Electrode sheet and electrochemical device

Abstract

Provided in the present disclosure are an electrode sheet and an electrochemical device. The electrode sheet comprises a current collector, wherein in an active material layer on at least one surface of the current collector, the difference between the mass percent of fluorine therein within a first region relative to the total mass of the active material layer in the first region and the mass percent of fluorine in the active material layer within a second region relative to the total mass of the active material layer in the second region is greater than 0, but less than or equal to 3%; the first region is a region between the side surface of the active material layer facing away from the current collector and a first side edge; the second region is a region between the side surface of the active material layer close to the current collector and a second side edge; the distance between the first side edge and the side surface of the active material layer facing away from the current collector is 1-10% of the total thickness of the active material layer; and the distance between the second side edge and the side surface of the active material layer close to the current collector is 1-10% of the total thickness of the active material layer. The electrode sheet provided in the present disclosure can improve the structural strength of the electrode sheet, and reduce the possibility of the electrode sheet cracking or shedding.

Description

Electrode and electrochemical device Technical Field

[0001] The present disclosure relates to the technical field of lithium-ion batteries, and in particular to a pole piece and an electrochemical device. Background Art

[0002] With the development of lithium-ion battery technology, lithium-ion batteries have been widely used in portable electronic products, new energy vehicles and other fields due to their good electrochemical properties.

[0003] With the further commercialization of lithium-ion batteries, the importance of high energy density batteries has become increasingly prominent. However, the energy density of batteries is significantly affected by their volume. Thick electrode design can improve the energy density of batteries by increasing the proportion of active material.

[0004] At present, there are many problems in the processing of thick electrodes. For example, during the processing of thick electrodes, the pole pieces often crack or even fall off, which seriously affects the performance of the battery.

[0005] Summary of the Invention

[0006] In view of this, the present disclosure is dedicated to providing a pole piece and an electrochemical device to improve the structural strength of the pole piece and reduce the possibility of cracking or powder loss of the pole piece.

[0007] A first aspect of the present disclosure provides a pole piece, comprising a current collector, wherein at least one surface of the current collector is provided with an active material layer;

[0008] The difference between the mass percentage of the fluorine element in the first region of the active material layer to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 3%;

[0009] The first region is a region between a surface of the active material layer facing away from the current collector and the first side edge;

[0010] The second region is a region between a surface of the active material layer close to the current collector and a second side edge;

[0011] The distance between the first side and the surface of the active material layer facing away from the current collector is 1-10% of the total thickness of the active material layer;

[0012] The distance between the second side and a surface of the active material layer close to the current collector is 1-10% of the total thickness of the active material layer.

[0013] Optionally, the difference between the mass percentage of the fluorine element in the first region of the active material layer to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 1%,

[0014] Preferably, the difference between the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 0.5%.

[0015] Optionally, the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is 0-18 wt %;

[0016] and / or; the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is 0-18 wt %.

[0017] Optionally, the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is 0.5-10 wt %;

[0018] and / or; the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is 0.5-10 wt %.

[0019] Optionally, the active material layer includes a third region, and the third region is a region between the third side and the fourth side;

[0020] The distance between the third side and the surface of the active material layer facing away from the current collector is 45%-50% of the total thickness of the active material layer;

[0021] The distance between the fourth side and the surface of the active material layer close to the current collector is 45%-50% of the total thickness of the active material layer;

[0022] The mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is greater than the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region, and / or the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is greater than the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region.

[0023] Optionally, a difference between a mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and a mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is ≤3%;

[0024] And / or; the difference between the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is ≤3%.

[0025] Optionally, a difference between a mass percentage of fluorine in the active material layer in the third region to the total mass of the active material layer in the third region and a mass percentage of fluorine in the active material layer in the first region to the total mass of the active material layer in the first region is ≤1.5%;

[0026] And / or; the difference between the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is ≤1.5%.

[0027] Optionally, the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is 0-21wt%. Preferably, the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is 1-12wt%.

[0028] Optionally, the active material layer includes a first binder and an active material;

[0029] A difference between a mass percentage ratio of the first binder to the active material in the first region and a mass percentage ratio of the first binder to the active material in the second region of the active material layer is ≤9%.

[0030] Optionally, a difference between a mass percentage ratio of the first binder to the active material in the first region of the active material layer and a mass percentage ratio of the first binder to the active material in the second region of the active material layer is ≤5%.

[0031] Optionally, the mass percentage ratio of the first binder to the active material in the first region of the active material layer is 0.5-10%;

[0032] and / or; the mass percentage ratio of the first binder to the active material in the second region of the active material layer is 0.5-10%.

[0033] Optionally, the first binder includes at least one of fibrous polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), (ETEF), fluorinated ethylene propylene copolymer (FEP), polyvinyl chloride (PVP), polyethylene oxide (PEO), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), polyacrylic acid (PAA), polypropylene (PP), polyethylene (PE) and polyacrylonitrile (PAN);

[0034] And / or, the active material includes LiCoO2, LiMn2O4, LiMnO2, LiNiO2, LiFePO4, LiMnPO4, LiCo x Ni 1-x O2(0≤x≤1), LiCo x Ni 1-x-y Al y At least one of O2 (0≤x≤1, 0≤y≤1);

[0035] And / or, the active material includes artificial graphite, natural graphite, soft carbon, hard carbon, mesocarbon microbeads, Si, SiOx, Si-C and SiO x -At least one of C.

[0036] Optionally, a bonding layer is further provided between the current collector and the active material layer, and the bonding layer includes a second bonding material and a conductive agent.

[0037] Optionally, the second adhesive material includes at least one of PVDF, SBR, PP, ethylene-vinyl acetate copolymer (EVA) hot melt adhesive, polyolefins (PO) hot melt adhesive, reactive polyurethane hot melt adhesive (PUR) hot melt adhesive, thermoplastic rubber (TPR) material, thermoplastic polyurethane elastomer (TPU) hot melt adhesive, polyamide (PA), polyester (PES), polyethylene and polyesteramide (PEA);

[0038] And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, conductive carbon fiber and conductive graphite.

[0039] Optionally, the thickness of the active material layer is greater than or equal to 20 μm and less than or equal to 3 mm; preferably, the thickness of the active material layer is greater than or equal to 30 μm and less than or equal to 300 μm.

[0040] and / or, the thickness of the bonding layer is greater than or equal to 500 nm and less than or equal to 3 μm;

[0041] And / or, the ratio of the thickness of the active material layer to the thickness of the bonding layer is greater than or equal to 10 and less than or equal to 6000; preferably, the ratio of the thickness of the active material layer to the thickness of the bonding layer is greater than or equal to 30 and less than or equal to 300.

[0042] A second aspect of the present disclosure provides an electrochemical device, comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet;

[0043] The positive electrode sheet and / or the negative electrode sheet are the electrode sheets as described in the first aspect.

[0044] The present disclosure provides a pole piece including a current collector, at least one surface of which is provided with an active material layer. By making the difference between the mass percentage of the fluorine element in the first region of the active material layer to the mass percentage of the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the mass percentage of the total mass of the active material layer in the second region greater than 0 and less than or equal to 3%, the fluorine element is an important component of the active material layer, and by controlling the difference between the fluorine element in the first region of the top surface and the second region of the bottom surface of the active material layer, that is, achieving a mass percentage of the fluorine element on the top surface and the bottom surface of the active material layer close, it means that the fluorine element is fully and evenly distributed in the thickness direction of the active material layer. The fluorine element has strong electrophilicity and chemical inertness, which can enhance the bonding force between the binder and the active material (i.e., active material) in the active material layer, so that the stress on the pole piece in the thickness direction is evenly distributed during processing, reducing the possibility of powder loss or cracking of the pole piece, thereby causing a short circuit in the battery; furthermore, the uniform distribution of the fluorine element in the thickness direction of the active material layer can make the active material layer have consistent chemical properties in the thickness direction, which helps to improve the performance and stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG1 shows one of the schematic diagrams of a pole piece provided by an embodiment of the present disclosure;

[0046] FIG2 shows a second schematic diagram of a pole piece provided by an embodiment of the present disclosure;

[0047] FIG3 shows a third schematic diagram of a pole piece provided by an embodiment of the present disclosure;

[0048] FIG4 is a fourth schematic diagram of a pole piece provided by an embodiment of the present disclosure;

[0049] FIG5 is a schematic diagram of a pole piece in Example 7 provided by the present disclosure;

[0050] FIG6 shows an element distribution diagram of the first region of the electrode in Example 7 provided by the present disclosure;

[0051] FIG7 shows an element distribution diagram of the third region of the electrode in Example 7 provided by the present disclosure;

[0052] FIG8 shows an element distribution diagram of the second region of the electrode in Example 7 provided by the present disclosure;

[0053] FIG9 is a schematic diagram of a pole piece in Example 3 provided by the present disclosure;

[0054] FIG10 shows an element distribution diagram of the first region of the electrode in Example 3 provided by the present disclosure;

[0055] FIG11 shows an element distribution diagram of the third region of the electrode in Example 3 provided by the present disclosure;

[0056] FIG12 shows an element distribution diagram of the second region of the electrode in Example 3 provided by the present disclosure. DETAILED DESCRIPTION

[0057] The following will clearly and completely describe the technical solutions in the embodiments of the present disclosure in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present disclosure without making any creative efforts shall fall within the scope of protection of the present disclosure.

[0058] 1 and 2 , the present disclosure provides a pole piece, including a current collector 10 , at least one surface of the current collector 10 being provided with an active material layer 11 ;

[0059] a difference between the mass percentage of the fluorine element in the first region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the first region and the mass percentage of the fluorine element in the second region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the second region is greater than 0 and less than or equal to 3%. For example, the difference may be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, or 3%.

[0060] The first region is the region between the surface of the active material layer 11 facing away from the current collector 10 and the first side edge;

[0061] The second region is the region between the surface of the active material layer 11 close to the current collector 10 and the second side edge;

[0062] The distance between the first side and the surface of the active material layer 11 facing away from the current collector 10 is 1-10% of the total thickness of the active material layer 11, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% of the total thickness;

[0063] The distance between the second side and the surface of the active material layer 11 close to the current collector 10 is 1-10% of the total thickness of the active material layer 11, for example, it can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10% of the total thickness.

[0064] The electrode sheet can be a positive electrode sheet or a negative electrode sheet. The active material layer 11 can be provided on only one surface of the current collector 10, as shown in FIG1 , or on both surfaces of the current collector 10, as shown in FIG2 .

[0065] Exemplarily, the distance between the first side and the surface of the active material layer 11 facing away from the current collector 10 and the distance between the second side and the surface of the active material layer 11 close to the current collector 10 may be the same or different. For example, the distance between the first side and the surface of the active material layer 11 facing away from the current collector 10 and the distance between the second side and the surface of the active material layer 11 close to the current collector 10 may both be 3%, 5%, or 8% of the total thickness of the active material layer 11; for another example, the distance between the first side and the surface of the active material layer 11 facing away from the current collector 10 is 5% of the total thickness of the active material layer 11, and the distance between the second side and the surface of the active material layer 11 close to the current collector 10 is 7% of the total thickness of the active material layer.

[0066] The first side mentioned above can be understood as the cross-section of the active material layer 11 at the first thickness position, and the first thickness refers to the distance between the first side and the surface of the active material layer 11 facing away from the current collector 10 in the thickness direction of the active material layer 11. The second side mentioned above can be understood as the cross-section of the active material layer 11 at the second thickness position, and the second thickness refers to the distance between the second side and the surface of the active material layer 11 close to the current collector 10 in the thickness direction of the active material layer 11. (Figures 1 and 2 are schematic longitudinal cross-sectional views of the electrode sheet)

[0067] The mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region refers to the percentage obtained by dividing the total mass of the fluorine element in the first region by the total mass of the active material layer 11 in the first region.

[0068] Similarly, the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region refers to the percentage obtained by dividing the total mass of the fluorine element in the second region by the total mass of the active material layer 11 in the second region.

[0069] The mass percentage of the fluorine element in the first region of the active material layer 11 to the total mass of the active material layer 11 in the first region can be detected by energy spectrum elemental analysis. Specifically, the cross section of the electrode can be cut by argon ion gas first, and then the element distribution of the first region and / or the second region of the active material layer can be tested using a scanning electron microscope energy spectrum elemental analysis based on the specific position of the first side and / or the second side, so that the mass fraction of the fluorine element in the total mass of the active material layer in the region at different positions can be measured. In one embodiment, the element distribution at 5 microns below the top surface of the active material layer (here refers to the surface of the active material layer facing away from the current collector) and 5 microns above the bottom surface of the active material layer (here refers to the surface of the active material layer close to the current collector) can be tested using a scanning electron microscope energy spectrum elemental analysis, and the mass fraction of the fluorine element in the total mass of the active material layer in the region at the corresponding position can be measured, as shown in Figures 6-8.

[0070] The mass percentage of the fluorine element in the first region of the active material layer 11 to the total mass of the active material layer 11 in the first region, and / or the mass percentage of the fluorine element in the second region of the active material layer 11 to the total mass of the active material layer 11 in the second region can be adjusted by adjusting the content of the binder, the type of binder or the thickness of the active material layer.

[0071] In the embodiment of the present disclosure, by making the difference between the mass percentage of the fluorine element in the first region of the active material layer 11 to the total mass of the active material layer 11 in the first region and the mass percentage of the fluorine element in the second region of the active material layer 11 to the total mass of the active material layer 11 in the second region greater than 0 and less than or equal to 3%, the fluorine element, as an important component of the active material layer, is controlled by controlling the difference between the fluorine element in the first region of the top surface and the second region of the bottom surface of the active material layer, that is, achieving a mass percentage of the fluorine element on the top surface and the bottom surface of the active material layer close, which means that the fluorine element is fully and evenly distributed in the thickness direction of the active material layer, so that during the processing, the stress on the electrode in the thickness direction is evenly distributed, thereby avoiding the electrode from falling off or cracking, thereby causing a short circuit in the battery; furthermore, the uniform distribution of the fluorine element in the thickness direction of the active material layer can make the active material layer have consistent chemical properties in the thickness direction, which helps to improve the performance and stability of the battery.

[0072] In one specific embodiment, the active material layer can be produced by a dry process, that is, the active material, the binder, and the conductive agent are dry-mixed without using a solvent to produce a composite material with a solid content concentration of substantially 100%. Dry mixing refers to the following method: the positive electrode active material and the binder are mixed without using a solvent in a state where the solid content concentration is substantially 100%. During the dry mixing, conductive materials other than the positive electrode active material and the binder can also be added. When materials other than the positive electrode active material and the binder are added, the solid content concentration in the dry mixing is also substantially 100%.

[0073] Subsequently, the composite material is rolled into a sheet to produce a sheet-shaped active material layer 11. Furthermore, the current collector and the active material layer 11 are stacked and hot-pressed to form an electrode sheet.

[0074] Compared with the existing wet method for preparing pole pieces, in its production process, it is necessary to mix the solvent, active material, and binder to form a slurry, and then apply the slurry to the surface of the current collector, and then dry the slurry coating on the surface of the current collector to volatilize the solvent. It can be seen that the drying process of the wet method makes the production process of the battery more complicated and reduces the production efficiency. In this embodiment, by adopting the dry method to prepare the pole piece, there is no need to use a solvent. The active material and the binder are mixed to form a slurry, and then the slurry is applied to the surface of the current collector 10. There is no need to dry the slurry coating on the surface of the current collector 10 to volatilize the solvent, thereby simplifying the production process of the pole piece and improving the production efficiency of the battery.

[0075] To make the fluorine element in the active material layer 11 more evenly distributed, thereby reducing the possibility of cracking and powder loss at the bottom of the active coating layer, optionally, the difference between the mass percentage of the fluorine element in the first region of the active material layer 11 to the total mass of the active material layer 11 in the first region and the mass percentage of the fluorine element in the second region of the active material layer 11 to the total mass of the active material layer 11 in the second region is greater than 0 and less than or equal to 1%. For example, the difference can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, or 1%.

[0076] Preferably, the difference between the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region and the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region is greater than 0 and less than or equal to 0.5%. For example, the difference can be 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%, thereby further improving the consistency of the electrical properties of the active material layer in the thickness direction and enhancing the electrochemical performance and safety performance of the battery.

[0077] Optionally, the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is 0-18 wt %, for example, 0.1 wt %, 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, 10 wt %, 11 wt %, 12 wt %, 13 wt %, 14 wt %, 15 wt %, 16 wt %, 17 wt %, or 18 wt %;

[0078] And / or; the mass percentage of fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region is 0-18wt%, for example, it can be 0.1wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%.

[0079] When the fluorine content of the active material layer 11 in the first region and the second region is within the above range, the binder can fully encapsulate the active material particles, reducing electrode rebound and powder loss during rolling and in the later stages of battery cycling, thereby improving energy density and battery safety. Specifically, the mass percentage of fluorine in the first region of the active material layer 11 to the total mass of the active material layer 11 in the first region can be 0.9wt%, 5wt%, 12wt%, or 18wt%.

[0080] The mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region may be 0.9 wt %, 5 wt %, 12 wt %, or 18 wt %.

[0081] Optionally, the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is 0.5-10 wt %, for example, 0.5 wt %, 1 wt %, 2 wt %, 3 wt %, 4 wt %, 5 wt %, 6 wt %, 7 wt %, 8 wt %, 9 wt %, or 10 wt %;

[0082] And / or; the mass percentage of fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region is 0.5-10wt%, for example, it can be 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%.

[0083] Furthermore, when the fluorine content of the active material layer 11 in the first region and the second region is within the above-mentioned smaller range, the mass of the active material per unit area can be improved, and the gram capacity of the electrode can be maximized while ensuring that the binder does not lose powder and reduces rebound, which is conducive to achieving high energy density product design.

[0084] Specifically, the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region may be 0.9 wt %, or 2 wt %, or 5 wt %, or 9 wt %;

[0085] The mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region may be 0.9 wt %, 2 wt %, 5 wt %, or 9 wt %.

[0086] If the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is less than 0.5wt%, the electrochemical performance of the electrode (such as capacity, cycle life, etc.) will be reduced; if the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is greater than 10wt%, the overall energy density of the battery will be reduced.

[0087] Optionally, the active material layer 11 includes a third region, which is a region between the third side and the fourth side;

[0088] The distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 is 45%-50% of the total thickness of the active material layer 11 , for example, 45%, 46%, 47%, 48%, 49%, or 50% of the total thickness;

[0089] The distance between the fourth side and the surface of the active material layer 11 close to the current collector 10 is 45%-50% of the total thickness of the active material layer 11, for example, 45%, 46%, 47%, 48%, 49%, or 50% of the total thickness;

[0090] The mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region is greater than the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region, and / or the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region is greater than the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region.

[0091] A third region is also provided in the active material layer 11, and the fluorine content in the third region is higher than the fluorine content in the first and second regions, that is, the mass percentage of the fluorine element in the middle region of the active material layer 11 is higher, which can enhance the bonding force between the binder and the active material (i.e., active material) in the active material layer, thereby reducing the possibility of cracking or disconnection in the middle region of the active material layer due to excessive stress, thereby improving the stability and cycle life of the electrode sheet (i.e., pole sheet).

[0092] For example, the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 and the distance between the fourth side and the surface of the active material layer 11 facing the current collector 10 may be the same or different. For example, the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 and the distance between the fourth side and the surface of the active material layer 11 facing the current collector 10 are both 45%, 48%, or 50% of the total thickness of the active material layer 11. For another example, the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 is 45% of the total thickness, and the distance between the fourth side and the surface of the active material layer 11 facing the current collector 10 is 49% of the total thickness. In one specific embodiment, the third region is the region between the third side extending upward by 5 microns from the middle of the active material layer in the thickness direction and the fourth side extending downward by 5 microns from the middle of the active material layer in the thickness direction (i.e., the region enclosed by the third and fourth sides in the thickness direction). In another embodiment, the third region, which is located at half the thickness of the active material layer 11 and is within ±10% of the thickness of the active material layer 11, can also be understood as the middle region of the active material layer 11 in the thickness direction.

[0093] The third side may be understood as a cross section of the active material layer 11 at a third thickness position, and the third thickness refers to the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 in the thickness direction of the active material layer 11. The fourth side may be understood as a cross section of the active material layer 11 at a fourth thickness position, and the fourth thickness refers to the distance between the fourth side and the surface of the active material layer 11 close to the current collector 10 in the thickness direction of the active material layer 11.

[0094] The mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region can also be determined by scanning electron microscope energy spectrum element analysis.

[0095] The mass percentage of fluorine in the third region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the third region is greater than the mass percentage of fluorine in the first region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the first region, and / or the mass percentage of fluorine in the third region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the third region is greater than the mass percentage of fluorine in the second region of the active material layer 11 as a percentage of the total mass of the active material layer 11 in the second region. In other words, the mass percentage of fluorine in the central region of the active material layer 11 is at least greater than the mass percentage of fluorine in a side edge region of the active material layer 11. Due to the higher mass percentage of fluorine in the central region of the active material layer 11 and the strong electrophilicity and chemical inertness of fluorine, the bonding force between the binder and the active material (i.e., active material) in the active material layer is enhanced, thereby reducing the possibility of cracking or disconnection in the central region of the active material layer due to excessive stress, thereby improving the stability and cycle life of the electrode sheet (i.e., pole piece).

[0096] To further enhance the stability and cycle life of the electrode sheet, optionally, the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is ≤3%, for example, the difference can be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, or 3%;

[0097] And / or; the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region is ≤3%, for example, the difference can be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, 3%.

[0098] Specifically, the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region can be 2% or 3%.

[0099] The difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the second region to the total mass of the active material layer 11 in the second region can be 2% or 3%.

[0100] In one embodiment, the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region is ≤1.5%. For example, the difference may be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, or 1.5%.

[0101] And / or; the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region is ≤1.5%, for example, the difference can be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.4%, 1.5%, 1.8%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, 3%.

[0102] Further reducing the difference between the fluorine content in the third region and the fluorine content in the first region and the second region can further improve the stability and cycle life of the electrode.

[0103] Specifically, the difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the first region to the total mass of the active material layer 11 in the first region can be 0.5%, or 0.9%, or 1.5%.

[0104] The difference between the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region and the mass percentage of the fluorine element in the active material layer 11 in the second region to the total mass of the active material layer 11 in the second region can be 0.5%, or 0.9%, or 1.5%.

[0105] Optionally, the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region is 0-21wt%, for example, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 4wt%, 5wt%, 6wt%, 8wt%, 10wt%, 12wt%, 14wt%, 15wt%, 16wt%, 18wt%, 20wt%, or 21wt%. When the fluorine content in the active material layer 11 in the third region is within the above range, the cohesion of the electrode can be effectively improved to prevent cracking and powder loss.

[0106] Specifically, the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region may be 0.9 wt %, 10 wt %, or 21 wt %.

[0107] Optionally, the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is 1-12wt%, for example, 0.1wt%, 0.5wt%, 1wt%, 2wt%, 4wt%, 5wt%, 6wt%, 8wt%, 10wt%, or 12wt%. Furthermore, when the fluorine content of the active material layer 11 in the third region is within the above-mentioned smaller range, while ensuring effective bonding between the main material particles, the main material particles per unit area are increased to ensure energy density. In addition, when the binder is too much, the transmission resistance of electrons and ions will increase, and the battery impedance will also increase.

[0108] Specifically, the mass percentage of the fluorine element in the active material layer 11 in the third region to the total mass of the active material layer 11 in the third region may be 1.2 wt %, 5 wt %, or 11 wt %.

[0109] Optionally, the active material layer 11 includes a first binder and an active material; the difference between the mass percentage ratio of the first binder and the active material in the first region of the active material layer 11 and the mass percentage ratio of the first binder and the active material in the second region of the active material layer 11 is ≤9%, for example, the difference can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, or 9%.

[0110] In this embodiment, by controlling the difference in mass percentage between the first binder in the first region of the active material layer 11 and the first binder in the second region of the active material layer 11, the first binder can be distributed more evenly across the different regions, reducing the likelihood of binder migration. This ensures that the stress in the thickness direction of the electrode sheet is substantially uniform, thereby reducing the likelihood of cracking of the active material layer 11 or even shedding from the surface of the current collector 10. This also reduces the electrode sheet's penetration resistance and improves the peel strength of the electrode composite sheet. Furthermore, the more even distribution of the first binder minimizes electrode sheet expansion during cycling, extending the battery's service life during cycling and reducing the cell's overall impedance.

[0111] Specifically, the difference between the mass percentage ratio of the first binder and the active material in the first region of the active material layer 11 and the mass percentage ratio of the first binder and the active material in the second region of the active material layer 11 can be 1%, or 3%, or 6%, or 9%.

[0112] In order to further reduce the through-hole resistance of the electrode sheet and improve the peel strength of the electrode composite material sheet, optionally, the difference between the mass percentage ratio of the first binder and the active material in the first region of the active material layer 11 and the mass percentage ratio of the first binder and the active material in the second region of the active material layer 11 is ≤5%.

[0113] Specifically, the difference between the mass percentage ratio of the first binder and the active material in the first region of the active material layer 11 and the mass percentage ratio of the first binder and the active material in the second region of the active material layer 11 can be 1%, or 2%, or 3%, or 4%, or 5%.

[0114] Optionally, the mass percentage ratio of the first binder to the active material in the first region of the active material layer 11 is 0.5-10%;

[0115] Specifically, the mass percentage ratio of the first binder and the active material in the first region of the active material layer 11 can be 0.5%, or 1%, or 2%, or 3%, or 4%, or 5%, or 6%, or 7%, or 8%, or 9%, or 10%.

[0116] Similarly, in order to take into account both the binding force and energy density of the active material layer 11 , optionally, the mass percentage ratio of the first binder to the active material in the second region of the active material layer 11 is 0.5%-10%.

[0117] Specifically, the mass percentage ratio of the first binder and the active material in the second region of the active material layer 11 can be 0.5%, or 1%, or 2%, or 3%, or 4%, or 5%, or 6%, or 7%, or 8%, or 9%, or 10%.

[0118] Optionally, the first binder includes at least one of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), ETEF, fluorinated ethylene propylene copolymer (FEP), PVP, polyethylene oxide (PEO), CMC, SBR, polyacrylic acid (PAA), PP, PE, and PAN.

[0119] In the case of a positive electrode, the active materials include LiCoO2, LiMn2O4, LiMnO2, LiNiO2, LiFePO4, LiMnPO4, LiCo x Ni 1-x O2(0≤x≤1), LiCo x Ni 1-x-y Al y At least one of O2 (0≤x≤1, 0≤y≤1);

[0120] In the case where the electrode is a negative electrode, the active materials include artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, Si, SiOx, Si-C, SiO x -At least one of C.

[0121] Optionally, referring to FIG. 3 and FIG. 4 , a bonding layer 12 is further provided between the current collector 10 and the active material layer 11 , and the bonding layer 12 includes a second bonding material and a conductive agent.

[0122] By providing the bonding layer 12 between the current collector 10 and the active material layer 11 , the bonding strength between the active material layer 11 and the current collector 10 can be increased, thereby reducing the possibility of the active material layer 11 falling off.

[0123] Optionally, the second adhesive material includes at least one of PVDF, SBR, PP, EVA hot melt adhesive, PO hot melt adhesive, PUR hot melt adhesive, TPR material, TPU hot melt adhesive, polyamide (PA, polyester (PES), polyethylene and polyester amide (PEA);

[0124] And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, conductive carbon fiber, and conductive graphite. By selecting a conductive agent with a large specific surface area (such as conductive carbon black, carbon nanotubes, or graphene), the roughness of the bonding layer 12 can be increased, thereby improving the bonding strength and peeling strength between the active material layer 11 and the adhesive layer 12.

[0125] Optionally, the thickness of the active material layer 11 is greater than or equal to 20 μm and less than or equal to 3 mm (for example, the thickness of the active material layer 11 can be 20 μm, 100 μm, 200 μm, 500 μm, 800 μm, 1 mm, 2 mm, 3 mm). Preferably, the thickness of the active material layer is greater than or equal to 30 μm and less than or equal to 300 μm (for example, the thickness of the active material layer 11 can be 30 μm, 50 μm, 80 μm, 100 μm, 150 μm, 200 μm, 250 μm, 300 μm). If the thickness of the active material layer 11 is less than 30 μm, the active material layer is prone to breakage during processing. If the thickness of the active material layer 11 is greater than 300 μm, the electrolyte is difficult to infiltrate, the lithium ion transmission path is too long, and the lithium ion transmission kinetics are insufficient, which seriously affects the electrochemical performance of the battery.

[0126] And / or, the thickness of the adhesive layer 12 is greater than or equal to 500nm and less than or equal to 3μm (for example, the thickness of the adhesive layer 12 is 500nm, 600nm, 800nm, 1μm, 1.5μm, 1.8μm, 2μm, 2.5μm, 2.8μm, 3μm); preferably, the thickness of the adhesive layer 12 is greater than or equal to 1μm and less than or equal to 2μm (for example, the thickness of the adhesive layer 12 is 1μm, 1.1μm, 1.2μm, 1.3μm, 1.4μm, 1.5μm, 1.6μm, 1.7μm, 1.8μm, 1.9μm, 2μm). By controlling the thickness of the adhesive layer 12 within 1-2μm, both the bonding force and the internal resistance of the electrode can be guaranteed. If the thickness of the adhesive layer 12 is greater than 2 μm, the internal resistance of the electrode will increase; if the thickness of the adhesive layer 12 is less than 1 μm, the active material layer will easily fall off.

[0127] And / or, the thickness ratio of the active material layer 11 to the bonding layer 12 is greater than or equal to 10 and less than or equal to 6000 (for example, the thickness ratio can be 10, 50, 100, 200, 500, 1000, 2000, 3000, 4000, 5000, 6000). Preferably, the thickness ratio of the active material layer 11 to the bonding layer 12 is greater than or equal to 30 and less than or equal to 300 (for example, the thickness ratio can be 30, 50, 80, 100, 120, 150, 160, 180, 200, 220, 250, 280, 300). By controlling the thickness ratio of the active material layer 11 to the bonding layer 12, the contact area between the current collector and the active material layer is increased without reducing the battery energy density, thereby improving the conductivity and cycle life of the battery, and also preventing problems such as peeling or short circuiting between the active material layer and the current collector due to mechanical vibration.

[0128] Specifically, the thickness of the active material layer 11 may be 30 μm, or 70 μm, or 1 mm; the thickness of the bonding layer 12 may be 500 nm, or 1 μm, or 3 μm;

[0129] The thickness ratio of the active material layer 11 to the adhesive layer 12 may be 10, 20, 100, 500, 1500, or 2000.

[0130] The content ratio of the first adhesive material in the active material layer 11 is smaller than the content ratio of the second adhesive material in the bonding layer 12 , which can improve the bonding strength between the bonding layer 12 and the active material layer 11 and between the bonding layer 12 and the current collector 10 .

[0131] Exemplarily, the electrode may be a positive electrode or a negative electrode.

[0132] The present disclosure also provides an electrochemical device, comprising a positive electrode sheet, a negative electrode sheet, and a separator located between the positive electrode sheet and the negative electrode sheet; the positive electrode sheet and / or the negative electrode sheet are the electrodes provided in the embodiments of the present disclosure.

[0133] In some embodiments, the electrochemical device may be a lithium-ion battery.

[0134] Since the electrochemical device provided by the present disclosure includes the electrode provided by the embodiment of the present disclosure, the electrochemical device provided by the embodiment of the present disclosure can achieve all the beneficial effects of the electrode provided by the embodiment of the present disclosure. To avoid repetition, they are not described here.

[0135] The present disclosure will be described in detail below through examples. The examples described in this disclosure are only some of the examples of the present disclosure, not all of them. Based on the examples in this disclosure, all other examples obtained by ordinary technicians in this field without making any creative work are within the scope of protection of this disclosure.

[0136] In the following examples, unless otherwise specified, all materials used were commercially available analytical materials.

[0137] Example 1: The electrode is a positive electrode

[0138] Positive electrode sheet preparation: 97wt% lithium cobalt oxide, 2wt% polytetrafluoroethylene, and 1wt% conductive carbon black are added to a dispersion device and thoroughly dry-mixed to obtain a uniformly dispersed positive electrode material. This positive electrode material is then added to a jet mill at a feed rate of 150g / min for 25 minutes to achieve thorough dispersion and fiberization of the polytetrafluoroethylene binder, resulting in a dough-like positive electrode material. The positive electrode material is then extruded and rolled using a screw pump to obtain an active material layer membrane (the composite material described above). Finally, the resulting active material layer membrane is laminated to a current collector without a glue layer through multi-stage hot calendering to produce the positive electrode sheet. The thickness of the active material layer membrane is 100 microns. The distance between the first side and the surface of the active material layer facing away from the current collector is 5% of the total thickness of the active material layer, and the distance between the second side and the surface of the active material layer close to the current collector is 7% of the total thickness of the active material layer; the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 is 45% of the total thickness of the active material layer, and the distance between the fourth side and the surface of the active material layer 11 close to the current collector 10 is 49% of the total thickness of the active material layer.

[0139] Example 2:

[0140] The difference from Example 1 is that the thickness of the pole piece is 300 μm;

[0141] Example 3:

[0142] The difference from Example 1 is that the thickness of the pole piece is 1000 μm;

[0143] Example 4:

[0144] The difference from Example 1 is that the positive electrode material ratio in the composite positive electrode material is 94 wt % lithium cobalt oxide, 5 wt % polytetrafluoroethylene, and 1 wt % conductive carbon black.

[0145] Example 5

[0146] The difference from Example 1 is that the binder in the composite positive electrode material is 2 wt% polyvinylidene fluoride, and the above mixed material is directly extruded into a film by a screw pump and composited on the current collector;

[0147] Example 6:

[0148] The difference from Example 1 is that the binder consists of 1 wt% polytetrafluoroethylene and 1 wt% polyvinylidene fluoride;

[0149] Example 7: The electrode is a negative electrode

[0150] Negative electrode sheet preparation: 97wt% graphite, 2wt% polytetrafluoroethylene, and 1wt% conductive carbon black were added to a dispersion device and thoroughly dry-mixed to obtain a uniformly dispersed negative electrode material. This negative electrode material was then added to a jet mill at a feed rate of 150g / min for 25 minutes to achieve thorough dispersion and fiberization of the polytetrafluoroethylene binder, resulting in a dough-like negative electrode material. The negative electrode material was then extruded and rolled using a screw pump to produce an active material layer membrane. Finally, the resulting active material layer membrane was laminated to a current collector without a glue layer through multi-stage hot calendering to produce the negative electrode sheet. The active material layer membrane had a thickness of 100 microns. The distance between the first side and the surface of the active material layer facing away from the current collector is 5% of the total thickness of the active material layer, and the distance between the second side and the surface of the active material layer close to the current collector is 7% of the total thickness of the active material layer; the distance between the third side and the surface of the active material layer 11 facing away from the current collector 10 is 45% of the total thickness of the active material layer, and the distance between the fourth side and the surface of the active material layer 11 close to the current collector 10 is 49% of the total thickness of the active material layer.

[0151] Example 8:

[0152] The same procedures as in Example 1 were followed, except that the prepared active material layer film was laminated to a current collector containing a glue layer (the adhesive layer comprised polyethylene and conductive carbon black in a mass ratio of 7:3) through multi-stage hot calendering to form a positive electrode sheet. The active material layer film had a thickness of 100 microns, and the adhesive layer had a thickness of 1 micron.

[0153] Example 9:

[0154] The negative electrode sheet was prepared by referring to Example 7, except that the prepared active material layer film was laminated to a current collector containing a glue layer (the adhesive layer comprised polyethylene and conductive carbon black in a mass ratio of 7:3) through multi-stage hot calendering. The thickness of the active material layer film was 100 μm, and the thickness of the adhesive layer was 1 μm.

[0155] Comparative Example 1a

[0156] The electrode sheet prepared in Comparative Example 1a is a positive electrode sheet, which is prepared in accordance with Example 1, except that:

[0157] 94wt% lithium cobaltate, 5wt% polyvinylidene fluoride, and 1wt% conductive carbon were dissolved in N-methylpyrrolidone, stirred evenly to form a positive electrode slurry, and then evenly coated on the positive electrode current collector aluminum foil. After drying, rolling, and slitting, the positive electrode sheet 1a was prepared.

[0158] Comparative Example 1b

[0159] The electrode sheet prepared in Comparative Example 1b is a negative electrode sheet, which is prepared in accordance with Example 7, except that:

[0160] 93wt% artificial graphite, 0.8wt% conductive carbon black, 4.2wt% polyvinylidene fluoride, and 2wt% sodium carboxymethyl cellulose were added to NMP and stirred evenly to prepare a negative electrode slurry. The prepared negative electrode slurry was evenly coated on the negative electrode current collector copper foil, and then dried, rolled, and cut to prepare the negative electrode sheet 1b.

[0161] Test Case

[0162] The electrodes obtained in the above embodiment and comparative example were subjected to the following tests, and the test results are shown in Table 1:

[0163] (1) Powder loss test

[0164] Fix the two ends of an electrode, select a certain bending radius (for example, 10mm, 15mm, 20mm), and perform positive and negative staggered bending (i.e., bend forward once and reverse once). After 200 consecutive positive and negative staggered bendings, visually check whether the electrode has lost powder. Then dry and weigh it. The weight is compared with the electrode before bending to calculate the weight retention rate and determine whether it has lost powder.

[0165] (2) Cracking test

[0166] A pole piece position was randomly selected, and after drying the pole piece, a CCD camera was used to observe and record the number of cracks on the pole piece surface.

[0167] Table 1

[0168] As shown in Table 1, the mass percentage of fluorine in the third region of Examples 1-7 was greater than that in at least one of the first or second regions. However, the mass percentage of fluorine in the third region of Comparative Examples 1a and 1b was less than that in the first and second regions of the corresponding electrode. Furthermore, the difference in mass percentage of fluorine between the first and second regions of Comparative Examples 1a and 1b was greater than 3. This resulted in uneven distribution of fluorine in the active material layer of the control group, causing uneven stress on the active material layer and resulting in powder loss and cracking.

[0169] The thickness of the active material layer of Examples 1, 2, and 3 gradually increases. For example, in Example 2, when the thickness of the active material layer is 300 microns, the difference in the mass percentage of the fluorine element between the first region and the second region gradually increases. When the thickness of the active material layer is increased to 1000 microns, the difference in the mass percentage of the fluorine element between the third region and the first region and / or the second region increases significantly. The stress distribution between the third region and the first region and / or the second region is uneven, resulting in slight powder loss and cracking of the electrode.

[0170] Comparing Examples 1, 5, and 6, it can be seen that when the binder is changed from PTFE to PVDF or PVDF and PTFE, the adhesion of PVDF deteriorates due to the point-like distribution of adhesion, resulting in slight cracking of the electrode and easy powder loss.

[0171] The difference between the mass percentage of the fluorine element in the first region of Example 1 and Example 7 and the mass fraction of the fluorine element in the second region is greater than 0 and less than or equal to 0.5wt%, and the difference between the mass percentage of the fluorine element in the third region of Example 1 and Example 7 and the mass fraction of the fluorine element in the first region and / or the second region is less than or equal to 1.5wt%, that is, the mass percentages of the fluorine element on the top surface, middle region and bottom surface of the active material layer are close, which means that the fluorine element is evenly distributed in the thickness direction of the active material layer, so that the stress on the electrode in the thickness direction is evenly distributed, the stress on the active material layer in the middle region is small, and the electrode will not shed powder or crack, thereby improving the stability and cycle life of the electrode.

[0172] It should be noted that, in this article, the terms "comprise", "include" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the statement "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device comprising the element. In addition, it should be noted that the scope of the methods and devices in the embodiments of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in the opposite order according to the functions involved. For example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0173] The above are only preferred embodiments of the present disclosure and are not intended to limit the present disclosure. Any modifications, equivalent replacements, etc. made within the spirit and principles of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A pole piece, characterized in that: A current collector is included, wherein at least one surface of the current collector is provided with an active material layer; The difference between the mass percentage of the fluorine element in the first region of the active material layer to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 3%; The first region is a region between a surface of the active material layer facing away from the current collector and a first side edge; The second region is a region between a surface of the active material layer on one side close to the current collector and a second side edge; The distance between the first side and the surface of the active material layer facing away from the current collector is 1-10% of the total thickness of the active material layer; The distance between the second side and a surface of the active material layer close to the current collector is 1-10% of the total thickness of the active material layer.

2. The pole piece according to claim 1, characterized in that: The difference between the mass percentage of the fluorine element in the first region of the active material layer to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 1%; Preferably, the difference between the mass percentage of the fluorine element in the first region of the active material layer to the total mass of the active material layer in the first region and the mass percentage of the fluorine element in the second region of the active material layer to the total mass of the active material layer in the second region is greater than 0 and less than or equal to 0.5%.

3. The pole piece according to claim 1 or 2, characterized in that: The mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is 0-18wt%; And / or, the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is 0-18 wt %.

4. The pole piece according to claim 1 or 2, characterized in that: The mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is 0.5-10wt%; And / or; the mass percentage of the fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is 0.5-10wt%.

5. The pole piece according to any one of claims 1 to 4, characterized in that: The active material layer includes a third region, and the third region is a region between the third side and the fourth side; The distance between the third side and the surface of the active material layer facing away from the current collector is 45%-50% of the total thickness of the active material layer; The distance between the fourth side and the surface of the active material layer close to the current collector is 45%-50% of the total thickness of the active material layer; The mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is greater than the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region; And / or, the mass percentage of fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is greater than the mass percentage of fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region.

6. The pole piece according to claim 5, characterized in that: The difference between the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is ≤ 3%; And / or, the fluorine element in the active material layer in the third region accounts for The difference between the mass percentage of the mass and the mass percentage of the fluorine element of the active material layer in the second region to the total mass of the active material layer in the second region is ≤3%.

7. The pole piece according to claim 5 or 6, characterized in that: The difference between the mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and the mass percentage of the fluorine element in the active material layer in the first region to the total mass of the active material layer in the first region is ≤1.5%; And / or, the difference between the mass percentage of fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region and the mass percentage of fluorine element in the active material layer in the second region to the total mass of the active material layer in the second region is ≤1.5%.

8. The pole piece according to any one of claims 5 to 7, characterized in that: The mass percentage of the fluorine element in the active material layer in the third region to the total mass of the active material layer in the third region is 0-21 wt %, preferably 1-12 wt %.

9. The pole piece according to any one of claims 1 to 8, characterized in that: The active material layer includes a first binder and an active material; The difference between the mass percentage ratio of the first binder to the active material in the first region of the active material layer and the mass percentage ratio of the first binder to the active material in the second region of the active material layer is ≤9%, preferably ≤5%.

10. The pole piece according to claim 9, characterized in that: The mass percentage ratio of the first binder and the active material in the first region of the active material layer is 0.5-10%; And / or, the mass percentage ratio of the first binder to the active material in the second region of the active material layer is 0.5-10%.

11. The pole piece according to claim 9 or 10, characterized in that: The first binder comprises at least one of fibrous PTFE, PVDF, ETEF, FEP, PVP, PEO, CMC, SBR, PAA, PP, PE, and PAN; And / or, the active material includes LiCoO2, LiMn2O4, LiMnO2, LiNiO2, LiFePO4, LiMnPO4, LiCo x Ni 1-x O2(0≤x≤1), LiCo x Ni 1-x-y Al y At least one of O2 (0≤x≤1, 0≤y≤1); And / or, the active material includes artificial graphite, natural graphite, soft carbon, hard carbon, mesophase carbon microspheres, Si, SiO x 、Si-C、SiO x -At least one of C.

12. The pole piece according to any one of claims 1 to 11, characterized in that: A bonding layer is further provided between the current collector and the active material layer, and the bonding layer includes a second bonding material and a conductive agent.

13. The pole piece according to claim 12, characterized in that: The second adhesive material includes at least one of PVDF, SBR, PP, EVA hot melt adhesive, PO hot melt adhesive, PUR hot melt adhesive, TPR material, TPU hot melt adhesive, polyamide PA, polyester PES, polyethylene and polyester amide PEA; And / or, the conductive agent includes at least one of conductive carbon black, carbon nanotubes, graphene, conductive carbon fibers and conductive graphite.

14. The pole piece according to claim 12 or 13, characterized in that: The thickness of the active material layer is greater than or equal to 20 μm and less than or equal to 3 mm; preferably, the thickness of the active material layer is greater than or equal to 30 μm and less than or equal to 300 μm; and / or, the thickness of the bonding layer is greater than or equal to 500 nm and less than or equal to 3 μm; And / or, the ratio of the thickness of the active material layer to the thickness of the bonding layer is greater than or equal to 10 and less than or equal to 6000. Preferably, the ratio of the thickness of the active material layer to the thickness of the bonding layer is greater than or equal to 30 and less than or equal to 300.

15. An electrochemical device, characterized in that: It includes a positive electrode sheet, a negative electrode sheet and a separator located between the positive electrode sheet and the negative electrode sheet; The positive electrode sheet and / or the negative electrode sheet is a sheet as claimed in any one of claims 1 to 14.

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