Electrode plate, preparation method thereof and battery

Abstract

The invention provides an electrode plate and a preparation method thereof and a battery, the electrode plate comprises a current collector and a material layer arranged on at least one surface of the current collector, the material layer comprises a first layer, a second layer and a third layer in the direction from the adjacent current collector to the far away from the current collector, the tortuosity of the first layer is A, the tortuosity of the second layer is B, the tortuosity of the third layer is A, and the tortuosity of the third layer is A; the tortuosity of the third layer is C, A is smaller than B, and C is smaller than B. According to the electrode plate provided by the invention, the tortuosity characteristics of different layer structures are limited, so that the problems of lithium ion diffusion and difficult electrolyte infiltration of a thick electrode can be improved, and the rate capability and the cyclicity of a battery can be improved.

Description

Technical Field

[0001] The present invention relates to the field of battery materials, and particularly to an electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] With the advent of the electronic information era, lithium-ion batteries have been widely popularized in fields such as electronic products and new energy vehicles. Consumers' increasing demand for devices with longer battery life and lighter weight has promoted the research and application of high-energy-density batteries.

[0003] In order to improve the energy density of lithium-ion batteries, the research on thick electrode technology has emerged. In the traditional lithium-ion battery manufacturing process, due to the inevitable evaporation process of solvents (such as NMP), wet thick electrode coatings are prone to cracking after baking. At the same time, the excessive thickness of the electrode sheet leads to uneven longitudinal lithium-ion concentration distribution, with a higher lithium-ion concentration near the separator side, making it difficult to fully diffuse to the current collector side, thus reducing battery performance. Dry electrodes do not require solvents, so it is more conducive to manufacturing thick electrodes in terms of process. However, traditional dry thick electrodes still have problems such as difficulty in lithium-ion diffusion and poor electrolyte infiltration, which affect the rate performance and cycle performance of the battery. Summary of the Invention

[0004] The present invention provides an electrode sheet. By defining the tortuosity characteristics of different layer structures, the electrode sheet provided by the present invention can improve the problems of lithium-ion diffusion and poor electrolyte infiltration in thick electrodes, which is beneficial to improving the rate performance and cycle performance of the battery.

[0005] The present invention also provides a preparation method of the above electrode sheet. This preparation method can prepare the above electrode sheet, and the process is simple.

[0006] The present invention also provides a battery. Since the battery includes the above electrode sheet, the battery has excellent rate performance and cycle performance.

[0007] In a first aspect, the present invention provides an electrode sheet, including a current collector and a material layer provided on at least one surface of the current collector. In the direction from adjacent to the current collector to away from the current collector, the material layer includes: a first layer, a second layer, and a third layer. The tortuosity of the first layer is A, the tortuosity of the second layer is B, and the tortuosity of the third layer is C, where A is less than B, and C is less than B.

[0008] Optionally, 1.2 ≤ A < 1.5;

[0009] And / or, 1.5 ≤ B ≤ 2.1;

[0010] And / or, 1.2 ≤ C < 1.5.

[0011] Optionally, the tortuosity of the electrode sheet is 2 - 2.52.

[0012] Optionally, the first layer includes a first active material, a first binder, and a first conductive agent; the second layer includes a second active material, a second binder, and a second conductive agent; the third layer includes a third active material, a third binder, and a third conductive agent; wherein, the first binder and the third binder include polyvinylidene fluoride, and the second binder includes polytetrafluoroethylene and / or a copolymer of tetrafluoroethylene.

[0013] Optionally, in the second layer, the mass ratio of the second binder is 1.5 wt% - 2.5 wt%;

[0014] and / or, the number average molecular weight of the second binder is 9.50×10 6 Da - 2×10 7 Da.

[0015] Optionally, the thickness of the first layer is 30 - 50 µm;

[0016] and / or, the thickness of the second layer is 20 - 60 µm;

[0017] and / or, the thickness of the third layer is 30 - 50 µm.

[0018] Optionally, the resistance of the electrode sheet is 470 mΩ - 540 mΩ;

[0019] and / or, the diffusion coefficient of the electrode sheet is 2.30×10 -13 cm 2 / s - 3.90×10 -13 cm 2 / s.

[0020] Optionally, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0021] In a second aspect, the present invention provides a method for preparing the above electrode sheet, comprising the following steps:

[0022] A first solid mixture containing a first active material, a first binder, and a first conductive agent is sprayed on at least one surface of a current collector, and after hot pressing and melting, a first layer is formed;

[0023] A second solid mixture containing a second active material, a second binder, and a second conductive agent is kneaded by a roller and rolled into a film sheet, and through hot pressing, the film sheet is bonded to the first layer to form a second layer;

[0024] A third solid mixture containing a third active material, a third binder, and a third conductive agent is sprayed on the surface of the second layer, and after hot pressing and melting, the electrode sheet is formed.

[0025] Optionally, spraying the first solid mixture on at least one surface of the current collector includes:

[0026] Atomizing and spraying the first solid mixture on at least one surface of the current collector by using a first air flow and a first static electricity;

[0027] And / or, spraying the third solid mixture on the surface of the second layer includes:

[0028] Atomizing and spraying the third solid mixture on the surface of the second layer by using a second air flow and a second static electricity.

[0029] Optionally, the pressure of the first air flow is 100 - 150 kPa, and the voltage of the first static electricity is 20 - 30 kV;

[0030] And / or, the pressure of the second air flow is 100 - 150 kPa, and the voltage of the second static electricity is 20 - 30 kV.

[0031] In a third aspect, the present invention provides a battery including the above-mentioned electrode sheet.

[0032] By defining the tortuosity characteristics of different layer structures, the electrode sheet provided by the present invention can improve the problems of lithium ion diffusion and difficult electrolyte infiltration in thick electrodes, thereby facilitating the improvement of the rate performance and cycle performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The accompanying drawings herein are incorporated into the specification and form a part of the specification, showing embodiments consistent with the present invention and used together with the specification to explain the principles of the present invention.

[0034] Figure 1 It is a schematic diagram of the microstructure of the positive electrode sheet in a specific embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0035] To enable those skilled in the art to better understand the solution of the present invention, the present invention will be further described in detail below. The specific embodiments listed below only describe the principles and features of the present invention, and the examples given are only used to explain the present invention and do not limit the scope of 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 efforts fall within the scope of protection of the present invention.

[0036] To solve the problems of difficult lithium-ion diffusion in dry thick electrodes and poor electrolyte infiltration, the traditional method is to use pore-forming agents to prepare electrode sheets with a pore structure of gradually decreasing gradient from the current collector to the electrode sheet surface. Therefore, the high porosity near the electrode sheet can provide more space for electrolyte penetration and ion movement, which is usually more conducive to improving the ionic conductivity of the material. However, the high porosity also results in a longer and more complex actual ion transport path, thus offsetting some of the benefits brought by the increased porosity. Therefore, the gradient-changing porosity cannot improve the inherent ion diffusion difficulty problem of thick electrodes.

[0037] For thick electrodes, there are the following three aspects of electrochemical processes inside the battery: 1. The liquid-phase transport of lithium ions in the pores of the electrode sheet, diffusion across the SEI film, and diffusion inside the solid particles. As the battery thickness increases, the lithium-ion transport process originally limited by solid-phase diffusion gradually shifts towards liquid-phase transport in the pores; 2. The transport of electrons at the current collector / electrode sheet interface and within the main body of the material layer. The transport rate of this process depends on the state of the current collector / electrode sheet interface and the conductive agent network. The electron current density is the largest at the current collector / electrode sheet interface, and it gradually decreases from this interface to the separator. This non-uniform gradient is more obvious in thick electrodes; 3. The charge transfer at the electrode / electrolyte interface, which is accompanied by the electric double layer and electrochemical reactions and is related to the specific surface area of the active material particles and the electrolyte wettability.

[0038] The tortuosity refers to the ratio of the actual ion transport path length to the straight-line distance and can be used to reflect the complexity of the ion transport path in a porous medium. To achieve a high liquid-phase diffusion rate in thick electrodes, excellent electron conduction in the main body of the electrode, and a relatively high charge transfer efficiency at the electrode / electrolyte interface, without changing the type of electrolyte and the cathode material, the above problems can be effectively solved by designing an electrode sheet with a hierarchical tortuosity. Specifically, the present invention adopts the following technical solutions:

[0039] In a first aspect, the present invention provides an electrode sheet, comprising a current collector and a material layer provided on at least one surface of the current collector. The material layer includes, from the direction adjacent to the current collector to the direction away from the current collector: a first layer, a second layer, and a third layer. The tortuosity of the first layer is A, the tortuosity of the second layer is B, and the tortuosity of the third layer is C, wherein A is less than B, and C is less than B.

[0040] By defining the tortuosity characteristics of different layer structures, the electrode sheet of the present invention can improve the problems of lithium-ion diffusion and poor electrolyte infiltration in thick electrodes, thereby facilitating the improvement of the rate performance and cycle performance of the battery. The main reasons include: when the above electrode sheet is used in a battery, in the direction from adjacent to the current collector to away from the current collector, the tortuosity of the material layer of the electrode sheet shows a regular change of low-medium-low. Among them, the first layer has low tortuosity characteristics and is in direct contact with the electrolyte, which can promote electrolyte infiltration, improve the charge transfer rate, and increase the interfacial specific surface area; when the second layer has a higher tortuosity, it has the following functions: it can increase the mechanical stability and structural integrity of the electrode sheet, which is beneficial to ensuring the stability of the battery, providing sufficient active materials to contact the electrolyte, ensuring the uniformity of the reaction, regulating the fluidity of the electrolyte, and preventing electrolyte loss caused by too fast penetration; due to the above functions, the second layer can cooperate with the first layer to enable the electrode to achieve better charge and discharge performance without sacrificing energy density; the third layer has low tortuosity characteristics and can cooperate with the low-tortuosity layer of the first layer to quickly respond to current changes, improve the wettability of the side of the electrode sheet adjacent to the electrolyte, the difference in the distribution of the lithium-ion concentration gradient on the inner side, and the problem of electron transport difficulties caused by the conductive agent network, thereby improving the kinetic performance and the rate performance of the battery.

[0041] In a specific embodiment, optionally, 1.2 ≤ A < 1.5;

[0042] and / or, 1.5 ≤ B ≤ 2.1;

[0043] and / or, 1.2 ≤ C < 1.5.

[0044] In the above-described embodiment, when the first layer and the third layer of the electrode sheet adopt the tortuosity within the above range, a more direct ion transport path can be provided, reducing the transport resistance, which helps to improve the reaction rate and overall conductivity of the electrode sheet; when the second layer of the electrode sheet adopts the tortuosity within the above range, the complexity of the ion transport path is increased, which helps to more evenly distribute the ions and current density, prevent local overreaction, and further ensure the stability of the electrode.

[0045] In some embodiments, the tortuosity of the electrode sheet is 2 - 2.52.

[0046] Exemplarily, the tortuosity of the electrode sheet is any value among 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.52, etc. or any range composed of any two of them.

[0047] In some embodiments, 1.326 ≤ A ≤ 1.473;

[0048] and / or, 1.593 ≤ B ≤ 2.019;

[0049] and / or, 1.326 ≤ C ≤ 1.473;

[0050] And / or, the tortuosity of the electrode sheet is 2.07 - 2.518.

[0051] A and C can be the same or different.

[0052] In some embodiments, the EIS test is performed by assembling a positive electrode symmetric button cell, and the tortuosity of the electrode sheet is calculated according to the test results.

[0053] It should be noted that when testing the tortuosity of different layers, the layer can be peeled off, and a small sample can be cut out. The area of one side surface of the small sample is close to 1000 mm 2 , and then the entire material layer on the small sample is tested.

[0054] In a specific embodiment, the first layer includes a first active material, a first binder, and a first conductive agent; the second layer includes a second active material, a second binder, and a second conductive agent; the third layer includes a third active material, a third binder, and a third conductive agent; wherein, the first binder and the third binder include polyvinylidene fluoride, and the second binder includes polytetrafluoroethylene and / or a polytetrafluoroethylene copolymer.

[0055] Exemplarily, the above-mentioned polytetrafluoroethylene copolymer includes but is not limited to: polytetrafluoroethylene - polyethylene copolymer, polytetrafluoroethylene - hexafluoropropylene copolymer, etc.

[0056] Polyvinylidene fluoride (PVDF) has good wettability to the electrolyte, which is beneficial to form a dense fiber network inside the first layer and the third layer, providing good conductivity for the active material layer, thereby reducing the tortuosity of the electrode sheet. Polytetrafluoroethylene (PTFE) has high rigidity and hydrophobicity, and can form a relatively irregular pore structure in the electrode material, thereby increasing the tortuosity of the second layer.

[0057] It can be understood that the electrode sheet of the present invention can be a positive electrode sheet or a negative electrode sheet. When it is a negative electrode sheet, exemplarily, the first active material, the second active material, and the third active material are each independently: at least one of graphite, tin-based materials (such as SnO2), lithium titanate, black phosphorus, and tin sulfide (SnS), etc.; the first conductive agent, the second conductive agent, and the third conductive agent are each independently: at least one of carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, metal powder, and graphene. In some embodiments, the first layer further includes a first dispersant, the second layer further includes a second dispersant, and the third layer further includes a third dispersant. The first dispersant, the second dispersant, and the second dispersant are each independently: at least one of sodium carboxymethyl cellulose, triethylhexyl phosphate, and sodium dodecyl sulfate.

[0058] In some embodiments, by mass, the mass ratio of the first active material, the first binder, and the first conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3); the mass ratio of the second active material, the second binder, and the second conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3); the mass ratio of the third active material, the third binder, and the third conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3).

[0059] When it is a positive electrode sheet, exemplarily, the first active material, the second active material, and the third active material are each independently one or more of: lithium nickelate, lithium titanate, lithium cobaltate, lithium nickel cobalt manganate, lithium nickel cobalt aluminate, lithium iron phosphate, lithium manganate; the first conductive agent, the second conductive agent, and the third conductive agent are each independently at least one of: carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotube, metal powder, graphene.

[0060] In some embodiments, by mass, the mass ratio of the first active material, the first binder, and the first conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3); the mass ratio of the second active material, the second binder, and the second conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3); the mass ratio of the third active material, the third binder, and the third conductive agent is (94 - 99):(0.5 - 5):(0.5 - 3).

[0061] In a specific embodiment, in the second layer, the mass fraction of the second binder is 1.5 wt% - 2.5 wt%.

[0062] Since polytetrafluoroethylene has higher rigidity and hydrophobicity than polyvinylidene fluoride, its content is controlled within the above range to balance its binding effect and electrolyte wettability.

[0063] Exemplarily, in the second layer, the mass fraction of the second binder is any value among 1.5 wt%, 1.6 wt%, 1.7 wt%, 1.8 wt%, 1.9 wt%, 2.0 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt%, 2.5 wt%, etc. or a range composed of any two of them.

[0064] In a specific embodiment, in the second layer, the number - average molecular weight of the second binder is 9.50×10 6 -2×10 7 Da.

[0065] The molecular weight of polytetrafluoroethylene or its copolymer affects its degree of fibrillation, and has an impact on the ease of preparation, stretchability, and tortuosity of the electrode sheet. The above - mentioned embodiment controls the number - average molecular weight of the second binder to be 9.50×10 6Da - 2×10 7 Da can further ensure that the adhesiveness, stretchability and tortuosity of the electrode are within a reasonable range.

[0066] Exemplarily, the number average molecular weight of polytetrafluoroethylene or its copolymer is 9.50×10 6 Da, 9.59×10 6 Da, 1.0×10 7 Da, 1.1×10 7 Da, 1.2×10 7 Da, 1.3×10 7 Da, 1.4×10 7 Da, 1.5×10 7 Da, 1.6×10 7 Da, 1.7×10 7 Da, 1.8×10 7 Da, 1.9×10 7 Da, 2.0×10 7 Da or any value within the range formed by any two of them.

[0067] In a specific embodiment, the thickness of the first layer is 30 - 50 µm.

[0068] In a specific embodiment, the thickness of the third layer is 30 - 50 µm.

[0069] By limiting the thickness of the first layer and the third layer, the above embodiments can further reduce the ion transport resistance and improve the reaction rate of the electrode sheet, thereby further ensuring the rate performance of the battery.

[0070] In a specific embodiment, the thickness of the second layer is 20 - 60 µm.

[0071] By limiting the thickness of the second layer, the above embodiments can improve the stability of the electrode, thereby further enhancing the durability of the battery during long-term cycling.

[0072] In some embodiments, the thicknesses of the first layer, the second layer and the third layer can be adjusted by means such as electrostatic spraying time and calendering roll gap, etc., and the present invention does not make specific limitations thereon.

[0073] By way of example and not limitation, the thickness of the first layer is any value among 30 µm, 32 µm, 35 µm, 37 µm, 39 µm, 40 µm, 42 µm, 45 µm, 47 µm, 49 µm, 50 µm, etc. or a range formed by any two of them. The thickness of the second layer is any value among 20 µm, 25 µm, 27 µm, 30 µm, 32 µm, 35 µm, 37 µm, 39 µm, 40 µm, 42 µm, 45 µm, 47 µm, 49 µm, 50 µm, 52 µm, 55 µm, 57 µm, 60 µm, etc. or a range formed by any two of them. The thickness of the third layer is any value among 30 µm, 32 µm, 35 µm, 37 µm, 39 µm, 40 µm, 42 µm, 45 µm, 47 µm, 49 µm, 50 µm, etc. or a range formed by any two of them.

[0074] In a specific embodiment, the resistance of the electrode sheet is 470 mΩ - 540 mΩ.

[0075] Among them, when the resistance of the electrode sheet is within the above range, the charge and discharge rate of the battery can be further ensured to meet the fast charging requirement.

[0076] Exemplarily, the resistance of the electrode sheet is any value among 470 mΩ, 480 mΩ, 490 mΩ, 500 mΩ, 510 mΩ, 520 mΩ, 530 mΩ, 540 mΩ, etc. or a range formed by any two of them.

[0077] In a specific embodiment, the diffusion coefficient of the electrode sheet is 2.30×10 -13 cm 2 / s - 3.90×10 -13 cm 2 / s.

[0078] Among them, when the diffusion coefficient of the electrode sheet is within the above range, the overall ion transport of the electrode sheet can be further improved, thereby ensuring the charge and discharge rate of the battery. At the same time, the concentration gradient and stress accumulation are reduced, and the cycle life of the battery is extended.

[0079] Exemplarily, the diffusion coefficient of the electrode sheet is 2.30×10 -13 cm 2 / s, 2.40×10 -13 cm 2 / s, 2.50×10 - 13 cm 2 / s, 2.60×10 -13 cm 2 / s, 2.70×10 -13 cm 2 / s, 2.80×10 -13 cm2 / s, 2.90×10 -13 cm 2 / s, 3.0×10 -13 cm 2 / s, 3.1×10 -13 cm 2 / s, 3.2×10 -13 cm 2 / s, 3.3×10 -13 cm 2 / s, 3.4×10 -13 cm 2 / s, 3.5×10 - 13 cm 2 / s, 3.6×10 -13 cm 2 / s, 3.7×10 -13 cm 2 / s, 3.8×10 -13 cm 2 / s, 3.9×10 -13 cm 2 Any value among / s, 3.9×10

[0080] In some embodiments, the method for testing the resistance of the electrode sheet includes the following process: after the electrode sheet is fabricated, the sheet resistance is tested using an ACCFILM film resistance tester, the test pressure is 0.04 t, and the holding time is 15 s.

[0081] In a specific embodiment, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0082] In a specific embodiment, the electrode sheet is a negative electrode sheet, and at least one of the first active material, the second active material, and the third active material includes graphite, and an isolation material is provided between the graphite and the polytetrafluoroethylene to isolate the graphite from the polytetrafluoroethylene.

[0083] Since an irreversible side reaction occurs between graphite and the polytetrafluoroethylene at a low potential, the above embodiment can avoid the occurrence of the side reaction by introducing the isolation material.

[0084] In some embodiments, the isolation material includes, but is not limited to, PEO, conductive carbon, extended metal, etc.

[0085] In some embodiments, the isolation material coats at least a part of the surface of the graphite.

[0086] In some embodiments, in any of the active materials, the proportion of the isolation material is 0.5 - 2.0 wt%.

[0087] In a second aspect, the present invention provides a method for preparing the above electrode sheet, comprising the following steps:

[0088] Spray a first solid mixture containing a first active material, a first binder, and a first conductive agent onto at least one surface of the current collector, and after hot pressing and melting, form a first layer;

[0089] Knead and roll a second solid mixture containing a second active material, a second binder, and a second conductive agent into a film, and through hot pressing, bond the film to the first layer to form a second layer;

[0090] Spray a third solid mixture containing a third active material, a third binder, and a third conductive agent onto the surface of the second layer, and after hot pressing and melting, form the electrode sheet.

[0091] In the above preparation method, the first layer and the third layer are formed by spraying on the current collector to form a loose accumulation, and then hot pressing and forming. In this process, the fibrillation of the binder is not involved, which can ensure a lower tortuosity. The second layer is formed by kneading the solid mixture in a roll, and the binder is turned into fibrils by shear force. The fibrils are connected to each other during the mixing process to form a network to fix the electrode powder together. The rolled film has a fibrillar connection structure, with a longer lithium ion diffusion path and a higher ionic resistance, thereby increasing the tortuosity of the second layer.

[0092] The electrode sheet prepared by the preparation method of a specific embodiment is as Figure 1 shown. Among them, the second layer is prepared by the binder fibrillation method and contains PTFE filaments. The wound and extended filaments carry the conductive agent and adhere between the active particles. Compared with the first layer and the third layer, it adds a long-range electron conduction path and has a lower electrode resistance. In an actual battery system, the third layer is close to the separator, and the liquid absorption rate of the separator is much higher than that of the positive and negative electrode sheets. During the infiltration process, the electrolyte infiltrates from the separator side along the third layer into the first layer. Therefore, the lithium ion concentration gradient increases from the third layer to the first layer. Based on the above phenomena, the third layer is designed as a low-tortuosity electrode sheet to promote electrolyte infiltration and increase the charge transfer rate at the interface. The second layer is a medium-tortuosity electrode sheet with excellent electronic conductivity to reduce the gradient difference of the electron current density. The first layer is a low-tortuosity electrode sheet to improve the phenomenon that the lithium ions cannot fully diffuse to the current collector side at high rates, resulting in poor rate performance.

[0093] In a specific embodiment, the spraying of the first solid mixture onto at least one surface of the current collector includes:

[0094] The step of atomizing and spraying the first solid mixture onto at least one surface of the current collector by using a first air flow and first static electricity;

[0095] And / or, the spraying of the third solid mixture onto the surface of the second layer includes:

[0096] The step of atomizing the third solid mixture by using a second air flow and a second static electricity and spraying it on the surface of the second layer.

[0097] In a specific embodiment, the pressure of the first air flow is 100 - 150 kPa, and the voltage of the first static electricity is 20 - 30 kV;

[0098] And / or, the pressure of the second air flow is 100 - 150 kPa, and the voltage of the second static electricity is 20 - 30 kV.

[0099] In a specific embodiment, the temperature of the hot pressing and melting is 180 - 300 °C.

[0100] In a third aspect, the present invention provides a battery including the above electrode sheet.

[0101] It can be understood that the above battery further includes a separator and an electrolyte.

[0102] The present invention has no particular limitation on the above separator, and any publicly known porous structure separator with electrochemical stability and chemical stability can be selected. For example, it can be at least one of glass fiber, non-woven fabric, polyethylene, polypropylene or polyvinylidene fluoride. The separator can be single-layer or multi-layer.

[0103] The above electrolyte includes an organic solvent and an electrolyte salt. The organic solvent serves as a medium for transporting ions in the electrochemical reaction, and an organic solvent known in the art for battery electrolytes can be used.

[0104] Exemplarily, the organic solvent can be at least one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), diethyl sulfone (ESE). In a specific embodiment, two or more of the above organic solvents can be selected.

[0105] As a source of ions, the electrolyte salt can be an electrolyte salt known in the art for use in battery electrolytes. Exemplarily, the electrolyte salt can be at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonate (LiSbF6), lithium difluorophosphate (LiPF2O2), lithium 4,5-dicyano-2-trifluoromethylimidazole (LiDTI), lithium bis(oxalato)borate (LiBOB), lithium trifluoromethanesulfonate (LiTFS), lithium bis(malonato)borate (LiBMB), lithium difluorooxalate borate (LiDFOB), lithium bis(difluoromalonato)borate (LiBDFMB), lithium (malonatooxalato)borate (LiMOB), lithium (difluoromalonatooxalato)borate (LiDFMOB), lithium tris(oxalato)phosphate (LiTOP), lithium tris(difluoromalonato)phosphate (LiTDFMP), lithium tetrafluorooxalate phosphate (LiTFOP), lithium difluorodioxalate phosphate (LiDFOP), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium (fluorosulfonyl)(trifluoromethanesulfonyl)imide (LiN(SO2F)(SO2CF3)), lithium nitrate (LiNO3), and lithium fluoride (LiF).

[0106] In some embodiments, the electrode sheet includes a positive electrode sheet and a negative electrode sheet. The above positive electrode sheet, separator, and negative electrode sheet can be stacked in sequence to obtain an electric core, or the above positive electrode sheet, separator, and negative electrode sheet can be stacked in sequence and then wound to obtain an electric core. The electric core is placed in a packaging battery film shell (such as an aluminum-plastic film shell), electrolyte is injected into the outer package and sealed to prepare the battery of the present invention.

[0107] The present invention will be further described below in conjunction with specific embodiments:

[0108] Example 1

[0109] This example provides a positive electrode sheet, and its preparation method includes:

[0110] (1) The first layer: The electrode powder is mixed at a high speed in the ratio of NCM811 (molecular formula: LiNi 0.8 Co 0.1 Mn 0.1 O2): carbon black (SP): PVDF = 95:2.5:2.5 (wt%). The premixed powder is accelerated with a high-pressure gas of 137.9 kPa and atomized and sprayed on the carbon-coated aluminum foil. At the same time, under a static voltage of 25 kV, the particulate powder adheres to the surface of the current collector. Subsequently, PVDF is melted by hot pressing at 200 °C to achieve the bonding of the active material, conductive agent, and current collector, forming a first layer with a thickness of 40 µm.

[0111] (2) The second layer: Mix the electrode materials in the ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%), where the number-average molecular weight of PTFE is 1.08×10 7 Da, and obtain the initially fibrillated electrode materials by high-speed stirring with a cutter mill; knead the fibrillated powder, with the roller temperature at 80 °C, the rotational speeds of rollers A and B at 1.5 m / min, the initial roller gap at 60 μm, pour the powder from above, after forming, adjust the roller gap, fold it in half and put it into the rollers for kneading multiple times until the powder becomes a diaphragm with regular shape, then adjust the rotational speeds of rollers A and B to 1.5 and 1.8 m / min, roll it transversely into the rollers until it is 90 μm thick, then adjust the speed difference to 0.3 m / min, roll it multiple times to reduce the thickness to form a uniform diaphragm with a thickness of 40 μm; finally, enter the current collector composite process, that is, place the diaphragm on the first layer, with the roller temperature at 200 °C, the roller gap at 120 μm, the roller speed at 1.4 m / min, and put the two into the rollers until the diaphragm adheres to the first layer;

[0112] (3) The third layer: High-speed mix the electrode powder in the ratio of NCM811:SP:PVDF = 95:2.5:2.5 (wt%), use high-pressure gas at 137.9 kPa to accelerate and atomize the premixed powder and spray it on the first two layers of electrode foils, and at the same time, under a static voltage of 25 kV, make the active particle powder adhere to its surface, then, melt PVDF by hot rolling at 200 °C to achieve the bonding of the third layer, with a thickness of 40 µm, and control the final surface density of the positive electrode foil to be 20 mg / cm by cold pressing 2 , and compact it to 3.4 g / cm 3 .

[0113] Example 2

[0114] It is basically the same as the preparation process of Example 1, the difference is that in step (2), the process is changed to NCM811:SP:PTFE = 95.5:2.5:2 (wt%).

[0115] Example 3

[0116] It is basically the same as the preparation process of Example 1, the difference is that in step (2), the process is changed to NCM811:SP:PTFE = 96:2.5:1.5 (wt%).

[0117] Example 4

[0118] It is basically the same as the preparation process of Example 1, the difference is that in step (2), the number-average molecular weight of PTFE used is changed to 2×10 7 Da.

[0119] Example 5

[0120] It is basically the same as the preparation process of Example 1, except that in step (2), the molecular weight of PTFE used is changed to 9.59×10 6 Da.

[0121] Example 6

[0122] It is basically the same as Example 1, except that the thickness ratio of the first, second, and third layer electrodes is 30μm: 60μm: 30μm.

[0123] Example 7

[0124] It is basically the same as Example 1, except that the thickness ratio of the first, second, and third layer electrodes is 50μm: 20μm: 50μm.

[0125] Example 8

[0126] It is basically the same as Example 1, except that the molecular weight of PTFE is 2.5×10 7 Da.

[0127] Example 9

[0128] It is basically the same as Example 1, except that the addition amount of PTFE is 3wt%.

[0129] Example 10

[0130] It is basically the same as Example 1, except that PTFE is replaced with ethylene-tetrafluoroethylene copolymer (ETFE), the molecular weight of ETFE is 1.04×10 7 Da, and the addition amount is 2.5wt%, see Table 1.

[0131] Example 11

[0132] It is basically the same as Example 3, except that the thickness ratio of the first, second, and third layer electrodes is 20μm: 80μm: 20μm.

[0133] Comparative Example 1

[0134] The method for preparing the electrode sheet in this example is as follows:

[0135] Mix PVDF 5130 with N-methylpyrrolidone (NMP) and stir for 240 min to obtain a PVDF glue solution. Add the NMP solvent to a stirring bucket, then sequentially add NCM811 and SP, and stir at a low speed (revolution speed 15 rpm, dispersion speed 200 rpm, stir for 10 min). Then add the PVDF glue solution and stir at a high speed (revolution speed 25 rpm, dispersion speed 2000 rpm, stir for 240 min) to prepare a positive electrode slurry, where NCM811:SP:PVDF = 95:2.5:2.5 (wt%). Extrusion coat the positive electrode slurry onto a current collector, bake the electrode at 110 °C for 12 h, and roll press to obtain a wet-process electrode with a surface density of 20 mg / cm 2 , and a compaction of 3.4 g / cm 3 of the wet-process electrode.

[0136] Comparative Example 2

[0137] The method for preparing the electrode in this example is as follows:

[0138] Mix the electrode materials in the ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%), and use a pill cutter to stir at a high speed to obtain initially fibrillated electrode materials; knead the fibrillated powder, the roller temperature is 80 °C, the rotation speeds of rollers A and B are 1.5 m / min, the initial roller gap is 60 μm, pour the powder from above, after forming, adjust the roller gap, fold in half and roll and knead multiple times until the powder becomes a regularly shaped membrane. Then adjust the rotation speeds of rollers A and B to 1.5 and 1.8 m / min, roll transversely to 90 μm, then adjust the speed difference to 0.3, and roll and thin multiple times to form a uniform membrane with a thickness of 120 μm and compound it with a current collector, and roll press to obtain a surface density of 20 mg / cm 2 , and a compaction of 3.4 g / cm 3 of the dry-process electrode.

[0139] Comparative Example 3

[0140] The method for preparing the electrode in this example is as follows:

[0141] Mix the electrode materials in the ratio of NCM811:SP:PVDF = 95:2.5:2.5 (wt%), set the voltages of the spray gun and the current collector to 25 KV, use compressed air to transport the dry-process powder in the hopper to the electrostatic spray gun for spraying, and hot press and compound the sprayed electrode at 175 °C and 6 kg / cm 2 pressure for 45 min to obtain a dry-process electrode, and roll press to increase the compaction to 3.4 g / cm 3 .

[0142] Comparative Example 4

[0143] This example provides a positive electrode sheet, and its preparation method includes:

[0144] (1) The first layer: High-speed mixing of electrode powder materials in the ratio of NCM811 (molecular formula: LiNi 0.8 Co 0.1 Mn 0.1 O2): carbon black (SP): PVDF = 95:2.5:2.5 (wt%). Accelerate the premixed powder with high-pressure gas at 137.9 kPa and atomize it to spray and adhere on the current collector. At the same time, under a static voltage of 25 kV, make the active particle powder adhere to the surface of the current collector. Subsequently, thermally press and melt PVDF to achieve the bonding of the active material, conductive agent, and current collector;

[0145] (2) The second layer: Mix the electrode materials in the ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%). Among them, the number-average molecular weight of PTFE is 1.08×10 7 Da. Use a pill cutter to stir at high speed to obtain the initially fibrillated electrode materials; Knead the fibrillated powder. The temperature of the roller is 80 °C, the rotation speeds of rollers A and B are 1.5 m / min, the initial roller spacing is 60 μm. Pour the powder from above. After forming, adjust the roller spacing, fold it in half and knead it into the roller multiple times until the powder becomes a film with regular shape. Then adjust the rotation speeds of rollers A and B to 1.5 and 1.8 m / min, roll it transversely until it reaches 90 μm, and then adjust the speed difference to 0.3 m / min. Roll it multiple times to thin it to form a uniform film with a thickness of 40 μm; Finally, enter the current collector composite process, that is, place the film on the first layer. The temperature of the roller is 200 °C, the roller spacing is 120 μm, and the roller speed is 1.4 m / min. The two enter the roller until the film adheres to the first layer;

[0146] (3) The third layer: Mix the electrode materials in the ratio of NCM811:SP:PTFE = 95:2.0:3.0 (wt%). Among them, the number-average molecular weight of PTFE is 2.5×10 7 Da. Use a pill cutter to stir at high speed to obtain the initially fibrillated electrode materials; Knead the fibrillated powder. The temperature of the roller is 80 °C, the rotation speeds of rollers A and B are 1.5 m / min, the initial roller spacing is 60 μm. Pour the powder from above. After forming, adjust the roller spacing, fold it in half and knead it into the roller multiple times until the powder becomes a film with regular shape. Then adjust the rotation speeds of rollers A and B to 1.5 and 1.8 m / min, roll it transversely until it reaches 90 μm, and then adjust the speed difference to 0.3 m / min. Roll it multiple times to thin it to form a uniform film with a thickness of 40 μm; Finally, enter the current collector composite process, that is, place the film on the second layer. The temperature of the roller is 200 °C, the roller spacing is 120 μm, and the roller speed is 1.4 m / min. The two enter the roller until the film adheres to the second layer; Control the final surface density of the positive electrode sheet to be 20 mg / cm2 and compacted to 3.4 g / cm 3 .

[0147] Comparative Example 5

[0148] (1) The first layer: The electrode powder materials were mixed at a high speed according to the ratio of NCM811 (molecular formula: LiNi 0.8 Co 0.1 Mn 0.1 O2): carbon black (SP): PVDF = 95:2.5:2.5 (wt%). The premixed powder was accelerated by high-pressure gas at 137.9 kPa and atomized and sprayed onto the current collector. At the same time, under a static voltage of 25 kV, the active particle powder adhered to the surface of the current collector. Subsequently, PVDF was melted by hot pressing to achieve the bonding of the active material, conductive agent, and current collector;

[0149] (2) The second layer: The electrode materials were mixed according to the ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%). Among them, the number-average molecular weight of PTFE was 1.08×10 7 Da. The electrode materials were initially fibrillated by high-speed stirring with a pill cutter; the fibrillated powder was kneaded. The roller temperature was 80 °C, the rotational speeds of rollers A and B were 1.5 m / min, the initial roller gap was 60 μm, and the powder was poured from above. After forming, the roller gap was adjusted, and the powder was folded and kneaded into the roller multiple times until the powder became a film with regular shape. Then, the rotational speeds of rollers A and B were adjusted to 1.5 and 1.8 m / min, and it was rolled transversely to 90 μm. Then, the speed difference was adjusted to 0.3 m / min, and it was rolled and thinned multiple times to form a uniform film with a thickness of 60 μm; finally, it entered the current collector composite process, that is, the film was placed on the first layer, the roller temperature was 200 °C, the roller gap was 120 μm, the roller speed was 1.4 m / min, and the two were rolled into the roller until the film adhered to the first layer;

[0150] Test Example

[0151] Batteries were prepared using the above positive electrode sheets, including the following steps:

[0152] (1) Preparation of the negative electrode sheet: The negative electrode material varied according to the test purpose. In the diffusion coefficient test experiment, the negative electrode was a lithium sheet; in the tortuosity test, the negative electrode was the same as the positive electrode; in the electrical performance test, raw materials were weighed according to the mass ratio of graphite material, sodium carboxymethyl cellulose (CMC), conductive carbon black (SP), styrene-butadiene rubber (SBR), and polyvinylpyrrolidone (PVP) of 95.2:1.5:1:2.2:0.1. First, CMC was dissolved in water to prepare a colloidal solution, then SP and PVP were mixed, and then the graphite material was added and mixed evenly. Finally, deionized water and SBR were added and stirred until the viscosity of the slurry reached 2000 - 4000 mPa·s. The slurry was coated on a copper foil current collector, baked, rolled, and cut to obtain the negative electrode sheet.

[0153] (2) Electrolyte: The solvent is a mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) with a volume ratio of 35:35:3. LiPF6 is added to make its concentration 1 mol / L, and the mixture is stirred evenly to obtain the electrolyte.

[0154] (3) Battery fabrication: The laminated soft-pack battery and coin cell are fabricated in a dry room and a glove box respectively. Fabrication process of the laminated soft-pack battery: Stacking is carried out in a zigzag pattern, that is, stacking in the order of separator - positive electrode - separator - negative electrode - separator - positive electrode - separator. The fabricated electrode cores are welded with positive and negative tabs respectively, and then placed into an aluminum-plastic shell for liquid injection, top and side sealing, aging, and formation, thus completing the fabrication of the soft-pack battery. Fabrication process of the coin cell: In the glove box, the positive electrode sheet is punched into a circular sheet with a diameter of 15 mm, and assembled in the order of positive electrode case - positive electrode - electrolyte - separator - negative electrode - gasket - spring sheet - negative electrode case, and pressed with a pressure of 50 kgf to obtain the coin cell.

[0155] Test Examples

[0156] The following tests are conducted on the positive active materials or batteries with test force of the examples and comparative examples:

[0157] 1. Diffusion coefficient test: Assemble a positive electrode vs. lithium coin cell, and use GITT to test the diffusion coefficient of the material. Before the test, charge and discharge at 0.2C for one week to activate the battery, let it stand for 1 h, adjust the SOC, charge with a 0.1C pulse for 10 s, let it stand for 1 h, and calculate the lithium ion diffusion coefficient according to the voltage differences before and after the pulse and before and after standing. The calculation method is shown in Formula 1:

[0158] ; Formula 1,

[0159] where V m is the molar volume of the active material, m B is the mass of the active material, M B is the relative molecular mass of the active material, S is the specific surface area of the electrode sheet, ΔE S is the voltage change during the 0.1C pulse charging section, and ΔE T is the voltage change when standing to equilibrium.

[0160] 2. Tortuosity test: Assemble a positive electrode symmetric coin cell, and use an electrochemical workstation to test the EIS curve, including the following process:

[0161] 1) Use a thickness gauge, such as a vernier caliper or a micrometer, to measure the thickness of the electrode sheet (d, cm);

[0162] 2) Obtain the porosity of the electrode sheet (ε, %) by mercury intrusion method or calculation formula: Calculation formula: , where,

[0163] ;

[0164] 3) Measurement of the ionic conductivity (K, mS / cm) of the electrolyte: Assemble a stainless-steel symmetric button cell, record the diaphragm thickness L and the stainless-steel area A (where the diaphragm thickness is 22 µm and the stainless-steel is a disc with a diameter of 15 mm), conduct an EIS test on the button cell (the test frequency range is 1 Hz - 1 MHz and the bias voltage is 5 mV), and fit the spectrum to obtain the solution resistance R b , substitute each parameter into the formula , and calculate to obtain the electrolyte conductivity K = 8 mS / cm;

[0165] 4) Measurement of the electrochemical impedance spectroscopy EIS, obtaining the high-frequency resistance and the ionic impedance in the pores (R ion , Ω) based on extrapolation in the low-frequency region: The EIS test frequency range is 10 5 Hz - 1 Hz and the bias voltage is 5 mV,

[0166] 5) Calculate the tortuosity of the electrode sheet according to formula 2:

[0167] Formula 2.

[0168] 3. Tensile strength test: Use a universal material testing machine to test the tensile strength of the diaphragm, and the tensile rate is 50 mm / min.

[0169] 4. Creeping liquid height test: Adopt the hanging method to test the wetting performance of the electrode sheet. Cut the electrode sheets of each example and comparative example into strips with a specification of 15 * 40 mm, then fix the electrode sheets vertically, immerse the lower end of the electrode sheet in the electrolyte, record the corresponding scale of time and liquid level height, and record the climbing height of the electrode sheet after 5 minutes.

[0170] 5. Battery electrical performance test: (1) Rate performance: Constant current and constant voltage charge at 0.33C until the full charge state cut-off voltage, the cut-off current is 0.05C, and then discharge at 0.1, 0.2, 0.5, 1, 0.1C rates to the empty charge state cut-off voltage. Based on the first 0.1C discharge capacity C0, compare the rate capacity retention rate of each example / comparative example battery according to the ratio of the 1C discharge capacity to C0; (2) Cycle performance: Constant current and constant voltage charge at 1C, the cut-off current is 0.05C, constant current discharge at 1C, cycle 100 times, and calculate the ratio of the capacity of the first cycle to the 100th cycle.

[0171] Summarize the above test results in Table 1 and Table 2.

[0172] Table 1:

[0173]

[0174] In the table, "-" represents that there is no result for this parameter.

[0175] Table 2:

[0176]

[0177] As can be seen from Table 1-2, compared with Comparative Examples 1-5, the electrodes of the examples have more excellent rate capacity retention and cycle capacity retention. The main reasons include: the thick electrode with a suitable hierarchical electrode structure has the advantages of high ion diffusion efficiency and electronic conductivity. During the charge and discharge process of the battery, ions pass through the inner / outer sprayed electrode layer. Since the outer layer with high porosity and low tortuosity contacts the electrolyte, it is beneficial to the infiltration of the electrolyte. At the same time, the inner layer with low tortuosity is used to weaken the difference in lithium ion concentration gradient, which is beneficial to improving the rate performance of the thick electrode. The fibrous dry electrode layer is inserted into the middle layer to optimize the conductive network and promote the improvement of the charge transfer efficiency. Therefore, the electrode design proposed in this paper can effectively improve the performance of the thick electrode.

[0178] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. An electrode sheet, comprising a current collector and a material layer disposed on at least one surface of the current collector, characterized in that, In the direction from adjacent to the current collector to away from the current collector, the material layer includes: a first layer, a second layer, and a third layer. The tortuosity of the first layer is A, the tortuosity of the second layer is B, and the tortuosity of the third layer is C, where A is less than B, and C is less than B.

2. The electrode sheet according to claim 1, wherein 1.2≤A<1.5； And / or, 1.5 ≤ B ≤ 2.1; And / or, 1.2 ≤ C < 1.

5.

3. The electrode sheet according to claim 1, characterized in that, The tortuosity of the electrode sheet is 2 - 2.

52.

4. The electrode sheet according to any one of claims 1-3, characterized in that, The first layer includes a first active material, a first binder, and a first conductive agent; the second layer includes a second active material, a second binder, and a second conductive agent; the third layer includes a third active material, a third binder, and a third conductive agent; where the first binder and the third binder include polyvinylidene fluoride, and the second binder includes polytetrafluoroethylene and / or a copolymer of tetrafluoroethylene.

5. The electrode sheet according to claim 4, wherein In the second layer, the mass ratio of the second binder is 1.5wt% - 2.5wt%; and / or, the number average molecular weight of the second binder is 9.50×10 6 Da - 2×10 7 Da.

6. The electrode sheet according to any one of claims 1-3, characterized in that, The thickness of the first layer is 30 - 50 µm; And / or, the thickness of the second layer is 20 - 60 µm; And / or, the thickness of the third layer is 30 - 50 µm.

7. The electrode sheet according to claim 6, characterized in that, The resistance of the electrode sheet is 470 mΩ - 540 mΩ; And / or, the diffusion coefficient of the electrode sheet is 2.30×10 -13 cm 2 / s - 3.90×10 -13 cm 2 / s.

8. The electrode sheet according to any one of claims 1-3, characterized in that, The electrode sheet is a positive electrode sheet or a negative electrode sheet.

9. A method for preparing an electrode sheet according to any one of claims 1-8, characterized in that, Including the following steps: A first solid mixture containing a first active material, a first binder, and a first conductive agent is sprayed on at least one surface of the current collector, and after hot pressing and melting, a first layer is formed; A second solid mixture containing a second active material, a second binder, and a second conductive agent is roll-kneaded and calendered into a film sheet, and through hot pressing, the film sheet is bonded to the first layer to form a second layer; A third solid mixture containing a third active material, a third binder, and a third conductive agent is sprayed on the surface of the second layer, and after hot pressing and melting, the electrode sheet is formed.

10. The preparation method according to claim 9, wherein, The spraying of the first solid mixture on at least one surface of the current collector includes: The step of atomizing the first solid mixture by using a first air flow and a first static electricity and spraying it on at least one surface of the current collector; And / or, the spraying of the third solid mixture on the surface of the second layer includes: The step of atomizing the third solid mixture by using a second air flow and a second static electricity and spraying it on the surface of the second layer.

11. The preparation method according to claim 10, wherein, The pressure of the first air flow is 100 - 150 kPa, and the voltage of the first static electricity is 20 - 30 kV; And / or, the pressure of the second air flow is 100 - 150 kPa, and the voltage of the second static electricity is 20 - 30 kV.

12. A battery, characterized in that, Including the electrode sheet according to any one of claims 1 - 8 or the electrode sheet prepared by the preparation method according to any one of claims 9 - 11.

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