein the positive electrode active material layer satisfies the following relational expression: Positive electrode sheet. (In the formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D ｖ５０ This indicates that the powder particle size of 50% of the active material in the positive electrode active material layer is less than or equal to this value, and is expressed in μm. LT is the longitudinal elongation rate of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s.

2. The aforementioned FD is 2.4 g / cc to 2.6 g / cc, and / or The aforementioned D ｖ５０ The positive electrode sheet according to claim 1, characterized in that the thickness is 0.6 μm to 1.2 μm.

3. The aforementioned LT ≤ 1%, and / or The positive electrode sheet according to claim 1, characterized in that the μ is 3 kPa·s to 10 kPa·s.

4. A method for designing a positive electrode sheet, The process involves preparing an active material into a slurry, applying it to at least one side of a positive electrode current collector, drying it, rolling it to form a positive electrode sheet, and measuring and calculating the current pressure density of the positive electrode active material layer on the positive electrode sheet. The steps include: calculating the target pressure density of the positive electrode active material layer on the positive electrode sheet according to the following relational expression based on the compacted powder density and powder particle size of the active material, the viscosity of the slurry, and the longitudinal elongation rate of the positive electrode sheet under a pressure of 15 tons; Based on the result of comparing the current pressure density of the positive electrode active material layer with the target pressure density, the preparation process parameters of the positive electrode sheet are controlled until the current pressure density of the positive electrode active material layer reaches 97% to 100% of the target pressure density. A method for designing a positive electrode sheet, characterized by including at least the following. (In the formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D ｖ５０ This indicates that the powder particle size of 50% of the active material in the positive electrode active material layer is less than or equal to this value, and is expressed in μm. LT is the longitudinal elongation rate of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s.

5. A step of selecting an active material according to pre-set requirements for compacted powder density and / or pre-set powder particle size, and / or, the step of preparing the active material into a slurry according to a predetermined viscosity requirement, and / or, the steps of preparing the active material into a slurry, applying it to at least one side of the positive electrode current collector, drying it according to a predetermined longitudinal elongation requirement, and rolling it to form the positive electrode sheet, The method for designing a positive electrode sheet according to claim 4, further comprising:

6. Based on a comparison of the current longitudinal elongation rate of the positive electrode sheet under a 15-ton pressure with a preset longitudinal elongation rate, the preparation process parameters of the positive electrode sheet are controlled until the current pressure density of the positive electrode active material layer reaches 97% to 100% of the target pressure density. The method for designing a positive electrode sheet according to claim 4, further comprising:

7. The preparation process parameters for the positive electrode sheet are: Roll rolling pressure, Roll rolling thickness, and Coating surface density Includes at least one of the following A method for designing a positive electrode sheet according to any one of claims 4 to 6, characterized in that

8. An electrode assembly characterized by comprising a positive electrode sheet according to any one of claims 1 to 3, or a positive electrode sheet designed by the method according to any one of claims 4 to 7.

9. A secondary battery characterized by including the electrode assembly described in claim 8.

10. A method for evaluating the performance of a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer located on at least one side of the positive electrode current collector, The steps include: calculating the theoretical pressure density of the positive electrode active material layer based on the following relational equation, The steps include: measuring and calculating the current pressure density of the positive electrode active material layer on the positive electrode sheet; The steps include: determining that the performance of the positive electrode sheet is excellent if the current pressure density reaches 97% to 100% of the theoretical pressure density, and determining that the performance of the positive electrode sheet is poor if the current pressure density does not reach or exceeds 97% to 100% of the theoretical pressure density; A method for evaluating the performance of a positive electrode sheet, characterized by comprising at least [a certain element]. (In the formula, PD is the pressure density of the positive electrode active material layer, expressed in g / cc. FD is the density of compacted active material in the positive electrode active material layer, expressed in g / cc. D ｖ５０ This indicates that the powder particle size of 50% of the active material in the positive electrode active material layer is less than or equal to this value, and is expressed in μm. LT is the longitudinal elongation rate of the positive electrode sheet under a 15-ton pressure, expressed as a percentage. μ is the viscosity of the slurry used to prepare the positive electrode active material layer, and is expressed in kPa·s.

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