Secondary battery and preparation method thereof

Abstract

The invention provides a secondary battery, which comprises a pole piece, a diaphragm and an electrolyte, the pole piece comprises a current collector and an active material layer arranged on at least one side of the current collector, and the active material layer comprises an active material, a conductive agent and a binder; the value range of the effective ion migration rate of the pole piece is 0.02 to 0.5; according to the invention, the pole piece prepared under high solid content is subjected to freeze drying or supercritical drying, so that the solvent in the pole piece leaves the pole piece, and meanwhile, the adhesive does not migrate along with the solvent, and the uniform distribution of the adhesive in the pole piece is maintained; meanwhile, under the condition that the electrode plate has higher surface density and higher compaction density, distribution of a pore structure in the electrode plate is maintained, due to the special process effect of freeze drying or supercritical drying, the high pore structure of the electrode plate can be maintained, or pores in the electrode plate can be further improved through pore forming, the migration rate of ions in the electrode is increased, and the service life of the electrode plate is prolonged. Therefore, the battery has relatively high energy density and better electrochemical performance.

Description

Technical Field

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

[0002] With the sharp increase in the global demand for sustainable energy, secondary batteries, as key components of energy storage solutions, the optimization of their performance and cost control have become the focus of research and development. The existing preparation of secondary battery slurries usually adopts a wet process, which requires a large amount of solvents such as deionized water for stirring and dispersion, and then the solvents need to be removed by drying. This process not only consumes a large amount of energy, but also during the drying process, the binder will float up due to easy volatilization, resulting in uneven distribution of the binder in the electrode sheet. The binder distribution is less at the bottom and more at the top perpendicular to the electrode sheet. The uneven distribution of the binder will cause several problems: 1. The adhesion between the layer and the current collector is reduced; 2. More binder in the upper layer will cause an increase in the electrode impedance, resulting in deterioration of the electrochemical performance; 3. It is easy to cause the layer to stick to the roller during the rolling process, resulting in processing difficulties.

[0003] To solve the above problems, dry electrode preparation is now commonly used. Although the dry electrode preparation process avoids the use of solvents, at present, the only available binder in the solid-phase batching process is PTFE. An external high shear force needs to be applied to the dry mixture to make it fibrillate and then form a network to bond the electrode powder. It has poor compatibility with the existing processes and equipment. At the same time, there are problems such as uneven dry mixing, insufficient binder content, film stretching and tearing, and PTFE is unstable at low potentials and will react irreversibly with lithium. When applied to the negative electrode, it will lithiate and consume active lithium, reducing the bonding effect. Using PTFE in the lithium iron phosphate system cannot fibrillate well, and high-performance electrode sheets cannot be obtained. Therefore, how to overcome the above existing technical problems and defects has become a key problem to be solved. Summary of the Invention

[0004] In view of the problem of uneven distribution of the binder in the existing electrode sheet during the preparation process, 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: In a first aspect of the present invention, there is provided a secondary battery, including an electrode sheet, a separator, and an electrolyte. The electrode sheet includes a current collector and an active material layer provided on at least one side of the current collector. The active material layer includes an active material, a conductive agent, and a binder; the effective ion migration rate of the electrode sheet is µ, and the value range of µ is 0.02 to 0.5.

[0006] Optionally, the test method for the ion migration rate µ of the electrode sheet includes the following steps: 1) Fabricate the said electrode into a symmetric cell, conduct electrochemical impedance spectroscopy (EIS) tests to obtain an EIS Nyquist plot with the characteristic shape of the intersection of the low-frequency region line segment and the high-frequency region line segment; 2) Extend the low-frequency line segment in the Nyquist plot until it intersects with the horizontal axis. Denote the difference between this intersection point and the intersection point of the high-frequency line segment and the horizontal axis as impedance R, with the unit of Ω; 3) Substitute R into the following formula to calculate the effective ion migration rate; where S is the effective reaction area of the electrode in the symmetric cell, with the unit of m 2 , d is the thickness of the active material layer, with the unit of um, and σ is the ionic conductivity of the electrolyte, with the unit of S / m.

[0007] Optionally, the density of the active material layer is ρ, and the value range of ρ is 1.2 - 4.5, with the unit of g / cm 3 ; the density of the active material is ρ A , and the ρ A value range is 2.0 - 5.2, with the unit of g / cm 3 ; ρ and the ρ A satisfy the relationship: 0.55 ≤ ρ / ρ A ≤ 0.88.

[0008] Optionally, the said electrode includes a positive electrode, and the positive electrode includes a positive current collector and a positive active material layer. The positive active material layer is disposed on at least one side of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, a positive binder, and a positive solvent. The effective ion migration rate of the positive electrode is µ c , and the value range of the µ c is 0.02 - 0.5.

[0009] Optionally, the density of the positive active material layer is ρ C , and the value range of the ρ C is 2.0 - 4.5, with the unit of g / cm 3 ; the density of the positive active material is ρ CA , and the value range of the ρ CA is 2.3 - 5.2, with the unit of g / cm 3 ; the ρ C and the ρ CA satisfy the relationship: 0.60 ≤ ρ C / ρ CA ≤ 0.88.

[0010] Optionally, the positive electrode active material includes one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium cobalt oxide, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide; one or more of lithium iron phosphate, lithium iron manganese 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 layered sodium-based positive electrodes such as sodium iron sulfate, sodium iron phosphate, sodium iron pyrophosphate, sodium iron pyrophosphate phosphate, sodium vanadium phosphate, sodium copper iron manganese, and sodium iron nickel manganese; Prussian blue positive electrodes; 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 its modified polymers; 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 microflakes.

[0011] Optionally, the thickness of the positive electrode active material layer is 50 µm to 200 µm.

[0012] Optionally, the electrode includes a negative electrode, and the negative electrode includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer is disposed 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. The effective ion migration rate of the negative electrode is µ A , the µ A The value range of is 0.025 to 0.35.

[0013] Optionally, the density of the negative electrode active material layer is ρ B , the ρ B The value range of is 1.2 - 2.2, with the unit of g / cm 3 ; The density of the negative electrode active material is ρ BA , the ρ BA The value range of is 2.0 - 3.5, with the unit of g / cm 3 ; The ρ B And the ρ BA Satisfy the relationship: 0.55 ≤ ρ B / ρ BA ≤ 0.82.

[0014] 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.; 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 its modified polymers, polyacrylonitrile, modified polyacrylonitrile, polymethyl methacrylate and its modified polymers; 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, graphite microflakes; Optionally, the thickness of the negative electrode active material layer is 40um to 150um.

[0015] In the second aspect of the present invention, a method for preparing a secondary battery is provided. The method for preparing the electrode includes the following steps: 1) Mix the active material, conductive agent, binder and solvent to obtain a precursor of the active material layer; 2) Thinning the precursor of the active material layer at least once to obtain the active material layer; 3) The thinning method in 2) can be thinning by screw extrusion or thinning by rolling; 4) Compound the active material layer with the current collector; 5) After compounding, perform freeze-drying or supercritical drying, and then roll to obtain the electrode.

[0016] 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, etc.

[0017] Optionally, in step 1), the precursor of the active material layer includes a precursor of the positive electrode active material layer and a precursor of the negative electrode active material layer. The solid content of the precursor of the positive electrode active material layer is X1, and the range of X1 is 70%≤X1<100%. The solid content of the precursor of the negative electrode active material layer is X2, and the range of X2 is 60%≤X2<100%.

[0018] Optionally, in step 5), the conditions for freeze-drying include: the temperature is below the melting point temperature of the solvent, and the pressure vacuum is 0.1Pa to 100Pa.

[0019] Optionally, in step 5), the conditions for supercritical drying include: the types of supercritical fluids used include any one or a combination of at least two of carbon dioxide, chlorotrifluoromethane, ethane or ethylene; the temperature is the critical temperature for maintaining the supercritical fluid state; the pressure is the critical pressure for maintaining the supercritical fluid state.

[0020] According to the electrode provided by the present invention, an electrode prepared from a mixture of a high-solid-content positive electrode or negative electrode, and then through freeze-drying or supercritical drying, when the solvent in the electrode leaves the electrode, the binder does not migrate with the solvent, maintaining the uniform distribution of the binder in the electrode; at the same time, when the electrode has a higher areal density and a higher tap density, the distribution of the pore structure in the electrode is maintained. Further, due to the special process effect of freeze-drying or supercritical drying, it is possible to maintain the high pore structure of the electrode, or create pores to further improve the pores in the electrode, and enhance the migration rate of ions in the electrode, enabling the battery to have a higher energy density and better electrochemical performance. Detailed implementation manners

[0021] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the protection scope of the present invention.

[0022] Unless otherwise specified, the test methods used in the following embodiments are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, are reagents and materials that can be obtained from commercial channels.

[0023] In one embodiment, the first aspect of the present invention provides a secondary battery, including an electrode, a separator and an electrolyte. The electrode includes a current collector and an active material layer provided on at least one side of the current collector. The active material layer includes an active material, a conductive agent and a binder; the effective ion migration rate of the electrode is μ, and the value range of μ is 0.02 to 0.5.

[0024] Specifically, the value range of μ is any one value or a range value composed of any two value points among 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45 or 0.50; in a preferred embodiment, the value range of μ is 0.1 to 0.5.

[0025] When the value range of μ is 0.02 - 0.5, the effective ion migration rate in the electrode is relatively 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, it will cause a large hindrance to the ion migration in the electrode, affect the ion transmission, and further affect 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 areal density or tap density of the active material or further reduce the binder content, etc. on the basis of the existing technical solutions, which will lead to a decrease in the energy density of the battery.

[0026] In this application, the electrode is prepared from a mixture of a high solid content cathode or anode, and then through freeze-drying or supercritical drying, when the solvent in the electrode leaves the electrode, the binder does not migrate with the solvent, maintaining the uniform distribution of the binder in the electrode; at the same time, with a higher areal density and higher tap density of the electrode, the distribution of the pore structure in the electrode is maintained. Further, due to the special process effects of freeze-drying or supercritical drying, it is possible to maintain the high pore structure of the electrode or create pores to further improve the pores in the electrode, enhance the ion migration rate in the electrode, and make the battery have a higher energy density and better electrochemical performance.

[0027] In one embodiment, the test method for the ion migration rate μ of the electrode includes the following steps: 1) Fabricate the electrode into a symmetric battery, perform electrochemical impedance spectroscopy test, and obtain an electrochemical impedance spectroscopy Nyquist diagram with the shape feature of the intersection of the low-frequency region line segment and the high-frequency region line segment; 2) Extend the low-frequency line segment in the Nyquist diagram until it intersects with the horizontal axis, and record the difference between this intersection point and the intersection point of the high-frequency line segment and the horizontal axis as the impedance R; 3) Substitute R into the following formula , and calculate to obtain the effective ion migration rate; where S is the effective reaction area of the electrode in the symmetric battery, with the unit of m 2 , d is the thickness of the active material layer, with the unit of μm, and σ is the ionic conductivity of the electrolyte, with the unit of S / m.

[0028] The present invention measures the μ value through the above method, and makes the solvent in the electrode leave the electrode in the form of sublimation or follow the supercritical fluid through freeze-drying or supercritical drying, so that the value range of μ is 0.02 - 0.5, thereby making the effective ion migration rate in the electrode relatively high, being conducive to the migration and transmission of ions in the electrode, having better kinetic performance, and being able to improve the electrochemical performance of the battery.

[0029] In one embodiment, the density of the active material layer is ρ, and the value range of ρ is 1.2 - 4.5, with the unit of g / cm 3 ; the density of the active material is ρ A , and the value range of ρ A is 2.0 - 5.2, with the unit of g / cm 3 ; ρ and ρ A satisfy the relationship: 0.55 ≤ ρ / ρ A ≤ 0.88.

[0030] Specifically, the value range of ρ is any point value within 1.2 g / cm 3 , 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 1.9 g / cm 3 , 2.0 g / cm 3 , 2.1 g / cm 3 , 2.2 g / cm 3 , 2.3 g / cm 3 , 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.1 g / cm 3 , 3.2 g / cm 3 , 3.3 g / cm 3 , 3.4 g / cm 3 , 3.5 g / cm 3 , 3.6 g / cm 3 , 3.7 g / cm 3 , 3.8 g / cm 3 , 3.9 g / cm 3 , 4.0 g / cm 3 , 4.1 g / cm 3 , 4.2 g / cm 3 , 4.3 g / cm 3 , 4.4 g / cm 3 or any range value composed of any two point values within 4.5 g / cm 3 ; in a preferred embodiment, the value range of ρ is 1.4 g / cm 3-4.2 g / cm 3 。

[0031] 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 the ρ is 1.2 g / cm 3 -4.5 g / cm 3 , each component in the active material layer can be in close contact, having good electron and ion conduction, and the electrode has a higher tap density, making the energy density of the battery high. When the value range of the ρ is less than 1.2 g / cm 3 , it will cause the components in the active material layer not to be in close contact, affecting the electron conduction between the components, and there are more pores in the active material layer, requiring additional electrolyte to fill the pores, which will lead to a decrease in the energy density of the battery and an increase in the manufacturing cost; when the value range of the ρ is greater than 4.5 g / cm 3 , it will cause the components in the active material layer to be in too close contact, reducing the pores in the active material layer and decreasing the effective ion migration rate, which will affect the electrochemical performance of the battery.

[0032] Specifically, the value range of the ρ A is 2.0 g / cm 3 , 2.1 g / cm 3 , 2.2 g / cm 3 , 2.3 g / cm 3 , 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.1 g / cm 3 , 3.2 g / cm 3 , 3.3 g / cm 3 , 3.4 g / cm 3 , 3.5 g / cm 3 , 3.6 g / cm 3 , 3.7 g / cm 3 , 3.8 g / cm 3 , 3.9 g / cm 3 , 4.0 g / cm 3 , 4.1 g / cm 3 , 4.2 g / cm 3 , 4.3 g / cm 3 , 4.4 g / cm 3 , 4.5 g / cm 3, 4.6 g / cm 3 , 4.7 g / cm 3 , 4.8 g / cm 3 , 4.9 g / cm 3 , 5.0 g / cm 3 , 5.1 g / cm 3 or 5.2 g / cm 3 Any point value or range value composed of any two point values among them; in a preferred embodiment, the ρ A has a value range of 2.3 g / cm 3 -5.1 g / cm 3 .

[0033] The weight of the active material per unit volume of the effective unit is the density of the active material; when the ρ A has a value range of 2.0 g / cm 3 -5.2 g / cm 3 , the active material itself has a relatively high density, and a relatively high compaction density of the electrode can be achieved. When the ρ A has a value range less than 1.2 g / cm 3 , it will cause a decrease in the compaction density that can be achieved by the electrode, affecting the energy density of the battery; when the ρ A has a value range greater than 5.2 g / cm 3 , it will cause pores in the active material layer, reducing the effective ion migration rate and affecting the electrochemical performance of the battery.

[0034] Specifically, the value range of ρ / ρ A is any point value or range value composed of any two point values among 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; in a preferred embodiment, the value range of ρ / ρ A is 0.65 - 0.85.

[0035] The ratio of the density of the active material layer to the density of the active material represents the volume ratio of the volume in the electrode except for the pores, corresponding to the porosity in the electrode. When the value range of ρ / ρ A is 0.55 - 0.88, the volumetric energy density of the electrode can be improved, and it has excellent ion migration ability; when the ρ / ρ AWhen the value range of is less than 0.55, it will affect the energy density of the lithium battery; when the ρ / ρ A is greater than 0.88, it will reduce the ion migration ability and affect the electrochemical performance of the battery.

[0036] In one embodiment, the electrode includes a positive electrode, and the positive electrode includes a positive current collector and a positive active material layer disposed on at least one side of the positive current collector. The positive active material layer includes a positive active material, a positive conductive agent, a positive binder, and a positive solvent. The effective ion migration rate of the positive electrode is µ C , and the µ C has a value range of 0.02 to 0.5.

[0037] Specifically, the value range of the µ C is any one value or a range value composed of any two point values among 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, or 0.50; in a preferred embodiment, the value range of the µ C is 0.1 to 0.5.

[0038] When the value range of the µ C is 0.02 to 0.5, the effective ion migration rate in the positive electrode is relatively 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 the µ C is less than 0.02, it will cause a large hindrance to the ion migration in the positive electrode, affect the ion transmission, and further affect the electrochemical performance of the battery; when the value range of the µ C is greater than 0.5, although the effective ion migration rate of the positive electrode is high and the kinetic performance of the electrode is good, it is necessary to reduce the areal density or tap density of the active material or further reduce the binder content, etc. on the basis of the existing technical solution, which will lead to a decrease in the energy density of the battery.

[0039] In one embodiment, the density of the positive active material layer is ρ C , and the value range of the ρ C is 2.0 - 4.5, with the unit of g / cm 3 ; the density of the positive active material is ρ CA , and the value range of the ρ CA is 2.3 - 5.2, with the unit of g / cm 3 ; the ρ C and the ρ CA satisfy the relationship: 0.60 ≤ ρ C / ρ CA ≤ 0.88.

[0040] Specifically, the value of ρ C ranges from 2.0 g / cm 3 to 2.1 g / cm 3 to 2.2 g / cm 3 to 2.3 g / cm 3 to 2.4 g / cm 3 to 2.5 g / cm 3 to 2.6 g / cm 3 to 2.7 g / cm 3 to 2.8 g / cm 3 to 2.9 g / cm 3 to 3.0 g / cm 3 to 3.1 g / cm 3 to 3.2 g / cm 3 to 3.3 g / cm 3 to 3.4 g / cm 3 to 3.5 g / cm 3 to 3.6 g / cm 3 to 3.7 g / cm 3 to 3.8 g / cm 3 to 3.9 g / cm 3 to 4.0 g / cm 3 to 4.1 g / cm 3 to 4.2 g / cm 3 to 4.3 g / cm 3 to 4.4 g / cm 3 or 4.5 g / cm 3 among any point value or range value composed of any two point values; in a preferred embodiment, the value of ρ C ranges from 2.3 g / cm 3 to 4.3 g / cm 3 .

[0041] When the value range of the ρ C is from 2.0 g / cm 3 to 4.5 g / cm 3 , the positive electrode active material itself has a relatively high density, and a relatively high compaction density of the positive electrode plate can be achieved; when the value range of the ρ C is less than 2.0 g / cm 3 , it will cause the components in the active material layer not to be in close contact, affecting the electron conduction between the components, and there are more pores in the active material layer, and more electrolyte needs to be added additionally to fill the pores, which will lead to a decrease in the energy density of the battery and an increase in the manufacturing cost; when the value range of the ρ C is greater than 4.5 g / cm 3When this occurs, the components in the active material layer come into contact too closely, reducing the pores in the active material layer and lowering the effective ion migration rate, which will affect the electrochemical performance of the battery.

[0042] Specifically, the value of ρ CA ranges from 2.3 g / cm 3 to 2.4 g / cm 3 to 2.5 g / cm 3 to 2.6 g / cm 3 to 2.7 g / cm 3 to 2.8 g / cm 3 to 2.9 g / cm 3 to 3.0 g / cm 3 to 3.1 g / cm 3 to 3.2 g / cm 3 to 3.3 g / cm 3 to 3.4 g / cm 3 to 3.5 g / cm 3 to 3.6 g / cm 3 to 3.7 g / cm 3 to 3.8 g / cm 3 to 3.9 g / cm 3 to 4.0 g / cm 3 to 4.1 g / cm 3 to 4.2 g / cm 3 to 4.3 g / cm 3 to 4.4 g / cm 3 to 4.5 g / cm 3 to 4.6 g / cm 3 to 4.7 g / cm 3 to 4.8 g / cm 3 to 4.9 g / cm 3 to 5.0 g / cm 3 to 5.1 g / cm 3 or any value within the range formed by any two of these values; in a preferred embodiment, the value of ρ 3 ranges from 3.4 g / cm CA to 5.1 g / cm 3 - 5.1 g / cm 3 .

[0043] When the value of ρ CA ranges from 2.3 g / cm 3 to 5.2 g / cm 3 , the positive electrode active material itself has a relatively high density, enabling a relatively high compaction density of the positive electrode plate. When the value of ρ CA is less than 2.3 g / cm 3When this occurs, the achievable compaction density of the positive electrode sheet will decrease, affecting the energy density of the battery; when the value of ρ CA is greater than 5.2 g / cm 3 it will result in pores in the active material layer, reducing the effective ion migration rate and affecting the electrochemical performance of the battery.

[0044] Specifically, the value range of ρ C / ρ CA is any one value or a range value composed of any two of the values 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; in a preferred embodiment, the value range of ρ C / ρ CA is 0.65 - 0.85.

[0045] When the value range of ρ C / ρ CA is 0.60 - 0.88, it can improve the volumetric energy density of the electrode sheet and has excellent ion migration ability. When the value range of ρ C / ρ CA is less than 0.60, there are more pores in the positive active material layer, and more electrolyte needs to be added additionally to fill the pores, which will lead to a decrease in the battery energy density and an increase in the manufacturing cost; when the value range of ρ C / ρ CA is greater than 0.88, it will cause a decrease in the migration energy of ions in the electrode sheet.

[0046] In one embodiment, the positive active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate; one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, lithium-rich manganese-based materials; or, one or more of layered sodium-based positive electrodes such as sodium iron sulfate, sodium iron phosphate, sodium pyrophosphate iron, sodium pyrophosphate phosphate iron, sodium vanadium phosphate, sodium copper iron manganese, sodium iron nickel manganese, and Prussian blue positive electrodes; 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 its modified polymers; 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 microflakes.

[0047] In one embodiment, the thickness of the positive electrode active material layer is 50 μm to 200 μm.

[0048] Specifically, the thickness of the positive electrode active material layer is any one value or a range value composed of any two point values among 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, 150 μm, 160 μm, 170 μm, 180 μm, 190 μm, or 200 μm; in a preferred embodiment, the thickness of the positive electrode active material layer is 70 μm to 150 μm.

[0049] When the thickness of the positive electrode active material layer is 50 μm to 200 μm, the positive electrode 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 is less than 50 μm, the energy density of the positive electrode is low; when the thickness of the positive electrode is greater than 200 μm, the effective ion migration rate decreases, and the battery kinetic performance deteriorates.

[0050] In one embodiment, the electrode includes a negative electrode, the negative electrode includes a negative current collector and a negative electrode active material layer disposed on at least one side of the negative 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 is μ A where the μ A has a value range of 0.025 to 0.35.

[0051] Specifically, the μ A has a value range of any one value or a range value composed of any two point values among 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; in a preferred embodiment, the μ A has a value range of 0.12 to 0.3.

[0052] When the μ AThe value range of is 0.025 - 0.35. The effective ion migration rate in the negative electrode sheet is relatively high, which is beneficial to the migration and transmission of ions in the sheet, has better kinetic performance, and can improve the electrochemical performance of the battery; when the μ A is less than 0.025, it will cause a large hindrance to the ion migration in the negative electrode sheet, affect the ion transmission, and further affect the electrochemical performance of the battery; when the μ A is greater than 0.35, although the effective ion migration rate of the negative electrode sheet is high and the kinetic performance of the sheet 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.

[0053] In one embodiment, the density of the negative active material layer is ρ B , and the value range of the ρ B is 1.2 - 2.2, with the unit of g / cm 3 ; the density of the negative active material is ρ BA , and the value range of the ρ BA is 2.0 - 3.5, with the unit of g / cm 3 ; the ρ B and the ρ BA satisfy the relationship: 0.55 ≤ ρ B / ρ BA ≤ 0.82.

[0054] Specifically, the value range of the ρ B is any point value or the range value composed of any two point values among 1.2 g / cm 3 , 1.3 g / cm 3 , 1.4 g / cm 3 , 1.5 g / cm 3 , 1.6 g / cm 3 , 1.7 g / cm 3 , 1.8 g / cm 3 , 1.9 g / cm 3 , 2.0 g / cm 3 , 2.1 g / cm 3 or 2.2 g / cm 3 ; in a preferred embodiment, the value range of the ρ B is 1.4 g / cm 3 - 2.0 g / cm 3 .

[0055] When the value range of the ρ B is 1.2 g / cm 3 - 2.2 g / cm 3, the negative electrode active material has a high density, the negative electrode sheet has a high tap density, has a high energy density, and has good electrochemical performance. When the ρ B is less than 1.2 g / cm 3 , it will cause the components in the active material layer not to be in close contact, affecting the electron conduction between the components, and there are more pores in the active material layer, and more electrolyte needs to be added additionally to fill the pores, which will lead to a decrease in the energy density of the battery and an increase in the manufacturing cost; when the ρ B is greater than 2.2 g / cm 3 , it will cause the components in the active material layer to be in too close contact, reducing the pores in the active material layer and reducing the effective ion migration rate, which will affect the electrochemical performance of the battery.

[0056] Specifically, the ρ BA is in the range of 2.0 g / cm 3 , 2.1 g / cm 3 , 2.2 g / cm 3 , 2.3 g / cm 3 , 2.4 g / cm 3 , 2.5 g / cm 3 , 2.6 g / cm 3 , 2.7 g / cm 3 , 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.1 g / cm 3 , 3.2 g / cm 3 , 3.3 g / cm 3 , 3.4 g / cm 3 , 3.5 g / cm 3 any point value or range value composed of any two point values in; in a preferred embodiment, the ρ BA is in the range of 2.2 g / cm 3 -3.2 g / cm 3 .

[0057] When the ρ BA is in the range of 2.0 g / cm 3 -3.5 g / cm 3 , the negative electrode active material itself has a high density, and a high tap density of the negative electrode sheet can be achieved. When the ρ BA is less than 2.0 g / cm 3 , it will cause the tap density that can be achieved by the negative electrode sheet to decrease, affecting the energy density of the battery; when the ρ BA is greater than 3.5 g / cm 3When this occurs, it will cause pores in the active material layer, reducing the effective ion migration rate and affecting the electrochemical performance of the battery.

[0058] Specifically, the value range of ρ B / ρ BA is any one value or a range value composed of any two point values among 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; in a preferred embodiment, the value range of ρ B / ρ BA is 0.6 - 0.8.

[0059] When the value range of ρ B / ρ BA is 0.55 - 0.82, it can improve the volume energy density of the electrode sheet and has excellent lithium ion migration ability; when the ρ B / ρ BA value range is less than 0.55, it will affect the energy density of the lithium battery; when the ρ B / ρ BA value range is greater than 0.82, it will reduce the ion migration ability and affect the electrochemical performance of the battery.

[0060] 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.; 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 its modified polymers, polyacrylonitrile, modified polyacrylonitrile, polymethyl methacrylate and its modified polymers; 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, graphite microflakes.

[0061] In one embodiment, the thickness of the negative electrode active material layer is 40um - 150um.

[0062] Specifically, the thickness of the negative electrode active material layer is any value among 40um, 50um, 60um, 70um, 80um, 90um, 100um, 110um, 120um, 130um, 140um or 150um, or a range value composed of any two of these values; in a preferred embodiment, the thickness of the negative electrode active material layer is 45um to 100um.

[0063] When the thickness of the negative electrode plate is 40um to 150um, the negative electrode plate has a relatively high energy density and a relatively high effective ion migration rate, and the electrode has good electrochemical performance; when the thickness of the negative electrode plate is less than 40um, it will result in a relatively low energy density of the negative electrode plate; when the thickness of the negative electrode plate is greater than 150um, it will result in a decrease in the effective ion migration rate and a deterioration of the battery kinetic performance.

[0064] In one embodiment, the second aspect of the present invention provides a method for preparing a secondary battery. The method for preparing the electrode plate includes the following steps: 1) Mix the active material, conductive agent, binder and solvent to obtain a precursor of the active material layer; 2) Thinning the precursor of the active material layer at least once to obtain the active material layer; 3) The thinning method in 2) can be screw extrusion thinning or rolling thinning; 4) Composite the active material layer with the current collector; 5) After composite, perform freeze-drying or supercritical drying, and then roll to obtain the electrode plate.

[0065] Specifically, mix the positive electrode active material, positive electrode binder and positive electrode conductive agent to form a positive electrode mixture, then add a positive electrode solvent and mix to obtain a precursor of the positive electrode active material; extrude or roll the precursor of the positive electrode active material through a die to obtain a positive electrode active material layer, composite the positive electrode active material layer with the positive electrode current collector, perform freeze-drying below the melting point temperature of the positive electrode solvent to remove the solvent, and roll to obtain the positive electrode plate; Mix the negative electrode active material, negative electrode binder and negative electrode conductive agent to form a negative electrode mixture, then add a negative electrode solvent and mix to obtain a precursor of the negative electrode active material; extrude or roll the precursor of the negative electrode active material through a die to obtain a negative electrode active material layer, composite the negative electrode active material layer with the negative electrode current collector, perform freeze-drying below the melting point temperature of the negative electrode solvent to remove the solvent, and roll to obtain the negative electrode plate; 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.

[0066] Specifically, the freezing temperature of deionized water < 0 °C, the freezing temperature of N-methylpyrrolidone < -24 °C, the freezing temperature of dimethyl sulfoxide < 18 °C, the freezing temperature of acetic acid < 16 °C, the freezing temperature of ethylene carbonate < 35 °C, the freezing temperature of propylene carbonate < -49 °C, the freezing temperature of dimethyl carbonate < 2 °C, the freezing temperature of diethyl carbonate < -43 °C, and the freezing temperature of ethyl methyl carbonate < -14 °C.

[0067] When freeze-drying the electrode sheet below the melting point of the solvent, the solvent in the active material layer of the electrode sheet can be transformed from a liquid state into a solid crystal, and then leave the electrode sheet in the form of sublimation. This can not only maintain the pore structure in the active material layer, but also avoid the migration of the binder caused by the direct transformation of the solvent from a liquid state to a gaseous state. It can improve the volumetric energy density of the positive electrode sheet and has excellent ion migration ability; when freeze-drying the electrode sheet above the melting point of the solvent, it will cause the solvent to not be transformed from a liquid state into a solid crystal, but directly be transformed from a liquid state into a gaseous state and leave the electrode sheet, which will cause the migration of the binder and the deterioration of the pore structure, affecting the electrochemical performance. Or, respectively compound the positive electrode active material layer with the positive electrode current collector, and the negative electrode active material layer with the negative electrode current collector, and then use a supercritical fluid to dry and remove the solvent under a supercritical state and roll-press to obtain the positive electrode sheet and the negative electrode sheet. Specifically, the types of supercritical fluids include any one or a combination of at least two of carbon dioxide, chlorotrifluoromethane, ethane, ethylene, or ammonia; when using carbon dioxide as the supercritical fluid, the drying temperature is ≥ 31 °C and the drying pressure is ≥ 7.38 Mpa; when using chlorotrifluoromethane as the supercritical fluid, the drying temperature is ≥ 28.8 °C and the drying pressure is ≥ 3.95 Mpa; when using ethane as the supercritical fluid, the drying temperature is ≥ 32.5 °C and the drying pressure is ≥ 4.6 Mpa; when using ethylene as the supercritical fluid, the drying temperature is ≥ 9.5 °C and the drying pressure is ≥ 5.06 Mpa. Specifically, mix the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder in a ratio of (94.5 - 98.5):(0.5 - 2.5):(1.0 - 3.0) to obtain a positive electrode mixture, and then add a positive electrode solvent. Based on the total mass of the positive electrode mixture being 100%, the mass ratio of the positive electrode solvent is α1, and the value range of α1 is 0 < α1 ≤ 20%. Mix the negative electrode active material, the negative electrode conductive agent, and the negative electrode binder in a ratio of (92.0~98.5):(0.5~3.0):(1.0~5.0) to obtain a negative electrode mixture, and then add a negative electrode solvent. Based on the total mass of the negative electrode mixture being 100%, the mass ratio of the negative electrode solvent is α2, and the value range of α2 is 0 < α2 ≤ 40%.

[0068] 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, where the range of X1 is 70% ≤ X1 < 100%. The solid content of the negative electrode active material layer precursor is X2, where the range of X2 is 60% ≤ X2 < 100%.

[0069] Specifically, the solid content of the positive electrode active material layer precursor is any one value or a range value composed of any two values among 70%, 75%, 80%, 85%, 90%, 95%, or 99%. In a preferred embodiment, the solid content of the positive electrode paste mass is 75% - 95%.

[0070] When the range of X1 is 70% ≤ X1 < 100%, the use of solvents can be reduced, and energy consumption in freeze-drying can be decreased. 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.

[0071] Specifically, the solid content of the negative electrode active material layer precursor is any one value or a range value composed of any two values among 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 99%. In a preferred embodiment, the solid content of the negative electrode paste is 65% - 90%.

[0072] When the range of X2 is 60% ≤ X1 < 100%, the use of solvents can be reduced, and energy consumption in freeze-drying or supercritical drying can be decreased. Also, the drying time can be reduced, and production efficiency can be improved. 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 both the energy consumption and time required for freeze-drying or supercritical drying, affecting production efficiency and increasing manufacturing costs.

[0073] In one embodiment, in step 5), the conditions for freeze-drying include: the temperature is below the melting point temperature of the solvent, and the pressure vacuum degree is 0.1 Pa - 100 Pa.

[0074] Specifically, by compounding the positive electrode active material layer with the positive electrode current collector and performing freeze-drying below the melting point temperature of the positive electrode solvent, the positive electrode solvent crystallizes and then sublimes through the low-temperature condition, which is different from the existing baking and volatilization process. It can reduce the migration of the positive electrode binder and the change in pores in the positive electrode sheet caused by the volatilization of the positive electrode solvent, ensuring that the original pore structure in the positive electrode sheet can be maintained during the removal of the positive electrode solvent. It can improve the volumetric energy density of the positive electrode sheet and has excellent ion migration ability.

[0075] The negative electrode active material layer is compounded with the negative electrode current collector, and freeze-drying is carried out 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, which is different from the existing baking and 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 pores in the negative electrode sheet, ensuring that the original pore structure in the negative electrode sheet can be maintained during the removal of the negative electrode solvent. Further, pores can be created to improve the pores in the electrode sheet; it can increase the volume energy density of the negative electrode sheet and has excellent ion migration ability.

[0076] Specifically, the pressure vacuum degree is any one value among 0.1 Pa, 1 Pa, 5 Pa, 10 Pa, 20 Pa, 30 Pa, 40 Pa, 50 Pa, 60 Pa, 70 Pa, 80 Pa, 90 Pa or 100 Pa, or a range value composed of any two point values. In a preferred embodiment, the pressure vacuum degree is 1 Pa to 80 Pa.

[0077] When the pressure vacuum degree is 0.1 Pa to 100 Pa, sublimation can occur simultaneously during the freezing and crystallization process of the solvent to leave the electrode sheet, ensuring that the original pore structure in the electrode sheet can be maintained during the removal of the solvent; when the pressure vacuum degree is less than 0.1 Pa, the energy consumption for maintaining the vacuum degree needs to be increased. More critically, too low a vacuum degree will also break the equilibrium process of the solvent from liquid to crystal to sublimation, thus affecting the pore structure in the electrode sheet; when the pressure vacuum degree is greater than 100 Pa, the sublimation rate after the solvent crystallization will be slow, thus prolonging the processing time and affecting the production efficiency.

[0078] In one embodiment, in step 5), the conditions for supercritical drying include: the types of supercritical fluids used include any one or a combination of at least two of carbon dioxide, chlorotrifluoromethane, ethane or ethylene; the temperature is the critical temperature for maintaining the supercritical fluid state; the pressure is the critical pressure for maintaining the supercritical fluid state.

[0079] Specifically, the positive electrode active material layer is compounded with the positive electrode current collector, and supercritical drying is carried out under the temperature and pressure conditions for maintaining the supercritical fluid state. The positive electrode solvent is removed by supercritical drying under supercritical conditions, which is different from the existing baking and 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 pores in the positive electrode sheet, ensuring that the original pore structure in the positive electrode sheet can be maintained during the removal of the positive electrode solvent; it can increase the volume energy density of the positive electrode sheet and has excellent ion migration ability.

[0080] The negative electrode active material layer is compounded with the negative electrode current collector, and supercritical drying is carried out under the temperature and pressure conditions that maintain the supercritical fluid state. The negative electrode solvent is removed by supercritical drying under supercritical conditions, which is different from the existing baking and volatilization process. It can reduce the migration of the negative electrode binder and the change of pores in the negative electrode sheet caused by the volatilization of the negative electrode solvent, 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.

[0081] The beneficial effects of the present invention are further described below in conjunction with embodiments.

[0082] In order to make the invention object, technical solution and beneficial technical effects of the present invention clearer, the present invention is further described in detail below in conjunction with embodiments. However, it should be understood that the embodiments of the present invention are only for explaining the present invention and not for limiting the present invention, and the embodiments of the present invention are not limited to the embodiments given in the specification. The specific experimental conditions or operating conditions not specified in the embodiments are made according to the conventional conditions or according to the conditions recommended by the material suppliers.

[0083] In addition, it should be understood that one or more method steps mentioned in the present invention do not exclude the existence of other method steps before and after the combined steps or the insertion of other method steps between these clearly mentioned steps, unless otherwise stated; it should also be understood that the combined connection relationship between one or more devices / devices mentioned in the present invention does not exclude the existence of other devices / devices before and after the combined devices / devices or the insertion of other devices / devices between these two clearly mentioned devices / devices, unless otherwise stated. Moreover, unless otherwise stated, the numbers of the method steps are only convenient tools for identifying the method steps, rather than limiting the arrangement order of the method steps or limiting the scope of implementation of the present invention. The change or adjustment of their relative relationship, without substantial change in the technical content, should also be regarded as the scope of implementation of the present invention.

[0084] In the following embodiments, the reagents, materials and instruments used can be commercially purchased or obtained by synthetic methods well-known in the art without special instructions.

[0085] Table 1 Design of the positive electrode sheets and negative electrode sheets of Examples 1-24 and Comparative Examples 1-7

[0086] Example 1 This example is used to illustrate the positive secondary battery disclosed by the present invention; it includes the following operation steps: Fabrication of the positive electrode sheet: Mix the positive electrode active material lithium iron phosphate, conductive agent carbon black, and binder PVDF in a mass ratio of 96.5:1.5:2.0; add the positive electrode solvent NMP to form a precursor of the positive electrode active material with a solid content of 80%. Extrude this active material precursor through a die head to obtain a positive electrode active material layer. Compound the positive electrode active material layer with the positive electrode current collector and freeze-dry it at a vacuum degree of 1 Pa and a temperature of -40 °C for 6 h, then roll-press it to obtain a positive electrode plate. The thickness of the positive electrode active material layer is 90 μm.

[0087] Fabrication of the negative electrode plate: Mix the negative electrode active material graphite negative electrode, conductive agent carbon black, binder styrene-butadiene rubber, and thickening agent sodium carboxymethyl cellulose in a mass ratio of 95.4:1.5:1.6:1.5. Add deionized water, the negative electrode solvent, and stir well to form a precursor of the negative electrode active material with a solid content of 80%. Extrude the negative electrode active material precursor through a die head to obtain a negative electrode active material layer. Compound the negative electrode active material layer with the negative electrode current collector and freeze-dry it at a vacuum degree of 1 Pa and a temperature of -10 °C for 8 h, then roll-press it to obtain a negative electrode plate. The thickness of the negative electrode active material layer is 75 μm.

[0088] Fabrication of the battery: Stack the positive electrode plate, separator, and negative electrode plate in sequence, with the separator in the middle of the positive and negative electrodes to play an isolation role. Repeat the stacking of the positive electrode plate, separator, and negative electrode plate to form a core. Then place the stacked core in an aluminum-plastic film bag that has been formed by punching the shell. Inject the electrolyte prepared above into the baked and dried battery cell respectively. After processes such as vacuum packaging, standing, and formation, a battery with a capacity of 1.2 Ah is obtained.

[0089] Examples 2 - 24 Examples 2 - 24 are used to illustrate the secondary battery disclosed in the present invention, including most of the operation steps in Example 1. The differences are as follows: The parameters for manufacturing the positive electrode plate and negative electrode plate shown in Table 1 are different.

[0090] Comparative Examples 1 - 7 Comparative Example 1 is used to illustrate the secondary battery disclosed in the present invention, including most of the operation steps in Example 1. The differences are as follows: The parameters for manufacturing the positive electrode plate and negative electrode plate shown in Table 1, and the drying method.

[0091] Performance testing Perform the following performance tests on the batteries prepared in the above Examples 1 - 24 and Comparative Examples 1 - 7: 1. Effective ion migration rate: Battery fabrication: After assembling two positive electrode plates with a state of charge of 50% and a separator placed between the two electrode plates, electrolyte is added and sealed to obtain a symmetric battery, and the area and thickness of the electrode plates are recorded; And / or, after assembling two negative electrode plates with a state of charge of 50% and a separator placed between the two electrode plates, electrolyte is added and sealed to obtain a symmetric battery, and the area and thickness of the electrode plates are recorded; Testing: Use an electrochemical workstation to perform electrochemical impedance spectroscopy testing on the above symmetric battery, with a perturbation voltage of 5 mV and a test frequency range of 100 kHz to 0.01 Hz, to obtain an electrochemical impedance spectroscopy Nyquist plot with the shape characteristics of the intersection of the low-frequency region line segment and the high-frequency region line segment; Calculating the ion migration rate: Extend the low-frequency line segment in the Nyquist plot 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 denoted as the impedance R, and substitute it into the following formula to calculate the effective ion migration rate.

[0092] 2. 2C discharge ratio: Charging: At 25 ± 2 °C, charge the battery at a rate of 0.5C at a constant current until 3.65 V and then switch to constant voltage charging, with the cut-off condition being that the current is less than 0.05C; Resting: At 25 ± 2 °C, rest for 10 min after charging; Discharging: At 25 ± 2 °C, then discharge at a constant current of 0.5C to 2.5 V, and record the discharge capacity as C0; Resting: At 25 ± 2 °C, rest for 10 min after discharging; Charging: At 25 ± 2 °C, charge the battery again at a rate of 0.5C at a constant current until 3.65 V and then switch to constant voltage charging, with the cut-off condition being that the current is less than 0.05C Resting: At 25 ± 2 °C, rest for 10 min after charging; Discharging: At 25 ± 2 °C, then discharge at a constant current of 2.0C to 2.5 V, and record the discharge capacity as C1; Discharge ratio: C1 / C0 * 100% is denoted as the 2C discharge ratio.

[0093] 3. 1C constant current charging ratio: Discharging: At 25 ± 2 °C, discharge the battery at a rate of 0.5C at a constant current to 2.5 V; Resting: At 25 ± 2 °C, rest for 10 min after discharging; Charging: At 25 ± 2 °C, then charge at a constant current of 1C to 3.65 V, and record the charging capacity as Ci; Charging: At 25 ± 2 °C, then charge at a constant voltage of 3.65 V until the current is less than 0.05C, and record the charging capacity as Cp; The constant current charging ratio: Ci / (Ci + Cp) * 100% is denoted as the 1C constant current charging ratio.

[0094] 4. DC internal resistance Adjust to 50% SOC: At 25 ± 2 °C, charge the battery at a rate of 0.5C at a constant current until 3.65V, then switch to constant voltage charging. The cut-off condition is that the current is less than 0.05C. After charging is completed, let it stand for 10 minutes, and then discharge it at a constant current of 0.5C to 50% SOC; DC resistance test: Let it stand for 60 minutes, record the end voltage V1, then discharge it with a current of 2C (2.4A) for 30s, and record the end voltage V2; DC internal resistance: (V1 - V2) / 2.4 * 1000 is denoted as the measured DC internal resistance value The test results are shown in Table 2.

[0095] Table 2 Electrochemical performance of secondary battery

[0096] As can be seen from Table 2, comparing Examples 1 - 3 with Comparative Examples 1 - 2, when the density ρ of the positive active material layer is 2.0 g / cm 3 - 4.5 g / cm 3 ; the density ρ of the active material A is 2.2 g / cm 3 - 5.2 g / cm 3 and ρ and ρ A satisfy the relationship: 0.60 ≤ ρ / ρ A ≤ 0.88, the volumetric energy density of the electrode can be improved, and it has excellent ion migration ability; when the value range of ρ is less than 2.0 g / cm 3 , or when the value range of ρ A is less than 2.2 g / cm 3 , or the value range of ρ / ρ A is less than 0.60, it will cause the achievable compaction density of the positive electrode to decrease, there are more pores in the positive active material layer, and more electrolyte needs to be added additionally to fill the pores, resulting in a decrease in the 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 the value range of ρ A is greater than 5.2 g / cm 3 , or the value range of ρ / ρ A is greater than 0.88, it will cause a decrease in the migration energy of ions in the electrode.

[0097] Comparing Example 3 and Examples 15 - 16 with Comparative Examples 3 - 4, when the density ρ of the negative active material layer is 1.2 g / cm 3 - 2.2 g / cm3 ; The density ρ of the active material A is 2.0 g / cm 3 -3.5 g / cm 3 , and ρ and ρ A satisfy the relationship: when 0.55 ≤ ρ / ρ A ≤ 0.82, the volumetric energy density of the electrode can be improved, and it has excellent ion migration ability; when the value range of ρ is less than 1.2 g / cm 3 , or when the value range of ρ A is less than 2.0 g / cm 3 , or ρ / ρ A is less than 0.55, it will cause the achievable compaction density of the negative electrode to decrease, there are more pores in the negative active material layer, and more electrolyte needs to be added additionally to fill the pores, resulting in a decrease in the battery energy density and an increase in the manufacturing cost; when the value range of ρ is greater than 2.2 g / cm 3 , or when the value range of ρ A is greater than 3.5 g / cm 3 , or ρ / ρ A is greater than 0.82, it will cause the migration rate of ions in the electrode to decrease.

[0098] Comparing Example 2 with Examples 4 - 6, when the solid content X1 of the positive active material layer precursor ranges from 70% ≤ X1 < 100%, the use of solvents can be reduced, and the energy loss in freeze-drying can be decreased; when X1 is less than 70%, the solvent content in the positive active material precursor is high, which will lead to an increase in the energy consumption required for freeze-drying.

[0099] Comparing Example 2 with Examples 17 - 19, when the solid content X2 of the negative active material layer precursor ranges from 60% ≤ X2 < 100%, the use of solvents can be reduced, and the energy loss in freeze-drying can be decreased; when X2 is less than 60%, the solvent content in the positive active material precursor is high, which will lead to an increase in the energy consumption required for freeze-drying.

[0100] Comparing Example 2 with Examples 7 - 10, when the thickness of the positive active material layer is 50 um - 200 um, the positive electrode has a higher energy density, a higher effective ion migration rate, and better electrochemical performance; when the thickness of the positive electrode is less than 50 um, the energy density of the positive electrode will be lower; when the thickness of the positive electrode is greater than 200 um, the effective ion migration rate will decrease, and the battery kinetic performance will deteriorate.

[0101] Comparing Comparative Example 2 with Examples 7 - 10, when the thickness of the negative electrode sheet is 40um - 150um, the negative electrode sheet has a relatively high energy density and a relatively high effective ion migration rate, and the electrode has good electrochemical performance; when the thickness of the negative electrode sheet is less than 40um, the energy density of the negative electrode sheet will be relatively low; when the thickness of the negative electrode sheet is greater than 150um, the effective ion migration rate will decrease and the battery kinetic performance will deteriorate.

[0102] Comparing Comparative Example 2 with Example 11, when freeze - drying the positive electrode sheet below the melting point of the positive electrode solvent, allowing the negative electrode solvent to crystallize and then sublimate out under low - temperature conditions, which is different from the existing baking and volatilization process, it can reduce the migration of the positive electrode binder and the change of pores in the positive electrode sheet caused by the volatilization of the positive electrode solvent, ensuring that the original pore structure in the positive electrode sheet can be maintained during the removal of the positive electrode solvent; it can improve the volumetric energy density of the positive electrode sheet and has excellent ion migration ability; when freeze - drying the positive electrode sheet above the melting point of the positive electrode solvent, the solvent in the positive electrode sheet will mainly leave the sheet in the form of gas, resulting in the migration of the positive electrode binder and the deterioration of the pore structure in the sheet, thus causing a decrease in the ion migration ability in the sheet, affecting the electrochemical performance, and will also lead to a significant extension of the drying time, affecting the drying efficiency.

[0103] Comparing Comparative Example 2 with Example 20, when freeze - drying the negative electrode sheet below the melting point of the negative electrode solvent, allowing the negative electrode solvent to crystallize and then sublimate out under low - temperature conditions, which is different from the existing baking and volatilization process, it can reduce the migration of the negative electrode binder and the change of pores in the negative electrode sheet caused by the volatilization of the negative electrode solvent, ensuring that the original pore structure in the negative electrode sheet can be maintained during the removal of the negative electrode solvent, and further can create pores to improve the pore structure; it can improve the volumetric energy density of the negative electrode sheet and has excellent ion migration ability; when freeze - drying the negative electrode sheet above the melting point of the negative electrode solvent, the solvent in the negative electrode sheet will mainly leave the sheet in the form of gas, resulting in the migration of the negative electrode binder and the deterioration of the pore structure in the sheet, thus causing a decrease in the ion migration ability in the sheet, affecting the electrochemical performance, and will also lead to a significant extension of the drying time, affecting the drying efficiency.

[0104] Comparing Comparative Example 2 with Examples 12 - 14, or comparing Comparative Example 2 with Examples 21 - 23; when the pressure vacuum is 0.1 Pa to 100 Pa, sublimation can occur simultaneously during the solvent freeze crystallization process to leave the electrode, ensuring that the original pore structure in the electrode can be maintained during the solvent removal process; when the pressure vacuum is less than 0.1 Pa, the energy consumption for maintaining the vacuum needs to be increased. More critically, too low a vacuum will also disrupt the equilibrium process of the solvent from liquid to crystal to sublimation, thereby affecting the pore structure in the electrode; when the pressure vacuum is greater than 100 Pa, it will lead to a slow sublimation rate after the solvent crystallizes, thereby prolonging the processing time and affecting the production efficiency. Comparing Examples 1 - 23 with Comparative Examples 5 - 7, compared with the traditional wet coating and drying process, the electrode prepared by the present invention with a high solid content and then freeze-dried has a higher ion migration rate, enabling the battery to have lower DC impedance, higher rate discharge and charge, and other electrochemical properties.

[0105] Comparing Example 24 with Comparative Examples 5 - 7, compared with the traditional wet coating and drying process, for the positive and negative electrodes prepared by the present invention with a high solid content and then supercritically dried, the solvent leaves the electrode with the supercritical fluid, avoiding the migration of the binder in the electrode when the solvent leaves the electrode, and being able to maintain the pore structure of the electrode, having a higher ion migration rate, enabling the battery to have lower DC impedance, higher rate discharge and charge, and other electrochemical properties.

[0106] In summary, the technical solution of the present invention, through the electrode prepared from a mixture of a high solid content positive or negative electrode, and then freeze-dried or supercritically dried, enables the solvent in the electrode to leave the electrode without the migration of the binder with the solvent, maintaining the uniform distribution of the binder in the electrode; at the same time, with a higher areal density and higher tap density of the electrode, the distribution of the pore structure in the electrode is maintained. Further, due to the special process effects of freeze-drying or supercritical drying, it is possible to maintain the high pore structure of the electrode, or create pores to further improve the pores in the electrode, enhancing the ion migration rate in the electrode, enabling the battery to have a high energy density and better electrochemical properties, and being suitable for large-scale application in the field of manufacturing secondary batteries with high energy density and high electrochemical properties.

[0107] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A secondary battery, characterized in that: It includes a pole piece, a separator and an electrolyte. The pole piece includes a current collector and an active material layer provided on at least one side of the current collector. The active material layer includes 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 to 0.

5.

2. The secondary battery according to claim 1, wherein: The test method for the ion migration rate μ of the pole piece includes the following steps: 1) Fabricate the pole piece into a symmetric cell and perform electrochemical impedance spectroscopy test to obtain an electrochemical impedance spectroscopy Nyquist diagram with the shape characteristics of the intersection of the low-frequency region line segment and the high-frequency region line segment; 2) Extend the low-frequency line segment in the Nyquist diagram until it intersects with the horizontal axis, and record the difference between the intersection point and the intersection point of the high-frequency line segment and the horizontal axis as the impedance R, with the unit of Ω; 3) Substitute R into the following formula , and calculate the effective ion migration rate; Among them, S is the effective reaction area of the electrode in the symmetric battery, with the unit of m 2 , d is the thickness of the active material layer, with the unit of um, and σ is the ionic conductivity of the electrolyte, with the unit of S / m.

3. The secondary battery according to claim 1, characterized in that: The density of the active material layer is ρ, and the value range of ρ is 1.2 - 4.5, with the unit of g / cm 3 ; The density of the active material is ρ A , and the value range of ρ A is 2.0 - 5.2, with the unit of g / cm 3 ; ρ and ρ A satisfy the relationship: 0.55 ≤ ρ / ρ A ≤ 0.

88.

4. The secondary battery according to claim 3, wherein: The electrode includes a positive electrode, the positive electrode includes a positive current collector and a positive active material layer, the positive active material layer is disposed on at least one side of the positive current collector, the positive active material layer includes a positive active material, a positive conductive agent, a positive binder, and a positive solvent, and the effective ion migration rate of the positive electrode is µ c , the µ c ranges from 0.02 to 0.

5.

5. The secondary battery according to claim 3, wherein: The density of the positive electrode active material layer is ρ C , and the value range of ρ C is 2.0 - 4.5, with the unit of g / cm 3 ; the density of the positive electrode active material is ρ CA , and the value range of ρ CA is 2.2 - 5.2, with the unit of g / cm 3 ; ρ C and ρ CA satisfy the relationship: 0.60 ≤ ρ C / ρ CA ≤ 0.

88.

6. The secondary battery according to claim 3, characterized in that: The positive active material includes one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate; one or more of lithium iron phosphate, lithium manganese iron phosphate, lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, and lithium-rich manganese-based materials; Or, one or more of layered sodium-ion battery cathodes such as sodium iron sulfate, sodium iron phosphate, sodium pyrophosphate iron, sodium pyrophosphate phosphate iron, sodium vanadium phosphate, sodium copper iron manganese, sodium iron nickel manganese, and Prussian blue cathodes; The positive 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 its modified polymers; The positive 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 microflakes.

7. The secondary battery according to claim 3, wherein: The thickness of the positive active material layer is 50 μm to 200 μm.

8. The secondary battery according to claim 3, wherein: The electrode includes a negative electrode, and the negative electrode includes a negative current collector and a negative active material layer. The negative active material layer is disposed on at least one side of the negative current collector. The negative active material layer includes a negative active material, a negative conductive agent, a negative binder, and a negative solvent. The effective ion migration rate of the negative electrode is µ A , and the value of the µ A ranges from 0.025 to 0.

35.

9. The secondary battery according to claim 3, characterized in that: The density of the negative electrode active material layer is ρ B , and the value range of ρ B is 1.2 - 2.2, with the unit of g / cm 3 ; the density of the negative electrode active material is ρ BA , and the value range of ρ BA is 2.0 - 3.5, with the unit of g / cm 3 ; ρ B and ρ BA satisfy the relation: 0.55 ≤ ρ B / ρ BA ≤ 0.

82.

10. The secondary battery according to claim 3, characterized in that: The negative active material includes one or more of artificial graphite, natural graphite, soft carbon, hard carbon, silicon, silicon oxide, silicon carbon, lithium titanate, etc.; The negative 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 its modified polymers, polyacrylonitrile, modified polyacrylonitrile, poly(methyl methacrylate) and its modified polymers; The negative 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 microflakes.

11. The secondary battery according to claim 3, characterized in that: The thickness of the negative active material layer is 40 μm to 150 μm.

12. A method for preparing a secondary battery according to any one of claims 1-11, characterized in that: The preparation method of the pole piece includes the following steps: 1) Mix the active material, conductive agent, binder and solvent to obtain a precursor of the active material layer; 2) Thinning the precursor of the active material layer at least once to obtain the active material layer; 3) The thinning method in 2) can be screw extrusion thinning or rolling thinning; 4) Composite the active material layer with the current collector; 5) After the composite, perform freeze-drying or supercritical drying, and then roll-press to obtain the electrode sheet.

13. The method for preparing a secondary battery according to claim 12, characterized in that: 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.

14. The method for preparing a secondary battery according to claim 12, 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%. The solid content of the negative electrode active material layer precursor is X2, and the range of X2 is 60% ≤ X2 < 100%.

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

16. The method for preparing a secondary battery according to claim 12, characterized in that: In step 5), the conditions for supercritical drying include: the types of supercritical fluids used include any one or a combination of at least two of carbon dioxide, chlorotrifluoromethane, ethane, or ethylene; the temperature is the critical temperature for maintaining the supercritical fluid state; the pressure is the critical pressure for maintaining the supercritical fluid state.

Images