A secondary battery and a method for preparing the same

Abstract

The present invention provides a secondary battery, comprising a pole piece, a diaphragm and an electrolyte, wherein the pole piece comprises a current collector and an active material layer arranged on at least one side of the current collector, wherein the active material layer comprises an active material, a conductive agent and a binder; the effective ion migration rate of the pole piece is µ, and the value range of µ is 0.02~0.5; the present invention prepares the pole piece under high solid content, and then freeze-dries or supercritically dries it, so that when the solvent in the pole piece leaves the pole piece, the binder does not migrate with the solvent, thereby maintaining a uniform distribution of the binder in the pole piece; at the same time, when the pole piece has a higher surface density and a higher compaction density, the distribution of the pore structure in the pole piece is maintained, and due to the special process effect of freeze-drying or supercritical drying, it is possible to maintain the high pore structure of the pole piece, or to create pores to further improve the pores in the pole piece, thereby improving the migration rate of ions in the electrode, so that the battery has a higher energy density and better electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a secondary battery and a preparation method thereof. Background Art

[0002] With the dramatic increase in global demand for sustainable energy, secondary batteries, as key components of energy storage solutions, have become a key focus of research and development, with performance optimization and cost control becoming key. Existing secondary battery slurry preparation typically utilizes a wet process, requiring large amounts of solvents, such as deionized water, for stirring and dispersion, followed by drying to remove the solvent. This process not only consumes significant energy but also volatilizes easily during drying, causing the binder to float upward, leading to uneven binder distribution within the electrode. Vertically perpendicular to the electrode, the binder is less at the bottom and more at the top. This uneven binder distribution can lead to several problems: 1. Reduced adhesion between the layer and the current collector; 2. Excessive binder in the upper layer increases electrode impedance, leading to degraded electrochemical performance; and 3. During the rolling process, the layer is prone to sticking to the rollers, making processing difficult.

[0003] To address the above-mentioned problems, dry-process electrode preparation is now commonly used. Although the dry-process electrode preparation process avoids the use of solvents, the only available binder in the solid-phase batching process is PTFE. External high shear force must be applied to the dry mixture to cause it to be fibrillated and then bonded to the electrode powder in a network-like manner. This has poor compatibility with existing processes and equipment. Furthermore, there are problems with uneven dry mixing and insufficient binder content, which can cause film stretching and tearing. PTFE is unstable at low potentials and will undergo an irreversible reaction with lithium. When used in the negative electrode, it will be lithiated and consume active lithium, reducing the bonding effect. The use of PTFE in the lithium iron phosphate system does not result in good fibrillation, and high-performance electrode plates cannot be obtained. Therefore, how to overcome the above-mentioned technical problems and defects has become a key issue that needs to be addressed. Summary of the Invention

[0004] In order to solve the problem of uneven distribution of binder in the preparation process of existing pole pieces, the present invention provides a secondary battery and a preparation method thereof.

[0005] The technical solutions adopted by the present invention to solve the above technical problems are as follows:

[0006] A first aspect of the present invention provides a secondary battery comprising a pole piece, a separator and an electrolyte, wherein the pole piece comprises a current collector and an active material layer arranged on at least one side of the current collector, wherein the active material layer comprises an active material, a conductive agent and a binder; the effective ion migration rate of the pole piece is µ, and the value range of µ is 0.02~0.5.

[0007] Optionally, the method for testing the ion migration rate µ of the electrode comprises the following steps:

[0008] 1) The electrode is fabricated into a symmetrical battery, and an electrochemical impedance spectroscopy test is performed to obtain a Nyquist plot of the electrochemical impedance spectroscopy having a characteristic shape of intersection of a low-frequency region line segment and a high-frequency region line segment;

[0009] 2) Extend the low-frequency line segment in the Nyquist diagram until it intersects the horizontal axis. The difference between this intersection point and the intersection point of the high-frequency line segment and the horizontal axis is recorded as the impedance R, in Ω.

[0010] 3) Substitute R into the following formula , the effective ion migration rate is calculated;

[0011] Where S is the effective reaction area of ​​the electrode in the symmetrical battery, in m 2 , d is the thickness of the active material layer, in μm, and σ is the ionic conductivity of the electrolyte, in S / m.

[0012] Optionally, the density of the active material layer is ρ, and the value range of ρ is 1.2-4.5, and the unit is g / cm 3 ; The density of the active material is ρ A , the ρ A The value range is 2.0-5.2, and the unit is g / cm 3 ; said ρ and said ρ A Satisfies the relationship: 0.55≤ρ / ρ A ≤0.88.

[0013] Optionally, the electrode sheet includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, the positive electrode active material layer is arranged on at least one side of the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, a positive electrode binder and a positive electrode solvent, and the effective ion migration rate of the positive electrode sheet is μ c , the µ c The value range is 0.02~0.5.

[0014] Optionally, the density of the positive electrode active material layer is p C , the ρ C The value range is 2.0-4.5, and the unit is g / cm 3 ; The density of the positive electrode active material is ρ CA , the ρ CA The value range is 2.3-5.2, and the unit is g / cm 3 ; said ρ C With the ρ CA Satisfy the relationship: 0.60≤ρ C / ρ CA ≤0.88.

[0015] Optionally, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganate, lithium nickel cobalt manganate, and lithium nickel cobalt aluminum oxide; one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials;

[0016] Or, one or more of layered sodium cathodes such as sodium ferric sulfate, sodium ferric phosphate, sodium ferric pyrophosphate, sodium ferric pyrophosphate, sodium vanadium phosphate, sodium copper iron manganate, sodium iron nickel manganate, and Prussian blue cathode;

[0017] The positive electrode binder includes one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile and modified polyacrylonitrile, polytetrafluoroethylene and modified polymers thereof;

[0018] The positive electrode conductive agent includes one or more of carbon black, acetylene black, Ketjen black, graphene microspheres, three-dimensional conductive metal organic frameworks, porous spherical carbon, conductive nanotubes, nanofibers, graphene, and graphite microsheets.

[0019] Optionally, the thickness of the positive electrode active material layer is 50um~200um.

[0020] Optionally, the electrode sheet includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is arranged on at least one side of the negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder and a negative electrode solvent, and the effective ion migration rate of the negative electrode sheet is μ A , the µ A The value range is 0.025~0.35.

[0021] Optionally, the density of the negative electrode active material layer is p B , the ρ B The value range is 1.2-2.2, and the unit is g / cm 3 ; The density of the negative electrode active material is ρ BA , the ρ BA The value range is 2.0-3.5, and the unit is g / cm 3 ; said ρ B With the ρ BA Satisfies the relationship: 0.55≤ρ B / ρ BA ≤0.82.

[0022] Optionally, the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon, silicon oxide, silicon carbon, lithium titanate, etc.;

[0023] The negative electrode binder is one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polyethylene, modified polyethylene, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polytetrafluoroethylene and modified polymers thereof, polyacrylonitrile, modified polyacrylonitrile, polymethyl methacrylate and modified polymers thereof;

[0024] The negative electrode conductive agent is one or more of carbon black, acetylene black, Ketjen black, graphene microspheres, three-dimensional conductive metal organic frameworks, porous spherical carbon, conductive nanotubes, nanofibers, graphene, and graphite microsheets;

[0025] Optionally, the thickness of the negative electrode active material layer is 40 μm to 150 μm.

[0026] A second aspect of the present invention provides a method for preparing a secondary battery, wherein the method for preparing a pole piece comprises the following steps:

[0027] 1) mixing an active material, a conductive agent, a binder, and a solvent to obtain an active material layer precursor;

[0028] 2) thinning the active material layer precursor at least once to obtain the active material layer;

[0029] 3) The thinning method in 2) can be screw extrusion thinning or pressing thinning;

[0030] 4) Compounding the active material layer with the current collector;

[0031] 5) After compounding, freeze drying or supercritical drying is performed, and then roller pressing is performed to obtain the electrode sheet.

[0032] Optionally, in step 1), the solvent includes one or more of deionized water, N-methylpyrrolidone, dimethyl sulfoxide, acetic acid, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and the like.

[0033] Optionally, in step 1), the active material layer precursor includes a positive electrode active material layer precursor and a negative electrode active material layer precursor, the solid content of the positive electrode active material layer precursor is X1, the range of X1 is 70%≤X1<100%, and the solid content of the negative electrode active material layer precursor is X2, the range of X2 is 60%≤X2<100%.

[0034] Optionally, in step 5), the freeze-drying conditions include: a temperature lower than the melting point of the solvent, and a pressure vacuum degree of 0.1 Pa to 100 Pa.

[0035] Optionally, in step 5), the supercritical drying conditions include: the type of supercritical fluid used includes any one of carbon dioxide, chlorotrifluoromethane, ethane or ethylene, or a combination of at least two; the temperature is the critical temperature for maintaining the supercritical fluid state; and the pressure is the critical pressure for maintaining the supercritical fluid state.

[0036] According to the electrode provided by the present invention, the electrode is prepared from a mixture of a high-solid content positive electrode or negative electrode, and then subjected to freeze drying or supercritical drying, so that when the solvent in the electrode leaves the electrode, the binder does not migrate with the solvent, thereby maintaining a uniform distribution of the binder in the electrode; at the same time, when the electrode has a higher surface density and a higher compaction density, the distribution of the pore structure in the electrode is maintained. Furthermore, due to the special process effects of freeze drying or supercritical drying, it is possible to maintain the high porosity structure of the electrode, or to form pores and further form pores to improve the pores in the electrode, thereby improving the migration rate of ions in the electrode, so that the battery has a higher energy density and better electrochemical performance. DETAILED DESCRIPTION

[0037] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention is further described in detail below with reference to the embodiments. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work are within the scope of protection of the present invention.

[0038] Unless otherwise specified, the experimental methods used in the following examples are conventional methods; the materials and reagents used are commercially available reagents and materials unless otherwise specified.

[0039] In one embodiment, the first aspect of the present invention provides a secondary battery, including a pole piece, a diaphragm and an electrolyte, the pole piece including a current collector and an active material layer arranged on at least one side of the current collector, the active material layer including an active material, a conductive agent and a binder; the effective ion migration rate of the pole piece is μ, and the value range of μ is 0.02~0.5.

[0040] Specifically, the value range of µ is any point value among 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50, or a range value consisting of any two point values; in a preferred embodiment, the value range of µ is 0.1~0.5.

[0041] When the value range of µ is 0.02-0.5, the effective ion migration rate in the electrode is high, which is conducive to the migration and transmission of ions in the electrode, has better kinetic performance, and can improve the electrochemical performance of the battery. When the value range of µ is less than 0.02, the ion migration in the electrode is greatly hindered, affecting the transmission of ions and thus the electrochemical performance of the battery; when the value range of µ is greater than 0.5, although the effective ion migration rate of the electrode is high and the kinetic performance of the electrode is good, it is necessary to reduce the surface density or compaction density of the active material or further reduce the binder content on the basis of the existing technical solution, which will lead to a decrease in the energy density of the battery.

[0042] The present application prepares an electrode from a mixture of a high-solid content positive electrode or negative electrode, and then freeze-dries or supercritically dries it, so that when the solvent in the electrode leaves the electrode, the binder does not migrate with the solvent, thereby maintaining a uniform distribution of the binder in the electrode; at the same time, when the electrode has a higher surface density and a higher compaction density, the distribution of the pore structure in the electrode is maintained. Furthermore, due to the special process effects of freeze-drying or supercritical drying, it is possible to maintain the high porosity structure of the electrode, or to create pores and further improve the pores in the electrode, thereby increasing the migration rate of ions in the electrode, so that the battery has a higher energy density and better electrochemical performance.

[0043] In one embodiment, the method for testing the ion migration rate μ of the electrode comprises the following steps:

[0044] 1) The electrode is fabricated into a symmetrical battery, and an electrochemical impedance spectroscopy test is performed to obtain a Nyquist plot of the electrochemical impedance spectroscopy having a characteristic shape of intersection of a low-frequency region line segment and a high-frequency region line segment;

[0045] 2) Extend the low-frequency line segment in the Nyquist diagram until it intersects the horizontal axis. The difference between this intersection point and the intersection point of the high-frequency line segment and the horizontal axis is recorded as the impedance R.

[0046] 3) Substitute R into the following formula , the effective ion migration rate is calculated;

[0047] Where S is the effective reaction area of ​​the electrode in the symmetrical battery, in m 2 , d is the thickness of the active material layer, in μm, and σ is the ionic conductivity of the electrolyte, in S / m.

[0048] The present invention measures the µ value by the above method, and uses freeze-drying or supercritical drying methods to allow the solvent in the electrode to leave the electrode in the form of sublimation or following the supercritical fluid, so that the value range of µ is 0.02~0.5, thereby making the effective ion migration rate in the electrode higher, which is beneficial to the migration and transmission of ions in the electrode, has better kinetic performance, and can improve the electrochemical performance of the battery.

[0049] In one embodiment, the density of the active material layer is ρ, and the value range of ρ is 1.2-4.5, and the unit is g / cm 3 ; The density of the active material is ρ A , the ρ A The value range is 2.0-5.2, and the unit is g / cm 3 ; said ρ and said ρ A Satisfy the relationship: 0.55≤ρ / ρ A ≤0.88.

[0050] Specifically, the value range of ρ is 1.2 g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 , 1.9g / cm 3 , 2.0g / cm 3 , 2.1g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 、3.9g / cm 3 , 4.0g / cm 3 , 4.1g / cm3 , 4.2g / cm 3 , 4.3g / cm 3 , 4.4g / cm 3 or 4.5g / cm 3 In a preferred embodiment, the value range of ρ is 1.4 g / cm 3 -4.2g / cm 3 .

[0051] The weight of the active material, conductive agent and binder contained in the active material layer per unit volume is the density of the active material layer; when the value range of ρ is 1.2 g / cm 3 -4.5g / cm 3 , each component in the active material layer can be in close contact, with good electronic and ionic conduction, and the electrode has a higher compaction density, so that the energy density of the battery is high. When the value range of ρ is less than 1.2g / cm 3 When the value of ρ is greater than 4.5 g / cm, the components in the active material layer cannot be in close contact, affecting the electronic conduction between the components. In addition, there are more pores in the active material layer, and more electrolyte needs to be added to fill the pores, which will lead to a decrease in battery energy density and an increase in manufacturing cost. 3 When the active material layer is too close, the components in the active material layer will be in too close contact, which will reduce the pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0052] Specifically, the A The value range is 2.0g / cm 3 , 2.1g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 、3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm3 、3.9g / cm 3 , 4.0g / cm 3 , 4.1g / cm 3 , 4.2g / cm 3 , 4.3g / cm 3 , 4.4g / cm 3 , 4.5g / cm 3 , 4.6g / cm 3 , 4.7g / cm 3 , 4.8g / cm 3 , 4.9g / cm 3 , 5.0g / cm 3 , 5.1g / cm 3 or 5.2g / cm 3 In a preferred embodiment, the ρ A The value range is 2.3g / cm 3 -5.1g / cm 3 .

[0053] The weight of the active substance per effective unit volume is the density of the active substance; when the ρ A The value range is 2.0g / cm 3 -5.2g / cm 3 , the active material itself has a high density, which can achieve a higher compaction density of the electrode. A The value range is less than 1.2g / cm 3 When ρ A The value range is greater than 5.2g / cm 3 When the active material layer is filled with ions, it will cause pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0054] Specifically, ρ / ρ A The value range of is any one of 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87 or 0.88, or a range consisting of any two of the values; in a preferred embodiment, ρ / ρ A The value range is 0.65~0.85.

[0055] The ratio of the density of the active material layer to the density of the active material represents the volume ratio of the electrode excluding the pores, which corresponds to the porosity in the electrode. When ρ / ρ A When the value range of ρ / ρ is 0.55-0.88, the volume energy density of the electrode can be improved, and the electrode has excellent ion migration ability; when the ρ / ρ A When the value range of is less than 0.55, it will affect the energy density of the lithium battery; when the ρ / ρ A When the value range is greater than 0.88, the ion migration ability will be reduced and the electrochemical performance of the battery will be affected.

[0056] In one embodiment, the electrode sheet includes a positive electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one side of the positive electrode current collector, the positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, a positive electrode binder and a positive electrode solvent, and the effective ion migration rate of the positive electrode sheet is μ C , the µ C The value range is 0.02~0.5.

[0057] Specifically, the μ C The value range of is any point value among 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50 or the range value consisting of any two point values; in a preferred embodiment, the μ C The value range is 0.1~0.5.

[0058] When the µ C The value range of μ is 0.02~0.5. The effective ion migration rate in the positive electrode is high, which is conducive to the migration and transmission of ions in the electrode, has better kinetic performance, and can improve the electrochemical performance of the battery. C When the value range of is less than 0.02, the ion migration in the positive electrode will be greatly hindered, affecting the transmission of ions and further affecting the electrochemical performance of the battery; when the μ C When the value range is greater than 0.5, although the effective ion migration rate of the positive electrode is high and the electrode kinetic performance is good, it is necessary to reduce the surface density or compaction density of the active material or further reduce the binder content on the basis of the existing technical solution, which will lead to a decrease in the energy density of the battery.

[0059] In one embodiment, the density of the positive electrode active material layer is p C , the ρ C The value range is 2.0-4.5, and the unit is g / cm 3 ; The density of the positive electrode active material is ρ CA , the ρ CAThe value range is 2.3-5.2, and the unit is g / cm 3 ; said ρ C With the ρ CA Satisfy the relationship: 0.60≤ρ C / ρ CA ≤0.88.

[0060] Specifically, the C The value range is 2.0g / cm 3 , 2.1g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 、3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 、3.9g / cm 3 , 4.0g / cm 3 , 4.1g / cm 3 , 4.2g / cm 3 , 4.3g / cm 3 , 4.4g / cm 3 or 4.5g / cm 3 In a preferred embodiment, ρ C The value range is 2.3g / cm 3 -4.3g / cm 3 .

[0061] When the ρ C The value range is 2.0g / cm 3 -4.5g / cm 3 , the positive electrode active material itself has a high density, which can achieve a higher compaction density of the positive electrode sheet; when the ρ C The value range is less than 2.0g / cm 3When ρ is used, the components in the active material layer cannot be in close contact, affecting the electronic conduction between the components. In addition, there are more pores in the active material layer, and more electrolyte needs to be added to fill the pores, which will lead to a decrease in battery energy density and an increase in manufacturing cost. C The value range is greater than 4.5g / cm 3 When the active material layer is too close, the components in the active material layer will be in too close contact, which will reduce the pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0062] Specifically, the CA The value range is 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 、3.9g / cm 3 , 4.0g / cm 3 , 4.1g / cm 3 , 4.2g / cm 3 , 4.3g / cm 3 , 4.4g / cm 3 , 4.5g / cm 3 , 4.6g / cm 3 , 4.7g / cm 3 , 4.8g / cm 3 , 4.9g / cm 3 , 5.0g / cm 3 , 5.1g / cm 3 or 5.2g / cm 3 In a preferred embodiment, the ρ CA The value range is 3.4g / cm 3 -5.1g / cm 3 .

[0063] When the ρCA The value range is 2.3g / cm 3 -5.2g / cm 3 , the positive electrode active material itself has a high density, which can achieve a higher compaction density of the positive electrode sheet. CA The value range is less than 2.3g / cm 3 When the positive electrode sheet is achievable, the compaction density will be reduced, affecting the energy density of the battery; when the ρ CA The value range is greater than 5.2g / cm 3 When the active material layer is filled with ions, it will cause pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0064] Specifically, ρ C / ρ CA The value range of is 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87 or 0.88, or a range consisting of any two point values; in a preferred embodiment, ρ C / ρ CA The value range is 0.65-0.85.

[0065] When ρ C / ρ CA The value range of ρ is 0.60-0.88, which can improve the volume energy density of the electrode and has excellent ion migration ability. C / ρ CA When the value range of is less than 0.60, there are more pores in the positive electrode active material layer, and more electrolyte needs to be added to fill the pores, which will lead to a decrease in battery energy density and an increase in manufacturing cost; when the ρ C / ρ CA When the value range of is greater than 0.88, the migration energy of ions in the electrode will be reduced.

[0066] In one embodiment, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide; one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials;

[0067] Or, one or more of layered sodium cathodes such as sodium ferric sulfate, sodium ferric phosphate, sodium ferric pyrophosphate, sodium ferric pyrophosphate, sodium vanadium phosphate, sodium copper iron manganate, sodium iron nickel manganate, and Prussian blue cathode;

[0068] The positive electrode binder includes one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile and modified polyacrylonitrile, polytetrafluoroethylene and modified polymers thereof;

[0069] The positive electrode conductive agent includes one or more of carbon black, acetylene black, Ketjen black, graphene microspheres, three-dimensional conductive metal organic frameworks, porous spherical carbon, conductive nanotubes, nanofibers, graphene, and graphite microsheets.

[0070] In one embodiment, the thickness of the positive electrode active material layer is 50 um to 200 um.

[0071] Specifically, the thickness of the positive electrode active material layer is any point value of 50um, 60um, 70um, 80um, 90um, 100um, 110um, 120um, 130um, 140um, 150um, 160um, 170um, 180um, 190um or 200um, or a range value consisting of any two point values; in a preferred embodiment, the thickness of the positive electrode active material layer is 70um~150um.

[0072] When the thickness of the positive electrode active material layer is 50um~200um, the positive electrode sheet has a higher energy density and a higher effective ion migration rate, and the electrode has better electrochemical performance; when the thickness of the positive electrode sheet is less than 50um, the energy density of the positive electrode sheet will be lower; when the thickness of the positive electrode sheet is greater than 200um, the effective ion migration rate will be reduced and the battery kinetic performance will deteriorate.

[0073] In one embodiment, the electrode sheet includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer provided on at least one side of the negative electrode current collector, the negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder and a negative electrode solvent, and the effective ion migration rate of the negative electrode sheet is μ A , the µ A The value range is 0.025~0.35.

[0074] Specifically, the μ AThe value range of is any one of 0.025, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34 or 0.35, or a range consisting of any two of the values; in a preferred embodiment, the µ A The value range is 0.12~0.3.

[0075] When the µ A The value range of µ is 0.025~0.35. The effective ion migration rate in the negative electrode is high, which is conducive to the migration and transmission of ions in the electrode, has better kinetic performance, and can improve the electrochemical performance of the battery. A When the value range of µ is less than 0.025, the ion migration in the negative electrode will be greatly hindered, affecting the transmission of ions and further affecting the electrochemical performance of the battery; when the µ A When the value range is greater than 0.35, although the effective ion migration rate of the negative electrode is high and the electrode kinetic performance is good, it is necessary to reduce the surface density or compaction density of the active material or further reduce the binder content on the basis of the existing technical solution, which will lead to a decrease in the energy density of the battery.

[0076] In one embodiment, the density of the negative electrode active material layer is p B , the ρ B The value range is 1.2-2.2, and the unit is g / cm 3 ; The density of the negative electrode active material is ρ BA , the ρ BA The value range is 2.0-3.5, and the unit is g / cm 3 ; said ρ B With the ρ BA Satisfies the relationship: 0.55≤ρ B / ρ BA ≤0.82.

[0077] Specifically, the B The value range is 1.2g / cm 3 , 1.3g / cm 3 , 1.4g / cm 3 , 1.5g / cm 3 , 1.6g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3, 1.9g / cm 3 , 2.0g / cm 3 , 2.1g / cm 3 or 2.2g / cm 3 In a preferred embodiment, the ρ B The value range is 1.4g / cm 3 -2.0g / cm 3 .

[0078] When the ρ B The value range is 1.2g / cm 3 -2.2g / cm 3 , the negative electrode active material has a higher density, the negative electrode sheet has a higher compaction density, has a high energy density, and has good electrochemical performance. B The value range is less than 1.2g / cm 3 When ρ is used, the components in the active material layer cannot be in close contact, affecting the electronic conduction between the components. In addition, there are more pores in the active material layer, and more electrolyte needs to be added to fill the pores, which will lead to a decrease in battery energy density and an increase in manufacturing cost. B The value range is greater than 2.2g / cm 3 When the active material layer is too close, the components in the active material layer will be in too close contact, which will reduce the pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0079] Specifically, the BA The value range is 2.0g / cm 3 , 2.1g / cm 3 , 2.2g / cm 3 , 2.3g / cm 3 , 2.4g / cm 3 , 2.5g / cm 3 , 2.6g / cm 3 , 2.7g / cm 3 , 2.8g / cm 3 , 2.9g / cm 3 , 3.0g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 In a preferred embodiment, the ρ BA The value range is 2.2g / cm3 -3.2g / cm 3 .

[0080] When the ρ BA The value range is 2.0g / cm 3 -3.5g / cm 3 , the negative electrode active material itself has a higher density, which can achieve a higher compaction density of the negative electrode. BA The value range is less than 2.0g / cm 3 When ρ BA The value range is greater than 3.5g / cm 3 When the active material layer is filled with ions, it will cause pores in the active material layer, reduce the effective ion migration rate, and affect the electrochemical performance of the battery.

[0081] Specifically, ρ B / ρ BA The value range of is 0.55, 0.56, 0.57, 0.58, 0.59, 0.60, 0.61, 0.62, 0.63, 0.64, 0.65, 0.66, 0.67, 0.68, 0.69, 0.7, 0.71, 0.72, 0.73, 0.74, 0.75, 0.76, 0.77, 0.78, 0.79, 0.8, 0.81 or 0.82, or a range consisting of any two points. In a preferred embodiment, ρ B / ρ BA The value range is 0.6~0.8.

[0082] When ρ B / ρ BA When the value range of ρ is 0.55-0.82, the volume energy density of the electrode can be improved, and the electrode has excellent lithium ion migration ability; when the ρ B / ρ BA When the value range of is less than 0.55, it will affect the energy density of the lithium battery; when the ρ B / ρ BA When the value range is greater than 0.82, the ion migration ability will be reduced and the electrochemical performance of the battery will be affected.

[0083] In one embodiment, the negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon, silicon oxide, silicon carbon, lithium titanate, etc.;

[0084] The negative electrode binder is one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polyethylene, modified polyethylene, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polytetrafluoroethylene and modified polymers thereof, polyacrylonitrile, modified polyacrylonitrile, polymethyl methacrylate and modified polymers thereof;

[0085] The negative electrode conductive agent is one or more of carbon black, acetylene black, Ketjen black, graphene microspheres, three-dimensional conductive metal organic frameworks, porous spherical carbon, conductive nanotubes, nanofibers, graphene, and graphite microsheets.

[0086] In one embodiment, the thickness of the negative electrode active material layer is 40 um to 150 um.

[0087] Specifically, the thickness of the negative electrode active material layer is any point value of 40um, 50um, 60um, 70um, 80um, 90um, 100um, 110um, 120um, 130um, 140um or 150um, or a range value consisting of any two point values; in a preferred embodiment, the thickness of the negative electrode active material layer is 45um~100um.

[0088] When the thickness of the negative electrode plate is 40um~150um, the negative electrode plate has a higher energy density and a higher effective ion migration rate, and the electrode has better electrochemical performance; when the thickness of the negative electrode plate is less than 40um, the energy density of the negative electrode plate will be lower; when the thickness of the negative electrode plate is greater than 150um, the effective ion migration rate will be reduced and the battery kinetic performance will deteriorate.

[0089] In one embodiment, the second aspect of the present invention provides a method for preparing a secondary battery, wherein the method for preparing a pole piece comprises the following steps:

[0090] 1) mixing an active material, a conductive agent, a binder, and a solvent to obtain an active material layer precursor;

[0091] 2) thinning the active material layer precursor at least once to obtain the active material layer;

[0092] 3) The thinning method in 2) can be screw extrusion thinning or pressing thinning;

[0093] 4) Compounding the active material layer with the current collector;

[0094] 5) After compounding, freeze drying or supercritical drying is performed, and then roller pressing is performed to obtain the electrode sheet.

[0095] Specifically, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed to form a positive electrode mixture, and a positive electrode solvent is subsequently added and mixed to obtain a positive electrode active material precursor; the positive electrode active material precursor is extruded or pressed through a die to obtain a positive electrode active material layer, the positive electrode active material layer is composited with a positive electrode current collector, freeze-dried at a temperature below the melting point of the positive electrode solvent to remove the solvent, and rolled to obtain a positive electrode sheet;

[0096] A negative electrode active material, a negative electrode binder, and a negative electrode conductive agent are mixed to form a negative electrode mixture, and then a negative electrode solvent is added and mixed to obtain a negative electrode active material precursor; the negative electrode active material precursor is extruded or pressed through a die to obtain a negative electrode active material layer, the negative electrode active material layer is composited with a negative electrode current collector, freeze-dried at a temperature below the melting point of the negative electrode solvent to remove the solvent, and rolled to obtain a negative electrode sheet; in one embodiment, in step 1), the solvent includes one or more of deionized water, N-methylpyrrolidone, dimethyl sulfoxide, acetic acid, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

[0097] Specifically, the freezing temperature of deionized water is less than 0°C, the freezing temperature of N-methylpyrrolidone is less than -24°C, the freezing temperature of dimethyl sulfoxide is less than 18°C, the freezing temperature of acetic acid is less than 16°C, the freezing temperature of ethylene carbonate is less than 35°C, the freezing temperature of propylene carbonate is less than -49°C, the freezing temperature of dimethyl carbonate is less than 2°C, the freezing temperature of diethyl carbonate is less than -43°C, and the freezing temperature of ethyl methyl carbonate is less than -14°C.

[0098] When the electrode is freeze-dried below the melting point of the solvent, the solvent in the active material layer of the electrode can be transformed from liquid to solid crystals, and then leave the electrode by sublimation. This not only maintains the pore structure in the active material layer, but also avoids the migration of the binder caused by the solvent directly volatilizing from liquid to gas, which can improve the volume energy density of the positive electrode and has excellent ion migration ability. When the electrode is freeze-dried above the melting point of the solvent, the solvent cannot be transformed from liquid to solid crystals, but directly volatilizes from liquid to gas and leaves the electrode, which will cause the migration of the binder and the deterioration of the pore structure, affecting the electrochemical performance.

[0099] Alternatively, the positive electrode active material layer is composited with the positive electrode current collector, and the negative electrode active material layer is composited with the negative electrode current collector, and then the solvent is removed by drying with a supercritical fluid under a supercritical state, and the positive electrode sheet and the negative electrode sheet are obtained by rolling;

[0100] Specifically, the type of supercritical fluid includes any one of carbon dioxide, chlorotrifluoromethane, ethane, ethylene or ammonia, or a combination of at least two thereof; when carbon dioxide is used as the supercritical fluid, the drying temperature is ≥31°C, and the drying pressure is ≥7.38 MPa; when chlorotrifluoromethane is used as the supercritical fluid, the drying temperature is ≥28.8°C, and the drying pressure is ≥3.95 MPa; when ethane is used as the supercritical fluid, the drying temperature is ≥32.5°C, and the drying pressure is ≥4.6 MPa; when ethylene is used as the supercritical fluid, the drying temperature is ≥9.5°C, and the drying pressure is ≥5.06 MPa;

[0101] Specifically, the positive electrode active material, the positive electrode conductive agent and the positive electrode binder are mixed in a ratio of (94.5-98.5): (0.5-2.5): (1.0-3.0) to form a positive electrode mixture, and then the positive electrode solvent is added. The total mass of the positive electrode mixture is 100%, and the mass proportion of the positive electrode solvent is α1, and the value range of α1 is 0<α1≤20%;

[0102] The negative electrode active material, the negative electrode conductive agent and the negative electrode binder are mixed in a ratio of (92.0~98.5):(0.5~3.0):(1.0~5.0) to obtain a negative electrode mixture, and then a negative electrode solvent is added. The total mass of the negative electrode mixture is 100%, and the mass proportion of the negative electrode solvent is α2, and the value range of α2 is 0<α2≤40%.

[0103] In one embodiment, in step 1), the active material layer precursor includes a positive electrode active material layer precursor and a negative electrode active material layer precursor, the solid content of the positive electrode active material layer precursor is X1, and the range of X1 is 70%≤X1<100%, and the solid content of the negative electrode active material layer precursor is X2, and the range of X2 is 60%≤X2<100%.

[0104] Specifically, the solid content of the positive electrode active material layer precursor is any point value of 70%, 75%, 80%, 85%, 90%, 95% or 99%, or a range value consisting of any two point values; in a preferred embodiment, the solid content of the positive electrode slurry group is 75%~95%.

[0105] When the range of X1 is 70%≤X1<100%, the use of solvent can be reduced and the energy loss in freeze drying can be reduced; when the range of X1 is less than 70%, the solvent content in the positive electrode active material precursor is high, which will lead to an increase in the energy consumption required for freeze drying.

[0106] Specifically, the solid content of the negative electrode active material layer precursor is any one of 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99%, or a range value consisting of any two of the values; in a preferred embodiment, the solid content of the negative electrode slurry is 65%~90%.

[0107] When the range of X2 is 60%≤X1<100%, the use of solvent can be reduced, the energy loss in freeze drying or supercritical drying can be reduced, and the drying time can be reduced to improve production efficiency; when the range of X2 is less than 60%, the solvent content in the negative electrode active material precursor is high, which will lead to an increase in the energy consumption and time required for freeze drying or supercritical drying, affecting production efficiency and increasing manufacturing costs.

[0108] In one embodiment, in step 5), the freeze-drying conditions include: a temperature lower than the melting point of the solvent, and a pressure vacuum degree of 0.1 Pa to 100 Pa.

[0109] Specifically, the positive electrode active material layer is compounded with the positive electrode current collector and freeze-dried at a temperature below the melting point of the positive electrode solvent. The positive electrode solvent is crystallized and then sublimated under low temperature conditions. Different from the existing baking volatilization process, it can reduce the migration of the positive electrode binder caused by the volatilization of the positive electrode solvent and the change of the pores in the positive electrode sheet, ensuring that the original pore structure in the positive electrode sheet can be maintained during the process of removing the positive electrode solvent; it can improve the volume energy density of the positive electrode sheet and has excellent ion migration ability.

[0110] The negative electrode active material layer is compounded with the negative electrode current collector and freeze-dried at a temperature below the melting point of the negative electrode solvent. The negative electrode solvent is crystallized and then sublimated under low temperature conditions. Different from the existing baking volatilization process, it can reduce the migration of the negative electrode binder caused by the volatilization of the negative electrode solvent and the change of the pores in the negative electrode plate, ensuring that the original pore structure in the negative electrode plate can be maintained during the process of removing the negative electrode solvent. Furthermore, pores can be created to improve the pores in the plate; the volume energy density of the negative electrode plate can be improved, and it has excellent ion migration ability.

[0111] Specifically, the pressure vacuum degree is any point value among 0.1Pa, 1Pa, 5Pa, 10Pa, 20Pa, 30Pa, 40Pa, 50Pa, 60Pa, 70Pa, 80Pa, 90Pa or 100Pa, or a range value consisting of any two point values. In a preferred embodiment, the pressure vacuum degree is 1Pa~80Pa.

[0112] When the pressure vacuum is 0.1Pa~100Pa, the solvent can sublime and leave the electrode at the same time as it freezes and crystallizes, ensuring that the original pore structure in the electrode can be maintained during the solvent removal process; when the pressure vacuum is less than 0.1Pa, the energy consumption for maintaining the vacuum needs to be increased. More importantly, too low a vacuum degree will also disrupt the equilibrium process of the solvent from liquid to crystallization and then to sublimation, thereby affecting the structure of the pores in the electrode; when the pressure vacuum is greater than 100Pa, the sublimation rate of the solvent after crystallization will be slow, thereby extending the processing time and affecting production efficiency.

[0113] In one embodiment, in step 5), the supercritical drying conditions include: the type of supercritical fluid used includes any one of carbon dioxide, chlorotrifluoromethane, ethane or ethylene, or a combination of at least two thereof; the temperature is the critical temperature for maintaining the supercritical fluid state; and the pressure is the critical pressure for maintaining the supercritical fluid state.

[0114] Specifically, the positive electrode active material layer is compounded with the positive electrode current collector and supercritically dried under temperature and pressure conditions that maintain a supercritical fluid state. The positive electrode solvent is removed through supercritical drying under supercritical conditions. Different from the existing baking volatilization process, it can reduce the migration of the positive electrode binder caused by the volatilization of the positive electrode solvent and the change of the pores in the positive electrode sheet, ensuring that the original pore structure in the positive electrode sheet can be maintained during the process of removing the positive electrode solvent; it can improve the volume energy density of the positive electrode sheet and has excellent ion migration ability.

[0115] The negative electrode active material layer is compounded with the negative electrode current collector and supercritically dried under temperature and pressure conditions that maintain a supercritical fluid state. The negative electrode solvent is removed through supercritical drying under supercritical conditions. Different from the existing baking volatilization process, it can reduce the migration of the negative electrode binder caused by the volatilization of the negative electrode solvent and the change of the pores in the negative electrode sheet, ensuring that the original pore structure in the negative electrode sheet can be maintained during the process of removing the negative electrode solvent; it can improve the volume energy density of the negative electrode sheet and has excellent ion migration ability.

[0116] The beneficial effects of the present invention are further illustrated below with reference to the examples.

[0117] In order to make the invention objectives, technical solutions and beneficial technical effects of the present invention clearer, the present invention is further described in detail below with reference to the examples. However, it should be understood that the examples of the present invention are only for the purpose of explaining the present invention and are not intended to limit the present invention, and the examples of the present invention are not limited to the examples given in the specification. In the examples, where no specific experimental conditions or operating conditions are specified, the products were prepared under conventional conditions or under the conditions recommended by the material supplier.

[0118] Furthermore, it should be understood that the one or more method steps mentioned in the present invention do not exclude the presence of other method steps before or after the combination step, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise specified. It should also be understood that the combination connection relationship between one or more devices / apparatuses mentioned in the present invention does not exclude the presence of other devices / apparatuses before or after the combination device / apparatus, or the insertion of other devices / apparatuses between two explicitly mentioned devices / apparatuses, unless otherwise specified. Furthermore, unless otherwise specified, the numbering of each method step is merely a convenient tool for identifying each method step, and is not intended to limit the order of arrangement of each method step or to define the scope of the present invention. Changes or adjustments to their relative relationships, without substantially changing the technical content, should also be considered within the scope of the present invention.

[0119] In the following examples, the reagents, materials and instruments used, unless otherwise specified, can be purchased commercially or obtained through synthetic methods known in the art.

[0120] Table 1 Design of positive and negative electrode sheets of Examples 1-24 and Comparative Examples 1-7;

[0121]

[0122] Example 1

[0123] This embodiment is used to illustrate the positive secondary battery disclosed in the present invention; it includes the following steps:

[0124] Production of positive electrode:

[0125] The positive electrode active material lithium iron phosphate, the conductive agent carbon black, and the binder PVDF are mixed in a mass ratio of 96.5:1.5:2.0; the positive electrode solvent NMP is added to form a positive electrode active material precursor with a solid content of 80%, and the active material precursor is extruded through a die to obtain a positive electrode active material layer. The positive electrode active material layer is compounded with the positive electrode current collector, freeze-dried at a vacuum degree of 1 Pa and a temperature of -40°C for 6 hours, and rolled to obtain a positive electrode sheet. The thickness of the positive electrode active material layer is 90 μm.

[0126] Production of negative electrode:

[0127] The negative electrode active material (graphite anode), the conductive agent (carbon black), the binder (styrene-butadiene rubber), and the thickener (sodium carboxymethyl cellulose) were mixed in a mass ratio of 95.4:1.5:1.6:1.5. Deionized water (negative electrode solvent) was added and thoroughly stirred to form a negative electrode active material precursor with a solid content of 80%. The negative electrode active material precursor was extruded through a die to form a negative electrode active material layer. The negative electrode active material layer was then combined with the negative electrode current collector, freeze-dried at a vacuum of 1 Pa and a temperature of -10°C for 8 hours, and roller-pressed to form a negative electrode sheet. The thickness of the negative electrode active material layer was 75 μm.

[0128] Battery production:

[0129] The positive electrode sheet, separator and negative electrode sheet are stacked in order, with the separator placed in the middle of the positive and negative electrodes to play an isolating role. The positive electrode sheet, separator and negative electrode sheet are repeatedly stacked to form a core, and then the stacked core is placed in an aluminum-plastic film bag that has been punched and formed. The electrolyte prepared above is injected into the baked and dried battery cells. After vacuum packaging, standing, formation and other processes, a battery with a capacity of 1.2Ah is obtained.

[0130] Examples 2-24

[0131] Examples 2-24 are used to illustrate the secondary battery disclosed in the present invention, and include most of the operating steps in Example 1, except that:

[0132] The parameters shown in Table 1 for manufacturing the positive electrode sheet and the negative electrode sheet are different.

[0133] Comparative Examples 1-7

[0134] Comparative Example 1 is used to illustrate the secondary battery disclosed in the present invention, and includes most of the operating steps in Example 1, except that:

[0135] Table 1 shows the parameters for manufacturing the positive electrode sheet and the negative electrode sheet, as well as the drying method.

[0136] Performance Testing

[0137] The batteries prepared in Examples 1-24 and Comparative Examples 1-7 were subjected to the following performance tests:

[0138] 1. Effective ion migration rate:

[0139] Battery production: Assemble two positive electrode sheets with a state of charge of 50% and a separator placed between the two sheets, add electrolyte, seal, and obtain a symmetrical battery. Record the area and thickness of the electrode sheets.

[0140] and / or, assembling two negative electrode sheets with a state of charge of 50% and a separator placed between the two sheets, adding electrolyte, sealing to obtain a symmetrical battery, and recording the area and thickness of the electrode sheets;

[0141] Testing: Electrochemical impedance spectroscopy (EIS) was performed on the symmetrical battery using an electrochemical workstation with a perturbation voltage of 5 mV and a test frequency range of 100 kHz to 0.01 Hz. The resulting Nyquist plot showed a characteristic shape where a line segment in the low-frequency region intersected with a line segment in the high-frequency region.

[0142] Calculate the ion migration rate: Extend the low-frequency line segment in the Nyquist diagram until it intersects the horizontal axis. The difference between this intersection and the intersection of the high-frequency line segment and the horizontal axis is recorded as the impedance R, and substitute it into the following formula , and the effective ion migration rate was calculated.

[0143] 2. 2C discharge rate:

[0144] Charging: At 25±2℃, charge the battery at a constant current rate of 0.5C to 3.65V, then switch to constant voltage charging. The cutoff condition is that the current is less than 0.05C;

[0145] Shelf: 25±2℃, shelf for 10 minutes after charging;

[0146] Discharge: 25±2℃, then discharge at 0.5C constant current to 2.5V, record the discharge capacity as C0;

[0147] Shelf: 25±2℃, shelf for 10min after discharge;

[0148] Charging: At 25±2℃, charge the battery again at a constant current rate of 0.5C to 3.65V, then switch to constant voltage charging. The cutoff condition is that the current is less than 0.05C.

[0149] Shelf: 25±2℃, shelf for 10 minutes after charging;

[0150] Discharge: 25±2℃, then discharge at 2.0C constant current to 2.5V, record the discharge capacity as C1;

[0151] Discharge ratio: C1 / C0*100% is recorded as 2C discharge ratio.

[0152] 3. 1C constant current charging ratio:

[0153] Discharge: At 25±2℃, discharge the battery at a constant current of 0.5C to 2.5V;

[0154] Shelf: 25±2℃, shelf for 10min after discharge;

[0155] Charging: 25±2℃, then charge at 1C constant current to 3.65V, record the charging capacity as Ci;

[0156] Charging: 25±2℃, then charge at 3.65V constant voltage until the current is less than 0.05C, and record the charging capacity as Cp;

[0157] Constant current charging ratio: Ci / (Ci+Cp)*100% is recorded as 1C constant current charging ratio.

[0158] 4. DC internal resistance

[0159] Adjust to 50% SOC: 25±2℃, charge the battery at a constant current rate of 0.5C to 3.65V, then switch to constant voltage charging. The cutoff condition is that the current is less than 0.05C. After charging, let it stand for 10 minutes, and then discharge it at a constant current of 0.5C to 50% SOC;

[0160] DC resistance test: Wait for 60 minutes, record the end voltage V1, then discharge at 2C (2.4A) for 30 seconds, record the end voltage V2;

[0161] DC internal resistance: (V1-V2) / 2.4*1000 is recorded as the measured DC internal resistance value

[0162] The test results are shown in Table 2.

[0163] Table 2 Electrochemical performance of secondary batteries

[0164]

[0165] As shown in Table 2, when the density ρ of the positive electrode active material layer is 2.0 g / cm 3 -4.5g / cm 3 ; Density of active material ρ A 2.2g / cm 3 -5.2g / cm 3 , and ρ and ρ A Satisfy the relationship: 0.60≤ρ / ρ A When ρ is less than 0.88, the volume energy density of the electrode can be improved, and it has excellent ion migration ability; when the value range of ρ is less than 2.0g / cm 3 , or when ρ A The value range is less than 2.2g / cm 3 , or ρ / ρ A When the value range of ρ is less than 0.60, the compaction density of the positive electrode sheet will be reduced, and there will be more pores in the positive active material layer. More electrolyte needs to be added to fill the pores, resulting in a decrease in battery energy density and an increase in manufacturing cost. When the value range of ρ is greater than 4.5 g / cm 3, or when ρ A The value range is greater than 5.2g / cm 3 , or ρ / ρ A When the value range of is greater than 0.88, the migration energy of ions in the electrode will be reduced.

[0166] Comparing Example 3 and Examples 15-16 with Comparative Examples 3-4, when the density ρ of the negative electrode active material layer is 1.2 g / cm 3 -2.2g / cm 3 ; Density of active material ρ A 2.0g / cm 3 -3.5g / cm 3 , and ρ and ρ A Satisfy the relationship: 0.55≤ρ / ρ A When ρ is less than 0.82, the volume energy density of the electrode can be improved, and it has excellent ion migration ability; when the value range of ρ is less than 1.2g / cm 3 , or when ρ A The value range is less than 2.0g / cm 3 , or ρ / ρ A When the value range of ρ is less than 0.55, the compaction density of the negative electrode sheet will be reduced, and there will be more pores in the negative active material layer. More electrolyte needs to be added to fill the pores, resulting in a decrease in battery energy density and an increase in manufacturing cost. When the value range of ρ is greater than 2.2 g / cm 3 , or when ρ A The value range is greater than 3.5g / cm 3 , or ρ / ρ A When the value range of is greater than 0.82, the migration rate of ions in the electrode will decrease.

[0167] Comparing Example 2 with Examples 4-6, when the solid content X1 of the positive electrode active material layer precursor is in the range of 70%≤X1<100%, the use of solvent can be reduced and the energy loss in freeze drying can be reduced; when the range of X1 is less than 70%, the solvent content in the positive electrode active material precursor is high, which will lead to an increase in the energy consumption required for freeze drying.

[0168] Comparing Example 2 with Examples 17-19, when the solid content X2 of the negative electrode active material layer precursor is in the range of 60%≤X2<100%, the use of solvent can be reduced and the energy loss in freeze drying can be reduced; when the range of X2 is less than 60%, the solvent content in the positive electrode active material precursor is high, which will lead to an increase in the energy consumption required for freeze drying.

[0169] Comparing Example 2 with Examples 7-10, when the thickness of the positive electrode active material layer is 50um~200um, the positive electrode plate has a higher energy density and a higher effective ion migration rate, and the electrode has better electrochemical properties; when the thickness of the positive electrode plate is less than 50um, the energy density of the positive electrode plate will be lower; when the thickness of the positive electrode plate is greater than 200um, the effective ion migration rate will be reduced and the battery kinetic performance will deteriorate.

[0170] Comparing Example 2 with Examples 7-10, when the thickness of the negative electrode plate is 40um~150um, the negative electrode plate has a higher energy density and a higher effective ion migration rate, and the electrode has better electrochemical properties; when the thickness of the negative electrode plate is less than 40um, the energy density of the negative electrode plate will be lower; when the thickness of the negative electrode plate is greater than 150um, the effective ion migration rate will be reduced and the battery kinetic performance will deteriorate.

[0171] Comparing Example 2 and Example 11, when the positive electrode plate is freeze-dried below the melting point of the positive electrode solvent, the negative electrode solvent is crystallized and then sublimated under low temperature conditions. Unlike the existing baking volatilization process, the migration of the positive electrode binder and the change of the pores in the positive electrode plate caused by the volatilization of the positive electrode solvent can be reduced, ensuring that the original pore structure in the positive electrode plate can be maintained during the process of removing the positive electrode solvent; the volume energy density of the positive electrode plate can be improved, and it has excellent ion migration ability; when the positive electrode plate is freeze-dried above the melting point of the positive electrode solvent, the solvent in the positive electrode plate will mainly leave the plate in the form of gas, resulting in the migration of the positive electrode binder and the deterioration of the pore structure in the plate, thereby causing a decrease in the ion migration ability in the plate, affecting the electrochemical performance, and causing a significant extension of the drying time, affecting the drying efficiency.

[0172] Comparing Example 2 and Example 20, when the negative electrode plate is freeze-dried below the melting point of the negative electrode solvent, the negative electrode solvent is crystallized and then sublimated under low temperature conditions. Unlike the existing baking volatilization process, the migration of the negative electrode binder caused by the volatilization of the negative electrode solvent and the change of the pores in the negative electrode plate can be reduced, ensuring that the original pore structure in the negative electrode plate can be maintained during the removal of the negative electrode solvent, and further pores can be formed to improve the pore structure; the volume energy density of the negative electrode plate can be improved, and it has excellent ion migration ability; when the negative electrode plate is freeze-dried above the melting point of the negative electrode solvent, the solvent in the negative electrode plate will mainly leave the plate in the form of gas, resulting in the migration of the negative electrode binder and the deterioration of the pore structure in the plate, thereby causing a decrease in the ion migration ability in the plate, affecting the electrochemical performance, and causing a significant extension of the drying time, affecting the drying efficiency.

[0173] Compare Example 2 with Examples 12-14, or Compare Example 2 with Examples 21-23; when the pressure vacuum is 0.1Pa~100Pa, the solvent can be sublimated and leave the electrode at the same time during the process of freezing and crystallizing, ensuring that the original pore structure in the electrode can be maintained during the process of removing the solvent; when the pressure vacuum is less than 0.1Pa, it is necessary to increase the energy consumption of maintaining the vacuum. More importantly, too low a vacuum will also disrupt the equilibrium process of the solvent from liquid to crystallization and then to sublimation, thereby affecting the structure of the pores in the electrode; when the pressure vacuum is greater than 100Pa, it will cause the sublimation rate of the solvent after crystallization to be slow, thereby extending the processing time and affecting production efficiency. Comparing Examples 1-23 and Comparative Examples 5-7, compared with the traditional wet coating and drying process, the electrode prepared by the present invention at a high solid content and then freeze-dried has a higher ion migration rate, so that the battery has lower DC impedance, higher rate discharge and charge and other electrochemical properties.

[0174] Comparing Example 24 with Comparative Examples 5-7, compared with the traditional wet coating and drying process, the present invention prepares the electrode sheets at a high solid content and then obtains the positive electrode sheets and negative electrode sheets through supercritical drying. The solvent leaves the electrode sheets with the supercritical fluid, avoiding the position migration of the binder in the electrode sheets when the solvent leaves the electrode sheets, and can maintain the pore structure of the electrode sheets, with a higher ion migration rate, so that the battery has lower DC impedance, higher rate discharge and charging and other electrochemical properties.

[0175] In summary, the technical solution of the present invention is to prepare an electrode from a mixture of a high-solid content positive electrode or negative electrode, and then freeze-dry or supercritically dry it, so that when the solvent in the electrode leaves the electrode, the binder does not migrate with the solvent, thereby maintaining a uniform distribution of the binder in the electrode; at the same time, when the electrode has a higher surface density and a higher compaction density, the distribution of the pore structure in the electrode is maintained. Furthermore, due to the special process effects of freeze-drying or supercritical drying, it is possible to maintain the high porosity structure of the electrode, or to form pores and further form pores to improve the pores in the electrode, thereby improving the migration rate of ions in the electrode, so that the battery has a higher energy density and better electrochemical performance, and is suitable for large-scale application in the field of generating and manufacturing secondary batteries with high energy density and high electrochemical performance.

[0176] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A secondary battery, characterized in that: The invention comprises a pole piece, a separator and an electrolyte, wherein the pole piece comprises a current collector and an active material layer arranged on at least one side of the current collector, wherein the active material layer comprises an active material, a conductive agent and a binder; the effective ion migration rate of the pole piece is μ, and the value range of μ is 0.02-0.5; the pole piece comprises a positive pole piece, wherein the positive pole piece comprises a positive current collector and a positive active material layer, wherein the positive active material layer is arranged on at least one side of the positive current collector, and the density of the positive active material layer is ρ C , the ρ C The value range is 2.0-4.5, and the unit is g / cm 3 ; The density of the positive electrode active material is ρ CA , the ρ CA The value range is 2.2-5.2, and the unit is g / cm 3 ; said ρ C With the ρ CA Satisfy the relationship: 0.60≤ρ C / ρ CA ≤0.88; The electrode sheet includes a negative electrode sheet, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is arranged on at least one side of the negative electrode current collector, and the density of the negative electrode active material layer is p B , the ρ B The value range is 1.2-2.2, and the unit is g / cm 3 ; The density of the negative electrode active material is ρ BA , the ρ BA The value range is 2.0-3.5, and the unit is g / cm 3 ; said ρ B With the ρ BA Satisfies the relationship: 0.55≤ρ B / ρ BA ≤0.

82.

2. The secondary battery according to claim 1, wherein: The method for testing the ion migration rate µ of the electrode comprises the following steps: The electrode is made into a symmetrical battery, and an electrochemical impedance spectroscopy test is performed to obtain an electrochemical impedance spectroscopy Nyquist plot having a shape characteristic of intersection of a low-frequency region line segment and a high-frequency region line segment; Extend the low-frequency line segment in the Nyquist diagram until it intersects the horizontal axis. The difference between the intersection of the low-frequency line segment and the horizontal axis and the intersection of the high-frequency line segment and the horizontal axis is recorded as the impedance R, in Ω. Substitute R into the following formula , the effective ion migration rate is calculated; Where S is the effective reaction area of ​​the electrode in the symmetrical battery, in m 2 , d is the thickness of the active material layer, in um, and σ is the ionic conductivity of the electrolyte, in S / m.

3. The secondary battery according to claim 1, wherein: The positive electrode active material layer includes a positive electrode active material, a positive electrode conductive agent, a positive electrode binder and a positive electrode solvent. The effective ion migration rate of the positive electrode plate is μ c , the µ c The value range is 0.02~0.

5.

4. The secondary battery according to claim 3, wherein: The positive electrode active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and lithium-rich manganese-based materials; or one or more of sodium ferric sulfate, sodium ferric phosphate, sodium ferric pyrophosphate, sodium vanadium phosphate, sodium copper iron manganate, sodium iron nickel manganate, and Prussian blue; The positive electrode binder includes one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polyvinyl alcohol, modified polyvinyl alcohol, polyvinyl butyral, modified polyvinyl butyral, polyacrylonitrile and modified polyacrylonitrile, polytetrafluoroethylene, and modified polytetrafluoroethylene; The positive electrode conductive agent includes one or more of carbon black, three-dimensional conductive metal organic framework, porous spherical carbon, conductive nanotubes, graphene, and graphite microsheets.

5. The secondary battery according to claim 1, wherein: The thickness of the positive electrode active material layer is 50 μm to 200 μm.

6. The secondary battery according to claim 1, wherein: The negative electrode active material layer includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder and a negative electrode solvent. The effective ion migration rate of the negative electrode plate is μ A , the µ A The value range is 0.025~0.

35.

7. The secondary battery according to claim 6, wherein: The negative electrode active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon, silicon oxide, silicon carbon, and lithium titanate; The negative electrode binder is one or more of rubber, modified rubber, polyvinylidene fluoride, modified polyvinylidene fluoride, polyimide, modified polyimide, polyethylene, modified polyethylene, polypropylene, modified polypropylene, polyacrylic acid, modified polyacrylic acid, polytetrafluoroethylene, modified polytetrafluoroethylene, polyacrylonitrile, modified polyacrylonitrile, polymethyl methacrylate, and modified polymethyl methacrylate; The negative electrode conductive agent is one or more of carbon black, three-dimensional conductive metal organic framework, porous spherical carbon, conductive nanotubes, graphene, and graphite microsheets.

8. The secondary battery according to claim 1, wherein: The thickness of the negative electrode active material layer is 40 μm to 150 μm.

9. A method for preparing a secondary battery according to any one of claims 1 to 8, characterized in that: The method for preparing the pole piece comprises the following steps: 1) mixing an active material, a conductive agent, a binder, and a solvent to obtain an active material layer precursor; 2) thinning the active material layer precursor at least once to obtain the active material layer; 3) The thinning method in 2) can be screw extrusion thinning or pressing thinning; 4) Compounding the active material layer with the current collector; 5) After compounding, freeze drying or supercritical drying is performed, and then roller pressing is performed to obtain the electrode sheet.

10. The method for preparing a secondary battery according to claim 9, wherein: In step 1), the solvent includes one or more of deionized water, N-methylpyrrolidone, dimethyl sulfoxide, acetic acid, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

11. The method for preparing a secondary battery according to claim 9, wherein: In step 1), the active material layer precursor includes a positive electrode active material layer precursor and a negative electrode active material layer precursor, the solid content of the positive electrode active material layer precursor is X1, and the range of X1 is 70%≤X1<100%, and the solid content of the negative electrode active material layer precursor is X2, and the range of X2 is 60%≤X2<100%.

12. The method for preparing a secondary battery according to claim 9, wherein: In step 5), the freeze-drying conditions include: a temperature lower than the melting point of the solvent, and a pressure vacuum degree of 0.1 Pa to 100 Pa.

13. The method for preparing a secondary battery according to claim 9, wherein: In step 5), the supercritical drying conditions include: the type of supercritical fluid used includes any one of carbon dioxide, chlorotrifluoromethane, ethane or ethylene, or a combination of at least two; the temperature is the critical temperature for maintaining the supercritical fluid state; and the pressure is the critical pressure for maintaining the supercritical fluid state.

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