An electrode sheet, a method of manufacturing the same, and a battery

Abstract

The application provides an electrode sheet, a preparation method thereof and a battery. The electrode sheet 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 from the direction adjacent to the current collector to the direction away from the current collector. 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. The electrode sheet provided by the application can improve the lithium ion diffusion and the problem of difficult electrolyte infiltration of the thick electrode by limiting the tortuosity characteristics of different layer structures, and is beneficial to improving the rate capability and cycle performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of battery materials, in particular to an electrode sheet, a preparation method thereof and a battery. BACKGROUND

[0002] With the advent of the electronic information age, lithium ion batteries are widely used in the fields of electronic products, new energy vehicles, etc. Consumers' demand for longer battery life and lighter devices is increasing, which promotes the research and application of high energy density batteries.

[0003] In order to improve the energy density of lithium ion batteries, thick electrode technology research has emerged as the times require. In the traditional lithium ion battery manufacturing process, due to the inevitable volatilization process of solvent (such as NMP), the wet thick electrode coating is prone to cracking after baking, and at the same time, the over-thick electrode leads to uneven distribution of lithium ion concentration in the longitudinal direction, the lithium ion concentration near the separator side is higher, and it is difficult to completely diffuse to the current collector side, which reduces the battery performance. However, dry electrodes do not require solvents, so it is more conducive to the manufacture of thick electrodes in the process. However, the traditional dry thick electrode still has its own problems of lithium ion diffusion difficulty and electrolyte difficult to soak, which affects the rate capability and cycle performance of the battery. SUMMARY

[0004] The present application provides an electrode sheet, which can improve the lithium ion diffusion and electrolyte soaking problems of thick electrodes by limiting the tortuosity characteristics of different layer structures, and is beneficial to improve the rate capability and cycle performance of the battery.

[0005] The present application also provides a preparation method of the above-mentioned electrode sheet, which can prepare the above-mentioned electrode sheet and has a simple process.

[0006] The present application also provides a battery, which has excellent rate capability and cycle performance due to the inclusion of the above-mentioned electrode sheet.

[0007] In the first aspect, the present application provides an electrode sheet, which 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 from the direction adjacent to the current collector to the direction away from the current collector, 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.

[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 comprises a first active material, a first binder and a first conductive agent; the second layer comprises a second active material, a second binder and a second conductive agent; the third layer comprises a third active material, a third binder and a third conductive agent; wherein the first binder and the third binder comprise polyvinylidene fluoride, and the second binder comprises polytetrafluoroethylene and / or a copolymer of tetrafluoroethylene.

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

[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 30-50µm;

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

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

[0018] Optionally, the resistance of the electrode sheet is 470mΩ-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 application provides a preparation method of the above electrode sheet, comprising the following steps:

[0022] Spraying a first solid mixture comprising a first active material, a first binder and a first conductive agent on at least one surface of the current collector, and after hot pressing and melting, forming a first layer;

[0023] Roll kneading and calendering a second solid mixture comprising a second active material, a second binder and a second conductive agent into a film sheet, and through hot pressing, bonding the film sheet with the first layer to form a second layer;

[0024] Spraying a third solid mixture comprising a third active material, a third binder and a third conductive agent on the surface of the second layer, and after hot pressing and melting, forming the electrode sheet.

[0025] Optionally, the first solid mixture is sprayed on at least one surface of the current collector, comprising:

[0026] a step of atomizing and spraying the first solid mixture on at least one surface of the current collector by using the first airflow and the first electrostatic field;

[0027] and / or, the third solid mixture is sprayed on the surface of the second layer, comprising:

[0028] a step of atomizing and spraying the third solid mixture on the surface of the second layer by using the second airflow and the second electrostatic field.

[0029] Optionally, the pressure of the first airflow is 100-150 kPa, and the voltage of the first electrostatic field is 20-30 kV;

[0030] and / or, the pressure of the second airflow is 100-150 kPa, and the voltage of the second electrostatic field is 20-30 kV.

[0031] In a third aspect, the present application provides a battery comprising the electrode sheet.

[0032] The electrode sheet provided by the present application can improve the lithium ion diffusion and electrolyte wettability of thick electrodes by limiting the tortuosity characteristics of different layer structures, thereby improving the rate capability and cycle performance of the battery. BRIEF DESCRIPTION OF DRAWINGS

[0033] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the present application and serve to explain the principles of the present application, together with the description.

[0034] Figure 1 A schematic diagram of the microstructure of the positive electrode sheet according to one specific embodiment of the present application. DETAILED DESCRIPTION

[0035] In order for those skilled in the art to better understand the scheme of the present application, the present application will be further described in detail below. The following specific embodiments are only used to describe the principles and characteristics of the present application, and the examples are used to explain the present application, but not to limit the scope of the present application. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of the present application.

[0036] To solve the problem of lithium ion diffusion difficulty and electrolyte difficult to infiltrate in the dry thick electrode, the traditional method is to use a pore-forming agent to prepare a gradient decreasing pore structure from the current collector to the surface of the electrode sheet, so 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 ion conductivity of the material, however, the high porosity also makes the actual transmission path of ions become longer and more complex, thus offsetting part of the benefits brought by the increase in porosity, therefore, the gradient change of porosity cannot improve the inherent ion diffusion difficulty problem of the thick electrode.

[0037] For the thick electrode, there are three aspects of electrochemical processes inside the battery: 1. Liquid phase transmission of lithium ions in the pores of the electrode sheet, diffusion across the SEI film and diffusion inside the solid particles, as the thickness of the battery increases, the originally limited rate step of lithium ion transmission process by solid phase diffusion gradually shifts to the liquid phase transmission in the pores; 2. Transmission of electrons at the current collector / electrode sheet interface and in the main body of the material layer, the transmission rate of this process depends on the current collector / electrode sheet interface and the state of the conductive agent network, the electron current density at the current collector / electrode sheet interface is the largest, and the electron current density gradually decreases from the interface to the separator, and this uneven gradient is more obvious in the thick electrode; 3. Charge transfer at the electrode / electrolyte, this process is accompanied by double layer and electrochemical reaction, and is related to the specific surface area of the active material particles and the electrolyte wettability.

[0038] Tortuosity refers to the ratio of the actual transmission path length of ions to the straight line distance, which can be used to reflect the complexity of the ion transmission path in the porous medium, in order to realize high liquid phase diffusion rate in the thick electrode, excellent electron conduction in the main body of the electrode and relatively high charge transfer efficiency at the electrode / electrolyte interface, without changing the type of electrolyte and the positive electrode material, the above problems can be effectively solved by designing an electrode sheet with hierarchical tortuosity, in detail, the present application adopts the following technical solutions:

[0039] In a first aspect, the present application provides an electrode sheet, comprising 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 from the direction adjacent to the current collector to the direction away from the current collector, 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] The electrode sheet of the present application can improve the lithium ion diffusion and electrolyte wettability of thick electrodes by defining the tortuosity characteristics of different layer structures, thereby improving the rate capability and cycle performance of the battery. The main reasons include: when the electrode sheet is used in a battery, the material layer tortuosity of the electrode sheet changes regularly from low to medium to low in the direction from the adjacent current collector to the direction away from the current collector. The first layer has low tortuosity characteristics and directly contacts the electrolyte, which can promote electrolyte wettability, improve charge transfer rate, and increase the interface specific surface area. The second layer has higher tortuosity, which can increase the mechanical stability and structural integrity of the electrode sheet, thereby ensuring the stability of the battery, providing sufficient active material and electrolyte contact, ensuring the uniformity of the reaction, adjusting the flowability of the electrolyte, and preventing electrolyte loss caused by too fast penetration. The second layer can work with the first layer to achieve better charge and discharge performance without sacrificing energy density. The third layer has low tortuosity characteristics and can work with the low tortuosity layer of the first layer to quickly respond to current changes, improve the wettability of the electrode sheet adjacent to the electrolyte, the difference in lithium ion concentration gradient distribution inside, and the electronic transmission difficulty caused by the conductive agent network, thereby improving the kinetic performance and improving 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] As described above, the first layer and the third layer of the electrode sheet have the above-mentioned range of tortuosity, which can provide a more direct ion transmission path and reduce transmission resistance, which helps to improve the reaction rate and overall conductivity of the electrode sheet. The second layer of the electrode sheet has the above-mentioned range of tortuosity, which increases the complexity of the ion transmission path, helps to more evenly distribute ions and current density, prevents local overreaction, and further ensures the stability of the electrode.

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

[0046] For example, the tortuosity of the electrode sheet is any one of 2, 2.1, 2.2, 2.3, 2.4, 2.5, 2.52, or a range formed by 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 symmetrically buckled, and the calculation of the electrode sheet tortuosity is performed 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 cut into a small sample, and the area of the single-sided surface of the small sample is close to 1000mm 2 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; and 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 polytetrafluoroethylene copolymer.

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

[0056] Polyvinylidene fluoride (PVDF) has good electrolyte wettability, which is beneficial to the formation of 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 application 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 material (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, graphene, etc. In some embodiments, the first layer further comprises a first dispersing agent, the second layer further comprises a second dispersing agent, and the third layer further comprises a third dispersing agent, and the first dispersing agent, the second dispersing agent, and the second dispersing agent are each independently at least one of: sodium carboxymethyl cellulose, triethylhexyl phosphoric acid, sodium dodecyl sulfate.

[0058] In some embodiments, 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) by mass; 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) by mass; and 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) by mass.

[0059] When being a positive electrode sheet, 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 manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, and lithium manganese oxide; and 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.

[0060] In some embodiments, 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) by mass; 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) by mass; and 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) by mass.

[0061] In a specific embodiment, the mass percentage of the second binder in the second layer 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] For example, the mass percentage of the second binder in the second layer is any one of 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%, or a range constituted by any two of them.

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

[0065] The molecular weight of polytetrafluoroethylene or its copolymer affects the degree of fiberization, which has an impact on the difficulty of electrode sheet preparation, stretchability, and tortuosity. The above embodiment controls the number average molecular weight of the second binder to be 9.50×10 6Da-2x10 7 Da, can further ensure that the adhesion, stretchability and tortuosity of the pole piece are within a reasonable range.

[0066] Exemplarily, the number average molecular weight of polytetrafluoroethylene or its copolymer is 9.50x10 6 Da, 9.59x10 6 Da, 1.0x10 7 Da, 1.1x10 7 Da, 1.2x10 7 Da, 1.3x10 7 Da, 1.4x10 7 Da, 1.5x10 7 Da, 1.6x10 7 Da, 1.7x10 7 Da, 1.8x10 7 Da, 1.9x10 7 Da, 2.0x10 7 Da, etc. or a range formed by 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] The above embodiments can further reduce the ion transmission resistance and improve the reaction rate of the electrode sheet by limiting the thickness of the first layer and the third layer, thereby further ensuring the rate capability of the battery.

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

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

[0072] In some embodiments, the thickness of the first layer, the second layer and the third layer can be adjusted by electrostatic spraying time and calender roll gap, etc., for which the present application does not make specific limitations.

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

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

[0075] Among them, the resistance of the electrode sheet is within the above range, which can further ensure the charge and discharge rate of the battery to meet the fast charging requirements.

[0076] Illustratively, the resistance of the electrode sheet is any one of 470 mΩ, 480 mΩ, 490 mΩ, 500 mΩ, 510 mΩ, 520 mΩ, 530 mΩ, 540 mΩ, etc., or a range consisting of both values.

[0077] In one 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, the diffusion coefficient of the electrode sheet is within the above range, which can further improve the overall ion transmission of the electrode sheet, thereby ensuring the charge and discharge rate of the battery, while reducing the concentration gradient and stress accumulation, and extending the cycle life of the battery.

[0079] For example, 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 x 10 -13 cm 2 / s, 3.0 x 10 -13 cm 2 / s, 3.1 x 10 -13 cm 2 / s, 3.2 x 10 -13 cm 2 / s, 3.3 x 10 -13 cm 2 / s, 3.4 x 10 -13 cm 2 / s, 3.5 x 10 - 13 cm 2 / s, 3.6 x 10 -13 cm 2 / s, 3.7 x 10 -13 cm 2 / s, 3.8 x 10 -13 cm 2 / s, 3.9 x 10 -13 cm 2 / s, etc. or a range formed by any two of the above.

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

[0081] In some embodiments, the electrode sheet is a positive electrode sheet or a negative electrode sheet.

[0082] In some embodiments, 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 the graphite and the polytetrafluoroethylene are separated by a separation material.

[0083] Since the graphite and the polytetrafluoroethylene have an irreversible side reaction at a low potential, the above-mentioned embodiments can avoid the occurrence of the side reaction by introducing the separation material.

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

[0085] In some embodiments, the separation material is coated on at least part of the surface of the graphite.

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

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

[0088] A first solid mixture comprising a first active material, a first binder and a first conductive agent is sprayed on at least one surface of the current collector and melted by hot pressing to form a first layer;

[0089] A second solid mixture comprising a second active material, a second binder and a second conductive agent is kneaded and rolled into a film, and the film is bonded to the first layer by heat pressing to form a second layer;

[0090] 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 melted by hot pressing to form the electrode sheet.

[0091] In the above preparation method, the first and third layers are sprayed on the current collector to form loose deposits, and then hot-pressed. This process does not involve the fiberization of the binder, which can ensure that its tortuosity is low. The second layer is made by kneading the solid mixture into a roller, and the binder is converted into fibrils through shear force. The fibrils are connected to each other during the mixing process to form a network to fix the electrode powder together. The calendered membrane has a fibrous connection structure, the lithium ion diffusion path is longer, and the ion resistance is higher, thereby increasing the tortuosity of the second layer.

[0092] The electrode sheet prepared by the preparation method of a specific embodiment is as follows Figure 1 As shown, the second layer is prepared by the binder fibrillation method and contains PTFE fiber filaments. The entangled and extended fiber filaments carry the conductive agent and adhere to the active particles. Compared with the first and third layers, a long-range electron path is added, and the electrode resistance is lower; in the actual battery system, the third layer is close to the diaphragm, and the diaphragm absorption rate is much higher than that of the positive and negative electrodes. During the infiltration process, the electrolyte infiltrates from the diaphragm side along the third layer into the first layer, so the lithium ion concentration gradient increases from the third layer to the first layer; based on the above phenomenon, the third layer is designed as a low-tortuosity electrode to promote electrolyte infiltration and increase the charge transfer rate at the interface. The second layer is a medium-tortuosity electrode with excellent electronic conductivity and reduces the gradient difference in electronic current density. The first layer is a low-tortuosity electrode to improve the phenomenon that ions cannot completely diffuse to the collector side at high rates, resulting in poor rate performance.

[0093] In one embodiment, the first solid mixture is sprayed onto at least one surface of a current collector, comprising:

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

[0095] And / or, the third solid mixture is sprayed on the surface of the second layer, comprising:

[0096] atomizing and spraying the third solid mixture on the surface of the second layer using a second gas stream and a second electrostatic force.

[0097] In one embodiment, the pressure of the first gas stream is 100-150 kPa, and the voltage of the first electrostatic force is 20-30 kV.

[0098] In one embodiment, the pressure of the second gas stream is 100-150 kPa, and the voltage of the second electrostatic force is 20-30 kV.

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

[0100] In a third aspect, the present application provides a battery comprising the electrode sheet described above.

[0101] It is understood that the battery described above further comprises a separator and an electrolyte.

[0102] The separator described above is not particularly limited, and any known porous separator having electrochemical stability and chemical stability can be used, for example, at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, or polyvinylidene fluoride. The separator can be single-layered or multi-layered.

[0103] The electrolyte described above comprises an organic solvent and an electrolyte salt. The organic solvent acts as a medium for transporting ions in an electrochemical reaction, and any known organic solvent for a battery electrolyte 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 one embodiment, two or more of the above organic solvents can be selected.

[0105] As a source of ions, the electrolyte salt may be an electrolyte salt known in the art for battery electrolyte. For example, the electrolyte salt may be lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium hexafluoroantimonate (LiSbF6), lithium difluorophosphate (LiPF2O2), 4,5-dicyano-2-trifluoromethylimidazolium lithium (LiDTI), lithium bis(oxalato)borate (LiBOB), lithium trifluoromethanesulfonate (LiTFS), lithium bis(malonate)borate (LiBMB), lithium difluorooxalatoborate (LiDFOB), bis(difluoropropane)sulfonate (LiPBT), lithium bis(tri ... At least one of lithium (difluoro)borate (LiBDFMB), lithium (malonate oxalate)borate (LiMOB), lithium (difluoromalonate oxalate)borate (LiDFMOB), lithium tris(oxalato)phosphate (LiTOP), lithium tris(difluoromalonate)phosphate (LiTDFMP), lithium tetrafluorooxalatophosphate (LiTFOP), lithium difluorobis(oxalato)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 sheets include positive and negative electrodes. The positive electrodes, separators, and negative electrodes may be stacked in sequence to obtain a battery cell, or the positive electrodes, separators, and negative electrodes may be stacked in sequence and then wound to obtain a battery cell. The battery cell is placed in a battery film shell (such as an aluminum-plastic film shell), and the electrolyte is injected into the outer packaging and sealed to prepare the battery of the present invention.

[0107] The present invention will be further described below with reference to specific embodiments:

[0108] Example 1

[0109] This example provides a positive electrode sheet, the preparation method of which includes:

[0110] (1) First layer: According to NCM811 (molecular formula: LiNi 0.8 Co 0.1 Mn 0.1 O2): Carbon black (SP): PVDF = 95:2.5:2.5 (wt%) ratio of electrode powders were mixed at high speed, and the premixed powders were accelerated with 137.9kPa high-pressure gas and atomized and sprayed onto carbon-coated aluminum foil. At the same time, under an electrostatic voltage of 25kV, the granular powders were attached to the surface of the current collector. Subsequently, the PVDF was hot-pressed and melted at 200°C to achieve the bonding of the active material, conductive agent and current collector, forming a first layer with a thickness of 40µm.

[0111] (2) The second layer: the electrode material is mixed in the ratio of NCM811: SP: PTFE = 95:2.5:2.5 (wt%), wherein the number average molecular weight of PTFE is 1.08*10 7 Da, and the initial fiberized electrode material is obtained by high-speed stirring of the cutting machine; the fiberized powder is kneaded, the roller temperature is 80 ℃, the A and B roller speeds are 1.5 m / min, the initial roller distance is 60 μm, the powder is poured from the top, after molding, the roller distance is adjusted, and the powder is folded into the roller kneading multiple times to form a regular-shaped film, then the A and B roller speeds are adjusted to 1.5, 1.8 m / min, and the transverse roller is rolled to 90 μm, then the speed difference is adjusted to 0.3 m / min, and the uniform film with a thickness of 40 μm is formed by multiple rolling thinning; finally, the film is placed on the first layer, the roller temperature is 200 ℃, the roller distance is 120 μm, and the roller speed is 1.4 m / min, and the two are rolled to the film adhered to the first layer;

[0112] (3) The third layer: the electrode powder is mixed at a high speed in the ratio of NCM811: SP: PVDF = 95:2.5:2.5 (wt%), the pre-mixed powder is accelerated by high-pressure gas of 137.9 kPa and sprayed on the previous two layers, and the active particle powder is attached to the surface under the electrostatic voltage of 25 kV, then the PVDF is melted by a hot roller at 200 ℃ to realize the bonding of the third layer, and the thickness is 40 μm, and the final areal density of the positive electrode sheet is controlled to be 20 mg / cm 2 , and the compaction is 3.4 g / cm 3 .

[0113] Example 2

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

[0115] Example 3

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

[0117] Example 4

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

[0119] Example 5

[0120] Preparation procedure substantially the same as Example 1, except that the PTFE used in step (2) has a molecular weight of 9.59 x 10 6 Da.

[0121] Example 6

[0122] Substantially the same as Example 1, except that the thickness ratio of the first, second and third electrode layers is 30 μm: 60 μm: 30 μm.

[0123] Example 7

[0124] Substantially the same as Example 1, except that the thickness ratio of the first, second and third electrode layers is 50 μm: 20 μm: 50 μm.

[0125] Example 8

[0126] Substantially the same as Example 1, except that the PTFE has a molecular weight of 2.5 x 10 7 Da.

[0127] Example 9

[0128] Substantially the same as Example 1, except that the PTFE is added in an amount of 3 wt%.

[0129] Example 10

[0130] Substantially the same as Example 1, except that the PTFE is replaced by ethylene-tetrafluoroethylene copolymer (ETFE), which has a molecular weight of 1.04 x 10 7 Da, and is added in an amount of 2.5 wt%, as shown in Table 1.

[0131] Example 11

[0132] Substantially the same as Example 3, except that the thickness ratio of the first, second and third electrode layers is 20 μm: 80 μm: 20 μm.

[0133] Comparative Example 1

[0134] The electrode sheet of this example was prepared as follows:

[0135] PVDF 5130 was mixed with N-methyl pyrrolidone (NMP) to obtain PVDF glue liquid by stirring for 240 min, NMP solvent was added in the stirring barrel, and then NCM811 and SP were added in sequence, low-speed stirring (revolution speed 15 rpm, dispersion speed 200 rpm, stirring for 10 min), followed by the addition of PVDF glue liquid, high-speed stirring (revolution speed 25 rpm, dispersion speed 2000 rpm, stirring for 240 min) to prepare a positive electrode slurry, wherein NCM811:SP:PVDF = 95:2.5:2.5 (wt%). The positive electrode slurry was extrusion coated on the current collector, the electrode piece was baked at 110°C for 12 h, and the roll was pressed to obtain a wet method electrode piece with a surface density of 20 mg / cm 2 , and a compaction of 3.4 g / cm 3 .

[0136] Comparative Example 2

[0137] The electrode piece preparation method of this example is as follows:

[0138] The electrode materials were mixed in a ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%), and the initial fibrous electrode material was obtained by high-speed stirring with a medicine cutter; the fibrous powder was kneaded, the roller temperature was 80°C, the A and B roller speeds were 1.5 m / min, the initial roller spacing was 60 μm, the powder was poured from the top, after shaping, the roller spacing was adjusted, and the powder was repeatedly folded into the roller and kneaded to form a regular film piece, then the A and B roller speeds were adjusted to 1.5 and 1.8 m / min, and the transverse roller was rolled to 90 μm, then the speed difference was adjusted to 0.3, and the uniform film piece with a thickness of 120 μm was repeatedly rolled and thinned to form a uniform film piece and composite current collector, and the roll was pressed to obtain a dry method electrode piece with a surface density of 20 mg / cm 2 , and a compaction of 3.4 g / cm 3 .

[0139] Comparative Example 3

[0140] The electrode piece preparation method of this example is as follows:

[0141] The electrode materials were mixed in a ratio of NCM811:SP:PVDF = 95:2.5:2.5 (wt%), the voltage of the spray gun and the current collector was set to 25 KV, and the dry powder in the hopper was transported to the electrostatic spray gun by compressed air for spraying, and the sprayed electrode was hot-pressed at 175°C and a pressure of 6 kg / cm 2 for 45 min to obtain a dry method electrode piece, and the roll was pressed to improve the compaction to 3.4 g / cm 3 .

[0142] Comparative Example 4

[0143] The example provides a positive electrode sheet, and a preparation method thereof includes:

[0144] (1) The first layer: the electrode material is mixed 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 electrode powder is mixed at high speed, the pre-mixed powder is accelerated by high-pressure gas and is atomized and sprayed on the current collector, and the active particle powder is attached to the surface of the current collector under the electrostatic voltage of 25 kV, then the PVDF is hot-pressed and melted to realize the bonding of the active material, the conductive agent and the current collector;

[0145] (2) The second layer: the electrode material is mixed according to the ratio of NCM811: SP: PTFE = 95:2.5:2.5 (wt%), wherein the number average molecular weight of PTFE is 1.08×10 7 Da, the initial fiberized electrode material is obtained by high-speed stirring of the cutting machine; the fiberized powder is kneaded, the roller temperature is 80 ℃, the A and B roller speeds are 1.5 m / min, the initial roller distance is 60 μm, the powder is poured from the top, after molding, the roller distance is adjusted, the powder becomes a regular-shaped membrane piece after multiple folding and roller kneading, then the A and B roller speeds are adjusted to 1.5 and 1.8 m / min, the transverse roller is calendered to 90 μm, then the speed difference is adjusted to 0.3 m / min, and the uniform membrane piece with a thickness of 40 μm is formed after multiple calendering and thinning; finally, the membrane piece is placed on the first layer in the current collector compounding process, the roller temperature is 200 ℃, the roller distance is 120 μm, and the roller speed is 1.4 m / min, and the two are rolled to make the membrane piece adhere to the first layer;

[0146] (3) The third layer: the electrode material is mixed according to the ratio of NCM811: SP: PTFE = 95:2.0:3.0 (wt%), wherein the number average molecular weight of PTFE is 2.5×10 7 Da, the initial fiberized electrode material is obtained by high-speed stirring of the cutting machine; the fiberized powder is kneaded, the roller temperature is 80 ℃, the A and B roller speeds are 1.5 m / min, the initial roller distance is 60 μm, the powder is poured from the top, after molding, the roller distance is adjusted, the powder becomes a regular-shaped membrane piece after multiple folding and roller kneading, then the A and B roller speeds are adjusted to 1.5 and 1.8 m / min, the transverse roller is calendered to 90 μm, then the speed difference is adjusted to 0.3 m / min, and the uniform membrane piece with a thickness of 40 μm is formed after multiple calendering and thinning; finally, the membrane piece is placed on the second layer in the current collector compounding process, the roller temperature is 200 ℃, the roller distance is 120 μm, and the roller speed is 1.4 m / min, and the two are rolled to make the membrane piece adhere to the second layer; the final area density of the positive electrode sheet is controlled to be 20 mg / cm2 , compaction 3.4 g / cm 3 .

[0147] Comparative Example 5

[0148] (1) First layer: NCM811 (molecular formula: LiNi 0.8 Co 0.1 Mn 0.1 O2): carbon black (SP): PVDF = 95:2.5:2.5 (wt%) ratio high-speed mixed electrode powder, the pre-mixed powder is accelerated by high-pressure gas of 137.9 kPa and is atomized and sprayed on the current collector, and at the same time, under the action of an electrostatic voltage of 25 kV, the active particle powder is attached to the surface of the current collector. Subsequently, the PVDF is hot-pressed and melted to realize the bonding of the active material, the conductive agent and the current collector;

[0149] (2) Second layer: mix the electrode material in a ratio of NCM811:SP:PTFE = 95:2.5:2.5 (wt%), wherein the number average molecular weight of PTFE is 1.08 x 10 7 Da, and high-speed stirring is performed by a cutting machine to obtain an initial fibrous electrode material; the fibrous powder is kneaded, the roller temperature is 80 ℃, the A and B roller speeds are 1.5 m / min, the initial roller gap is 60 μm, the powder is poured from the top, after shaping, the roller gap is adjusted, and the powder is repeatedly folded and rolled into a regular film piece, then the A and B roller speeds are adjusted to 1.5 and 1.8 m / min, and the transverse roller is calendered to 90 μm, then the speed difference is adjusted to 0.3 m / min, and the film piece is repeatedly calendered to a thickness of 60 μm. Finally, enter the current collector compounding process, that is, place the film piece on the first layer, the roller temperature is 200 ℃, the roller gap is 120 μm, and the roller speed is 1.4 m / min, and the two are rolled into the film piece adhered to the first layer;

[0150] Test Example

[0151] The above positive electrode sheet is used to prepare a battery, including the following steps:

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

[0153] (2) Electrolyte: The solvent was mixed by volume ratio of 35:35:3 of ethylene carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC), and LiPF6 was added to make the concentration 1 mol / L, and the electrolyte was obtained by uniform mixing.

[0154] (3) Battery production: The production of laminated soft pack battery and button cell was completed in the drying room and glove box, respectively. The production process of laminated soft pack battery: according to the Z-shaped laminating method, i.e. in the order of separator-positive electrode-separator-negative electrode-separator-positive electrode-separator, the laminating was performed, the prepared battery core was welded with positive and negative tabs, and then was put into an aluminum plastic shell for liquid injection, top side sealing, aging, and formation, i.e. the production of soft pack battery was completed. The production process of button cell: in the glove box, the positive electrode sheet was punched into a circular sheet with a diameter of 15 mm, and assembled in the order of positive electrode shell-positive electrode-electrolyte-separator-negative electrode-gasket-spring sheet-negative electrode shell, and the button cell was obtained by pressing with a pressure of 50 kgf.

[0155] Test Example

[0156] The following tests were performed on the positive electrode active material or the battery of the test force of the examples and comparative examples:

[0157] 1. Diffusion coefficient test: assemble the positive electrode against lithium button cell, use GITT to test the diffusion coefficient of the material, activate the battery by 0.2C charging and discharging for one week before testing, adjust the SOC, 0.1C pulse charging for 10s, and stand for 1h, calculate the lithium ion diffusion coefficient according to the voltage difference before and after pulse and before and after standing, the calculation method is shown in formula 1:

[0158] Formula 1,

[0159] Wherein, 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 in the 0.1C pulse charging section, ΔE T is the voltage change when standing to equilibrium.

[0160] 2. Tortuosity test: assemble the positive electrode symmetrical button cell, use the electrochemical workstation to test the EIS curve, including the following processes:

[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) Use mercury intrusion method or calculation formula to obtain the porosity of the electrode sheet (ε, %): calculation formula: Wherein,

[0163] ;

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

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

[0166] 5) Calculate the tab tortuosity according to formula 2:

[0167] Formula 2.

[0168] 3. Tensile strength test: The tensile strength of the membrane was tested using a universal material testing machine, and the tensile rate was 50 mm / min.

[0169] 4. Liquid climbing height test: The wicking performance of the tab was tested using the hanging method, the tab of each example and comparative example was cut into a 15*40 mm strip, then the tab was fixed vertically, the lower end of the tab was immersed in the electrolyte, and the time and liquid level corresponding scale were recorded, and the climbing height of the tab was recorded after 5 min.

[0170] 5. Battery performance test: (1) Rate performance: 0.33C constant current and constant voltage charging to full charge state cutoff voltage, cutoff current is 0.05C, then discharging at 0.1, 0.2, 0.5, 1, 0.1C rate to empty state cutoff voltage, taking the first 0.1C discharge capacity C0 as the basis, comparing the rate capacity retention rate of each example / comparative example battery according to the ratio of 1C discharge capacity to C0; (2) Cycle performance: 1C constant current and constant voltage charging, cutoff current is 0.05C, 1C constant current discharging, cycle 100 times, calculate the capacity ratio of the first cycle and the 100th cycle.

[0171] The above test results are summarized in Table 1 and Table 2.

[0172] Table 1:

[0173]

[0174] In the table, "-" represents no result of the parameter.

[0175] Table 2:

[0176]

[0177] From Tables 1-2, it can be seen that the electrodes of the examples have more excellent rate capacity retention and cycle capacity retention than Comparative Examples 1-5, and the main reasons include: the thick electrode with a suitable hierarchical electrode structure has high ion diffusion efficiency and electronic conductivity advantage, in the process of battery charging and discharging, ions pass through the inner / outer sprayed electrode layer, and due to the high porosity and low tortuosity of the outer layer, the electrolyte is beneficial to the infiltration of the electrolyte, at the same time, the low tortuosity of the inner layer weakens the difference in lithium ion concentration gradient, which is beneficial to improve the rate performance of the thick electrode, the middle layer inserts the fibrous dry electrode layer, optimizes the conductive network, and promotes the improvement of charge transfer efficiency, therefore, the electrode tab 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 application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part 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 application.

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: 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, wherein 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 smaller than B, and C is smaller than B; The thickness of the third layer is 30-50 μm.

2. The electrode sheet according to claim 1, characterized in that 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 to 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; wherein 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, characterized in that In the second layer, the mass proportion 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 to 3, characterized in that: The thickness of the first layer is 30-50µm; And / or, the thickness of the second layer is 20-60 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 to 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 to 8, characterized in that: The following steps are involved: A first solid mixture comprising a first active material, a first binder and a first conductive agent is sprayed on at least one surface of a current collector and melted by hot pressing to form a first layer; A second solid mixture comprising a second active material, a second binder and a second conductive agent is kneaded and rolled into a film, and the film is bonded to the first layer by heat pressing 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 melted by hot pressing to form the electrode sheet.

10. The preparation method according to claim 9, characterized in that The first solid mixture is sprayed on at least one surface of a current collector, comprising: A step of atomizing the first solid mixture by using a first airflow and a first static electricity and spraying the atomized solid mixture onto at least one surface of a current collector; And / or, the third solid mixture is sprayed on the surface of the second layer, comprising: The step of utilizing a second airflow and second static electricity to atomize the third solid mixture and spraying it on the surface of the second layer.

11. The preparation method according to claim 10, characterized in that: 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 airflow is 100-150 kPa, and the voltage of the second static electricity is 20-30 kV.

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

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