Positive electrode sheet and preparation method thereof, lithium-ion battery, battery pack, and electrical equipment

Abstract

The present invention relates to the technical field of lithium-ion batteries, and particularly to a positive electrode sheet, a preparation method thereof, a lithium-ion battery, a battery pack, and an electrical device. The positive electrode sheet includes a current collector and an active layer stacked in sequence. The part of the active layer that extends a fixed thickness from the first outer surface into the active layer along its own thickness direction is a functional layer, and the ratio of the thickness of the functional layer to the thickness of the active layer is less than or equal to 1; the functional layer includes a plurality of first active particles containing manganese elements and a plurality of second active particles containing iron elements, and at least part of the plurality of second active particles adheres to the surface of the first active particles. The average particle size of the first active particles is 7 μm to 10 μm, and the ratio of the average particle size of the second active particles to the average particle size of the first active particles is 0.07 to 0.1; the molecular formula of the first active particles is xLi2MnO3·(1-x)LiMO2, where M = Ni, Mn, 0 < x < 1, and the molecular formula of the second active particles is LiMn y Fe 1‑ y PO4, where 0

Description

Technical Field

[0001] The present invention relates to the technical field of lithium - ion batteries, and particularly to a positive electrode sheet, a preparation method thereof, a lithium - ion battery, a battery pack, and an electrical device. Background Art

[0002] The lithium - rich manganese - based material has advantages such as high specific capacity and high working voltage, and is considered to be one of the most potential positive electrode sheet materials for the next generation of lithium - ion batteries. However, during the high - voltage charge - discharge cycle of the lithium - rich manganese - based material, irreversible capacity loss and voltage performance attenuation will occur, which further makes the lithium - ion battery have a high safety risk and restricts its practical application. Summary of the Invention

[0003] Embodiments of the present invention disclose a positive electrode sheet, a preparation method thereof, a lithium - ion battery, a battery pack, and an electrical device. The positive electrode sheet can reduce the irreversible capacity loss and voltage attenuation of the lithium - rich manganese - based material, and ensure the safety of the lithium - ion battery.

[0004] In a first aspect, in order to achieve the above object, the present invention discloses a positive electrode sheet, which includes:

[0005] A current collector;

[0006] An active layer, the active layer is stacked on the current collector, the active layer includes a first outer surface, the first outer surface is the outer surface of the active layer facing away from the current collector, the active layer includes a functional layer, the functional layer is a part of the active layer that extends a fixed thickness from the first outer surface into the active layer along the thickness direction of the active layer, and the ratio of the thickness of the functional layer to the thickness of the active layer is less than or equal to 1;

[0007] The functional layer includes multiple first active particles containing manganese elements and multiple second active particles containing iron elements, at least part of the multiple second active particles adheres to the surface of the first active particles, the average particle size Dv50 of the first active particles is 7 μm to 10 μm, and the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles is 0.07 to 0.1;

[0008] The molecular formula of the first active particles is xLi2MnO3·(1 - x)LiMO2, where M = Ni, Mn, 0 < x < 1, and the molecular formula of the second active particles is LiMn y Fe 1-y PO4, where 0 < y < 1.

[0009] As an optional implementation manner, in the embodiments of the present invention, the average particle size Dv50 of the second active particles is 0.5 μm to 0.8 μm.

[0010] As an optional embodiment, in an embodiment of the present invention, the functional layer further includes a first binder, the first active particles are 80wt% to 90wt%, the second active particles are 5wt% to 15wt%, and the first binder is 2wt% to 5wt%.

[0011] As an optional embodiment, in an embodiment of the present invention, the active layer also includes a transition layer, the transition layer includes a plurality of second active particles, the second active particles are 90wt% to 95wt%, the transition layer is stacked on the current collector and is located between the current collector and the functional layer, and the ratio of the thickness of the functional layer to the thickness of the transition layer is (1.5 to 2.5):1.

[0012] As an optional implementation manner, in an embodiment of the present invention, the thickness of the functional layer is 30 μm to 100 μm, and / or the thickness of the transition layer is 10 μm to 40 μm.

[0013] As an optional implementation, in an embodiment of the present invention, the functional layer has a first main portion and a first thinned portion connected to each other, and the first main portion and the first thinned portion cover the surface of the transition layer.

[0014] As an optional embodiment, in an embodiment of the present invention, the functional layer has a first main body portion and a first thinned portion connected to each other, and the transition layer has a second main body portion and a second thinned portion connected to each other, the first main body portion and the first thinned portion cover the surface of the second main body portion, and the second thinned portion is exposed outside the functional layer.

[0015] In a second aspect, an embodiment of the present application further provides a method for preparing a positive electrode sheet, wherein the positive electrode sheet is the positive electrode sheet described in the first aspect, and the method for preparing the positive electrode sheet comprises the following steps:

[0016] providing the current collector;

[0017] A plurality of the first active particles, a plurality of the second active particles, a first conductive agent and a first binder are mixed and coated on the current collector, and then dried to prepare the active layer.

[0018] As an optional embodiment, in an embodiment of the present invention, the current collector is provided;

[0019] disposing a conductive layer on the current collector;

[0020] Disposing a transition layer on a side of the conductive layer away from the current collector;

[0021] A plurality of the first active particles, a plurality of the second active particles, a first conductive agent and a first binder are mixed and coated on a side of the transition layer away from the current collector, and then dried to prepare the active layer.

[0022] As an optional embodiment, in an embodiment of the present invention, the step of disposing the conductive layer on the current collector comprises: applying a first slurry obtained by mixing 90% to 95% by mass of acetylene black and 5% to 10% by mass of polyacrylate on the current collector, and drying the slurry to prepare the conductive layer; and / or,

[0023] The step of setting the transition layer on the side of the conductive layer away from the current collector is: mixing 90% to 95% by mass of second active particles, 2% to 5% by mass of a second conductive agent and 3% to 5% by mass of a second binder to obtain a second slurry, coating it on the conductive layer, and drying it to prepare the transition layer.

[0024] In a third aspect, an embodiment of the present application further provides a lithium-ion battery, wherein the lithium-ion battery includes the positive electrode sheet as described in the first aspect.

[0025] In a fourth aspect, an embodiment of the present application further provides a battery pack, comprising a box body and a lithium-ion battery as described in the third aspect disposed in the box body.

[0026] In a fifth aspect, an embodiment of the present application further provides an electrical device, which includes an electrical device body and a lithium-ion battery as described in the third aspect arranged in the electrical device body, and the lithium-ion battery is used to power the electrical device body.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] The positive electrode sheet provided by the embodiment of the present application includes: a current collector and an active layer. The active layer is stacked on the current collector. The active layer includes a first outer surface, which is the outer surface of the active layer facing away from the current collector. The active layer includes a functional layer, and the functional layer is a part of the active layer that extends a fixed thickness from the first outer surface into the active layer along its own thickness direction. The ratio of the thickness of the functional layer to the thickness of the active layer is less than or equal to 1. The functional layer includes multiple first active particles containing manganese elements and multiple second active particles containing iron elements. At least part of the multiple second active particles adheres to the surface of the first active particles. The average particle size Dv50 of the first active particles is 7 μm to 10 μm, and the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles is 0.07 to 0.1. The molecular formula of the first active particles is xLi2MnO3·(1 - x)LiMO2, where M = Ni, Mn, 0 < x < 1, and the molecular formula of the second active particles is LiMn y Fe 1-y PO4, where 0 < y < 1. Controlling the ratio of the average particle size of the second active particles to the average particle size of the first active particles to be 0.07 to 0.1 enables the second active particles to have a good adhesion effect on the first active particles and form a coating layer. On the one hand, it can reduce the direct corrosion of the first active particles containing manganese elements by the electrolyte. In addition, the attached second active particles are equivalent to the protective structure framework on the outer layer of the first active particles, which can improve the crystal structure stability of the first active particles containing manganese elements, thereby inhibiting problems such as irreversible capacity loss and voltage decay of the latter, and ensuring its safety while increasing the capacity and cycle life of the lithium-ion battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0030] Figure 1 is a schematic structural diagram of a positive electrode sheet disclosed in an embodiment of the present invention;

[0031] Figure 2 is a schematic structural diagram of only the thinned functional layer disclosed in an embodiment of the present invention;

[0032] Figure 3 is a schematic structural diagram of the simultaneously thinned functional layer and transition layer disclosed in an embodiment of the present invention;

[0033] Figure 4 is a schematic flowchart of a preparation method of a positive electrode sheet disclosed in an embodiment of the present invention;

[0034] Figure 5 (a) is a scanning electron microscope image of the lithium iron manganese phosphate particles in Example 1 of the present invention;

[0035] Figure 5 (b) is a scanning electron microscope image of the lithium-rich manganese-based particles in Example 1 of the present invention;

[0036] Figure 6 is a scanning electron microscope image of the functional layer in Example 1 of the present invention;

[0037] Figure 7 This is a microscopic morphology diagram of the cross section of the positive electrode sheet in Example 1 of the present invention.

[0038] Icon: 100, positive electrode sheet; 10, current collector; 20, active layer; 21, conductive layer; 22, transition layer; 22a, second main body; 22b, second thinned portion; 23, functional layer; 23a, first main body; 23b, first thinned portion. DETAILED DESCRIPTION

[0039] The technical solution of the present invention will be further described below in conjunction with embodiments and drawings.

[0040] Lithium-rich manganese-based materials with layered structures provide two-dimensional tunnels for lithium ion intercalation and deintercalation. Compared with the spiky stone structure, the diffusion coefficient is larger and the intercalation and deintercalation of lithium ions is faster. Lithium-rich manganese-based materials have a high energy density after initial charge activation and are regarded as potential materials for lithium battery positive electrodes. However, they still have problems such as poor cycle stability and low compaction density. Specifically, after multiple charge and discharge cycles, the electrolyte will continue to corrode the lithium-rich manganese-based materials, causing the manganese element to continuously dissolve. In addition, the lithium-rich manganese-based materials undergo oxygen evolution reactions, and the lattice defects are obvious. The crystal structure, grain morphology and size will all change, resulting in irreversible capacity loss and voltage decay in the lithium-rich manganese-based materials, which will seriously affect the cycle stability and rate performance of lithium-ion batteries and reduce the safety of lithium-ion batteries.

[0041] First, the present application provides a positive electrode sheet that can reduce electrolyte corrosion on lithium-rich manganese-based materials, improve the structural stability of lithium-rich manganese-based materials, and reduce the dissolution of manganese in lithium-rich manganese-based materials. It also reduces irreversible capacity loss and voltage decay in lithium-rich manganese-based materials, thereby increasing the capacity and cycle life of lithium-ion batteries while ensuring safety.

[0042] See also Figure 1, this application provides a positive electrode sheet 100, comprising: a current collector 10 and an active layer 20. The active layer 20 is stacked on the current collector 10. The active layer 20 includes a first outer surface, which is the outer surface of the active layer facing away from the current collector 20. The active layer 20 includes a functional layer 21, and the functional layer 21 is a part of the active layer 20 that extends inward by a fixed thickness along the thickness direction of the active layer 20 from the first outer surface. The ratio of the thickness of the functional layer 21 to the thickness of the active layer 20 is less than or equal to 1. The functional layer 21 includes a plurality of first active particles containing manganese elements and a plurality of second active particles containing iron elements. At least a part of the plurality of second active particles adheres to the surface of the first active particles. The average particle size Dv50 of the first active particles is 7 μm to 10 μm, and the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles is 0.07 to 0.1. The molecular formula of the first active particles is xLi2MnO3·(1 - x)LiMO2, where M = Ni, Mn, 0 < x < 1, and the molecular formula of the second active particles is LiMn y Fe 1- y PO4, where 0 < y < 1.

[0043] The second active particles with small particle size have a large specific surface area and can adhere to the surface of the first active particles. By controlling the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles to be 0.07 to 0.1, the second active particles have a good adhesion effect on the first active particles, can form a coating layer, thereby reducing the direct corrosion of the first active particles by the electrolyte, and can play a role in supporting and protecting the structure of the first active particles, inhibiting problems such as irreversible capacity loss and voltage decay of the lithium-rich manganese-based material, improving the capacity and cycle life of the lithium-ion battery, and ensuring safety.

[0044] The ratio of the thickness of the functional layer 21 to the thickness of the active layer 20 is less than or equal to 1. When the ratio is less than 1, the active layer 20 can be provided with not only the functional layer 21 but also other film layers; when the ratio is equal to 1, the active layer 20 is the functional layer 21.

[0045] There are several ways for the second active particles to adhere to the surface of the first active particles, but not limited to: for example, the second active particles adhere to a part of the surface of the first active particles, or the second active particles coat the surface of the first active particles.

[0046] The ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles is 0.07 to 0.1. On the one hand, it can improve the adhesion effect of the second active particles, form a complete coating layer, and enable the first active particles to obtain more sufficient structural support, so that the second active particles have a better protection and support effect on the first active particles. On the other hand, due to the small average particle size Dv50 of the second active particles, the specific surface area of the second active particles increases, which can increase the contact area with other substances, thereby increasing the bonding force and compaction density of the functional layer, and thus can improve the capacity and cycle life of the lithium-ion battery.

[0047] Exemplarily, the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles can be any point value within the above range. For example, it can be 0.07, 0.071, 0.072, 0.073, 0.074, 0.075, 0.076, 0.077, 0.078, 0.079, 0.08, 0.081, 0.082, 0.083, 0.084, 0.085, 0.086, 0.087, 0.088, 0.089, 0.09, 0.091, 0.092, 0.093, 0.094, 0.095, 0.1, etc.

[0048] The first active particles are a lithium-rich manganese-based material, and the molecular formula of the first active particles is xLi2MnO³·(1 - x)LiMO², where M = Ni, Mn, 0 < x < 1. The lithium-rich manganese-based material has a relatively high initial discharge specific capacity, high energy density, and high working voltage. The lithium-rich manganese-based material does not contain expensive Co elements, has low cost, small environmental pollution, and low toxicity.

[0049] Exemplarily, x can be any point value within the range of 0 to 1, including but not limited to: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc., and the corresponding 1 - x can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. For example, the molecular formula of the lithium-rich manganese-based material can be the following: 0.1Li2MnO³0.9LiMnO², 0.2Li2MnO³0.8LiNiO², 0.3Li2MnO³0.7LiMnO², 0.4Li2MnO³0.6LiNi - MnO², 0.5Li2MnO³0.5LiNi - MnO², 0.6Li2MnO³0.4LiNiO², 0.7Li2MnO³0.3LiMnO², 0.8Li2MnO³0.2LiNiO², 0.9Li2MnO³0.1LiNi - MnCoO². This embodiment does not make specific limitations on this.

[0050] The second active particle is lithium iron manganese phosphate, and the molecular formula of lithium iron manganese phosphate is LiMn y Fe 1-y PO4, where 0 < y < 1. The elemental composition of the lithium iron manganese phosphate material is stable. It has an olivine-type structure with a stable crystal structure. During the process of lithium ion insertion and extraction during charge and discharge, the structure will not collapse, ensuring good safety and excellent cycle stability.

[0051] Exemplarily, y can be any point value within the range of 0 to 1, including but not limited to: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. Correspondingly, 1 - y can be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, etc. For example, the molecular formula of lithium iron manganese phosphate is LiMn 0.1 Fe 0.9 PO4, LiMn 0.2 Fe 0.8 PO4, LiMn 0.3 Fe 0.7 PO4, LiMn 0.4 Fe 0.6 PO4, LiMn 0.5 Fe 0.5 PO4, LiMn 0.6 Fe 0.4 PO4, LiMn 0.7 Fe<着 0.3 PO4, LiMn 0.8 Fe 0.2 PO4, LiMn 0.9 Fe 0.1 PO4, and this embodiment does not make specific limitations on this.

[0052] The average particle size Dv50 of the first active particle is 7μm to 10μm, and the average particle size Dv50 of the first active particle is 7μm to 10μm. The crystal structure of the lithium-rich manganese-based material within this particle size range is relatively complete, with clear boundaries and relatively uniform particle distribution. If the particle size of the first active particle is less than 7μm, the specific surface area increases, the corrosion of the electrolyte to the lithium-rich manganese-based material increases, and side reactions increase; if the particle size of the first active particle is greater than 10μm, the compaction density of the positive electrode sheet decreases.

[0053] Exemplarily, the average particle size of the lithium-rich manganese-based material can be any point value within the above range, for example, it can be: 7μm, 7.2μm, 7.5μm, 7.7μm, 8μm, 8.2μm, 8.5μm, 8.7μm, 9μm, 9.2μm, 9.5μm, 9.7μm, 10μm, etc.

[0054] In some embodiments, the average particle size Dv50 of the second active particles is 0.5μm to 0.8μm. The second particles have a micron-sized particle size, a small particle size, a large specific surface area, and can increase the contact area with other materials. Being located in the functional layer can increase the compaction density of the functional layer and improve the adhesion of the functional layer. If the average particle size Dv50 of the second active particles is less than 0.5μm, agglomeration will occur, increasing the internal resistance of the positive electrode sheet; if the average particle size Dv50 of the second active particles is greater than 0.8μm, the average particle size is too large, the specific surface area of ​​the second active particles is reduced, the contact area with the first active particles is reduced, the adhesion effect is reduced, and it is difficult to form a coating layer.

[0055] Exemplarily, the average particle size Dv50 of the second active particles may be any value within the range, including but not limited to: 0.5 μm, 0.52 μm, 0.55 μm, 0.57 μm, 0.6 μm, 0.62 μm, 0.65 μm, 0.67 μm, 0.7 μm, 0.72 μm, 0.75 μm, 0.77 μm, 0.8 μm, etc.

[0056] See also Figure 1 The specific structure of the positive electrode sheet 100 provided in the present application is introduced in detail below: In some embodiments, the positive electrode sheet 100 includes a current collector 10 and an active layer 20, and the active layer 20 includes a conductive layer 21, a transition layer 22 and a functional layer 23 stacked in sequence.

[0057] In some embodiments, the functional layer 23 further comprises a first binder, wherein the first active particles are 80 wt% to 90 wt%, the second active particles are 5 wt% to 15 wt%, and the first binder is 2 wt% to 5 wt%. The second active particles have a better coating effect on the first active particles under the action of the first binder. Controlling the first active particles within the above range is conducive to achieving its advantages of high specific capacity and high energy density. Controlling the second active particles within the above range can, on the one hand, prevent excessive addition from causing agglomeration between particles and affecting the performance of capacity, and on the other hand, prevent too little addition from causing the second active particles to have an insignificant coating effect on the first active particles.

[0058] For example, when expressed as a percentage by weight, the percentage includes any value within the range, for example, the first active particles are 80 wt%, 82 wt%, 84 wt%, 86 wt%, 88 wt%, 90 wt%, etc. The second active particles are 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, 10 wt%, 11 wt%, 12 wt%, 13 wt%, 14 wt%, 15 wt%, etc. This application is not limited to specific parameter values.

[0059] In some embodiments, the functional layer 23 further comprises a first conductive agent, with the first conductive agent comprising 2 wt% to 5 wt%. The first conductive agent can enhance the conductivity of the functional layer 23. The first conductive agent comprises 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc. The first binder comprises 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc. This application does not limit the specific parameter values.

[0060] Modifying lithium-rich manganese-based materials using bulk doping and surface modification methods cannot solve the problem of structural instability of lithium-rich manganese-based materials. In addition, the particle size of lithium-rich manganese-based materials prepared by conventional methods is very large, resulting in a low compaction density of the material and poor adhesion between the lithium-rich manganese-based material and the current collector, which is not conducive to the electrochemical performance of lithium-ion batteries. However, the present application can effectively improve the adhesion and compaction density of lithium-ion batteries by using second active particles and providing a transition layer.

[0061] In some embodiments, the transition layer 22 includes a plurality of second active particles, with the second active particles comprising 90 wt% to 95 wt%. The transition layer 22 includes a plurality of small-sized second active particles, accounting for more than 90%, which can increase the contact area with the functional layer 23, thereby increasing the bonding force, ensuring a strong bonding effect between the conductive layer 21 and the functional layer 23, and increasing the compaction density. This can significantly improve the contact between the functional layer and the conductive layer, thereby reducing contact resistance, directly improving the electrochemical performance of the lithium-ion battery, increasing the charge and discharge capacity and cycle stability of the lithium-ion battery, reducing the peeling and shedding of the first active particles in the positive electrode sheet, and improving the capacity and cycle life of the lithium-ion battery. For example, in terms of mass percentage, the percentage includes any value within this range, for example: the second active particles are 90 wt%, 91 wt%, 92 wt%, 93 wt%, 94 wt%, 95 wt%, etc.

[0062] In some embodiments, the transition layer 22 further includes a second conductive agent and a second adhesive, wherein the second conductive agent comprises 2 wt% to 5 wt% and the second adhesive comprises 3 wt% to 5 wt%. The provision of the second conductive agent can improve the conductivity of the transition layer 22, and the provision of the second adhesive can enhance the adhesiveness of the transition layer 22. For example, the second conductive agent comprises 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc. The second adhesive comprises 3 wt%, 4 wt%, 5 wt%, etc. This application does not limit the specific parameter values.

[0063] The first and second conductive agents may be any one of conductive carbon black, carbon nanotubes, graphene, carbon fibers, conductive polymers, acetylene black, artificial graphite, and natural graphite, or a combination of at least two thereof; and / or the first and second binders may be any one of polyvinylidene fluoride, a copolymer of vinylidene fluoride and hexafluoropropylene, and polytetrafluoroethylene, or a combination of at least two thereof. The use of the aforementioned first conductive agents and first binders can achieve excellent conductivity and bonding effects.

[0064] Similarly, the first conductive agent and the second conductive agent can be made of the same material or different materials. When different materials are selected, the amount of the materials can be the same or different. The first binder and the second binder can also be made of the same material or different materials. When different materials are selected, the amount of the materials can be the same or different. Preferably, the first conductive agent and the second conductive agent are made of the same material. For example, the first conductive agent and the second conductive agent are both made of conductive carbon black, and the first binder and the second binder are both made of polyvinylidene fluoride. The conductivity of the same conductive agent is more matched, and the conductive performance is better. Similarly, the bonding effect of the same binder is also more matched, and the bonding effect is better.

[0065] In some embodiments, the transition layer 22 is stacked on the current collector 10 and located between the current collector 10 and the functional layer 23. The ratio of the thickness of the functional layer 23 to the thickness of the transition layer 22 is (1.5-2.5):1. Controlling the thickness of the functional layer 23 and the thickness of the transition layer 22 within the above range not only allows for the formation of gaps and channels between the functional layer and the transition layer, but also facilitates electrolyte infiltration, allowing more first active particles and second active particles in the active layer to participate in the electrochemical reaction. It also reduces the occurrence of peeling between the transition layer and the functional layer.

[0066] For example, the ratio of the thickness of the functional layer to the thickness of the transition layer can be any value within the above range of 1.5:1, 1.6:1, 1.7:1, 1.8:1, 1.9:1, 2.0:1, 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, etc.

[0067] In some embodiments, the thickness of the functional layer is 30 μm to 100 μm, and the thickness of the transition layer is 10 μm to 40 μm. The thickness of the transition layer can be any value within this range, including but not limited to: 40 μm, 38 μm, 35 μm, 33 μm, 30 μm, 27 μm, 25 μm, 22 μm, 20 μm, 17 μm, 15 μm, 12 μm, 10 μm, etc. The thickness of the functional layer can be any value within this range, including but not limited to: 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, etc.

[0068] See also Figure 2 In some embodiments, the functional layer 23 includes a first main portion 23a and a first thinned portion 23b that are connected to each other, and the first main portion 23a and the first thinned portion 23b cover the surface of the transition layer 23. It is understood that when thinning the positive electrode sheet 100, only thinning the functional layer 23 can retain more first active particles in the functional layer 23, which is beneficial to maintaining the energy density of the lithium-ion battery.

[0069] See also Figure 3 In some embodiments, the functional layer 23 includes a first main portion 23a and a first thinned portion 23b connected to each other, and the transition layer 22 includes a second main portion 22a and a second thinned portion 22b connected to each other. The first main portion 22a and the first thinned portion 22b cover the surface of the second main portion 23a, and the second thinned portion 22b is exposed outside the functional layer 23. Therefore, when the positive electrode sheet 100 is thinned, the functional layer 23 and the transition layer 22 are thinned simultaneously, increasing the interfacial gap between the two layers, facilitating the infiltration of the electrolyte from the interface between the functional layer and the transition layer into the interior of the active layer, and thus improving the electrochemical performance of the lithium-ion battery.

[0070] In some embodiments, the conductive layer 21 is stacked on the current collector 10 and located between the current collector 10 and the transition layer 22. The conductive layer 21 comprises acetylene black and polyacrylate, with the acetylene black comprising 90% to 95% by mass and the polyacrylate comprising 5% to 10% by mass. Furthermore, the thickness of the conductive layer 21 is 1 μm to 3 μm. The high proportion of acetylene black enhances the conductivity of the conductive layer. Acetylene black has good dispersion in polyacrylate and is less susceptible to sedimentation, facilitating uniform bonding of the acetylene black to the current collector. Furthermore, during the electrochemical reaction and deintercalation of lithium, the polyacrylate has a low swelling coefficient, stabilizing the internal structure of the positive electrode sheet. As a binder, polyacrylate has excellent tensile mechanical strength, facilitating machining. Conductive layer thicknesses below 1 μm exhibit poor conductivity. Maintaining the thickness within 3 μm achieves excellent conductivity, conserves consumables, and controls the thickness of the lithium-ion battery.

[0071] For example, the mass percentage of acetylene black can be any value within this range, including but not limited to 90%, 91%, 92%, 93%, 94%, 95%, and so on. The mass percentage of polyacrylate can be any value within this range, including but not limited to 5%, 6%, 7%, 8%, 9%, 10%, and so on. The polyacrylate can be polymethyl acrylate, polyethyl acrylate, polybutyl acrylate, polyhydroxyethyl methacrylate, and so on. The thickness of the conductive layer can be any value within this range, including but not limited to 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, and so on.

[0072] In some embodiments, the current collector 10 is any one of Series 1 aluminum foil, Series 3 aluminum foil, and Series 8 aluminum foil; and / or the thickness of the current collector 10 is 15 μm to 20 μm. Using these types of aluminum foil can provide more electron channels for electrochemical reactions, accelerate charge transfer, reduce electrochemical polarization, improve charge and discharge efficiency, and has no significant corrosion effect, making the lithium-ion battery more stable.

[0073] For example, the thickness of the current collector 10 may be any value within the range, including but not limited to: 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, etc.

[0074] It should be noted that the structure of the positive electrode sheet in the related art is usually a two-layer structure, that is, one layer is the current collector, and the other layer is the active film layer to achieve electrochemical performance. In this structure, since the active film layer includes a large amount of other additives in addition to the active substance, the mass proportion of the active substance in the active film layer is reduced. In this case, in order to increase the total amount of active material added to the positive electrode sheet so that it meets the chemical properties of the lithium-ion battery, the active film layer needs to be coated with a thicker thickness. However, precisely because the thickness of the active film layer is thicker, it is difficult for the electrolyte to completely penetrate into the film layer, the wettability of the electrolyte in the positive electrode sheet is poor, and the part of the positive electrode sheet that is not in contact with the electrolyte cannot participate in the electrochemical reaction of the battery, and the interface resistance increases, affecting the rate performance, discharge capacity and service life of the lithium battery.

[0075] The positive electrode sheet 100 of the present application has a four-layer structure consisting of a functional layer 23, a transition layer 22, a conductive layer 21, and a current collector 10. In some embodiments, the thickness of the current collector 10 is 15 μm to 20 μm, the thickness of the conductive layer 21 is 1 μm to 3 μm, the thickness of the transition layer 22 is less than or equal to 40 μm, and the thickness of the functional layer 23 is 30 μm to 100 μm. The resulting overall thickness of the four-layer structure is less than or equal to 163 μm. This is not significantly different from the thickness of lithium-ion battery positive electrode sheets in the related art (100 μm to 200 μm), but the positive electrode sheet 100 of the present application can overcome the disadvantage of poor wettability.

[0076] Specifically, compared with only setting up a relatively dense active membrane layer, there are relatively more micro-pores between the four membrane layers stacked in sequence. The micro-pores between the layers can provide more channels for lithium ions to shuttle freely, the electrolyte wettability is enhanced, the electrolyte infiltration area is increased, the lithium ion extraction and embedding are smoother, and each contact surface of each membrane layer of the positive electrode can fully participate in the electrochemical reaction of the lithium battery. The interface resistance between the layers is significantly reduced, which can effectively improve the rate performance, discharge capacity and service life of the lithium battery.

[0077] It should be emphasized that, on the one hand, the positive electrode sheet of the present application can not only provide more micro-pores for more electrolyte infiltration, thereby increasing the infiltration area of ​​the positive electrode sheet; on the other hand, the provision of the transition layer 22 can improve the adhesion between the functional layer, the conductive layer and the current collector, thereby achieving more stable bonding between the four film layers, thereby enhancing the stability of the overall layered structure of the lithium-ion battery, which is conducive to further improving the capacity and cycle life of the lithium-ion battery.

[0078] Second, see Figure 4 The present application also provides a method for preparing a positive electrode sheet, the method comprising the following steps:

[0079] providing the current collector;

[0080] A plurality of the first active particles, a plurality of the second active particles, a first conductive agent and a first binder are mixed and coated on the current collector, and then dried to prepare the active layer.

[0081] In some embodiments, the method for preparing the positive electrode sheet includes the following steps:

[0082] providing a current collector;

[0083] disposing a conductive layer on the current collector;

[0084] The transition layer is arranged on the side of the conductive layer facing away from the current collector;

[0085] A plurality of first active particles, a plurality of second active particles, a first conductive agent and a first binder are mixed and coated on a side of the transition layer away from the current collector, and then dried to prepare an active layer.

[0086] In some embodiments, the step of disposing the conductive layer on the current collector comprises: applying a first slurry obtained by mixing 90% to 95% by mass of acetylene black and 5% to 10% by mass of polyacrylate on the current collector, and drying the slurry to prepare the conductive layer; and / or,

[0087] The steps of setting the transition layer on the side of the conductive layer away from the current collector are: mixing 90% to 95% by mass of second active particles, 2% to 5% by mass of a second conductive agent and 3% to 5% by mass of a second binder to obtain a second slurry, coating it on the conductive layer, and drying it to prepare the transition layer.

[0088] Specifically, in the step of preparing the conductive layer, the corresponding acetylene black and polyacrylate and deionized water are weighed according to the corresponding mass percentage and mechanically stirred for 2 hours to obtain a first slurry. The first slurry is evenly coated on the current collector and placed in a vacuum oven and dried at 150°C for 10 hours to prepare a conductive layer on the current collector.

[0089] In the step of preparing the transition layer, a second slurry obtained by mixing the second active particles, the second conductive agent, the second binder and an appropriate amount of solvent in corresponding mass percentages is mechanically stirred for 6 hours to obtain a second slurry, and the second slurry is evenly coated on the conductive layer, placed in a vacuum oven, and dried at 150°C for 10 hours to prepare a transition layer on the current collector.

[0090] In the step of preparing the functional layer, a third slurry obtained by mixing the first active particles, the first conductive agent and the first binder and an appropriate amount of solvent in corresponding mass percentages is mechanically stirred for 6 hours to obtain a third slurry, and the third slurry is evenly coated on the transition layer, placed in a vacuum oven, and dried at 150°C for 10 hours to prepare a functional layer on the transition layer.

[0091] In a third aspect, the present application further provides a lithium-ion battery, which includes the positive electrode sheet mentioned above and prepared by the above method, and also includes a negative electrode sheet, a separator, an electrolyte and a battery shell.

[0092] In a fifth aspect, embodiments of the present application further provide a battery pack. The battery pack includes a housing and a secondary battery as described in the third aspect, disposed within the housing. Multiple secondary batteries are housed within the housing, and the multiple secondary batteries are connected in at least one of series and parallel configurations. The battery pack secures and protects the secondary batteries.

[0093] In a sixth aspect, an embodiment of the present application further provides an electric device, which may be an energy storage base station, an electric vehicle, a ship, a bus, and the like. The electric device includes an electric device body and a secondary battery such as the third aspect disposed in the electric device body, and the secondary battery is used to power the electric device body. The electric device body includes a positive electrode of the device and a negative electrode of the device, the positive electrode sheet of the secondary battery is electrically connected to the positive electrode of the device, and the negative electrode sheet is electrically connected to the negative electrode of the device. Based on this, the present application further provides a method for preparing the lithium-ion battery, which, based on the above-mentioned method for preparing the positive electrode sheet, further provides a negative electrode sheet, and obtains a lithium-ion battery through post-processing.

[0094] Specifically: in the step of providing the negative electrode sheet, corresponding amounts of artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose are weighed in a stirring tank at a mass percentage of 95%:2.5%:2.5%, and an appropriate amount of deionized water is added and stirred for 5 hours to obtain a uniform slurry with appropriate viscosity. The slurry is then coated on a copper foil with a thickness of 10 μm, placed in a vacuum oven, and dried at 150° C. for 15 hours to obtain the negative electrode sheet.

[0095] In the post-processing step, the positive electrode sheet and the negative electrode sheet are placed in a press for pressing, and then a puncher is used to cut out Φ15mm positive electrode discs and Φ18mm negative electrode discs respectively. The positive electrode sheet and the negative electrode disc are placed in a glove box filled with an argon protective atmosphere for battery assembly, wherein a solution obtained by dissolving 1 mol / L lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate with a molar ratio of 1:1 is used as an electrolyte. The positive electrode disc, the negative electrode disc, the polyethylene separator and the battery casing are assembled together, and then the electrolyte is injected to finally produce a button-type lithium-ion battery.

[0096] The following is a further introduction to the solution of this application with reference to specific examples and experimental data:

[0097] Example 1

[0098] This embodiment provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, an electrolyte, and a battery case.

[0099] The preparation method of the lithium ion battery comprises the following steps:

[0100] Provide positive electrode sheet;

[0101] A negative electrode sheet is provided. Appropriate amounts of artificial graphite, conductive carbon black, and sodium carboxymethyl cellulose are weighed in a mixing tank at a mass percentage of 95%:2.5%:2.5%. An appropriate amount of deionized water is added and stirred for 5 hours to obtain a uniform slurry with appropriate viscosity. The slurry is then coated on a 10 μm thick copper foil and placed in a vacuum oven. Drying at 150°C for 15 hours yields a negative electrode sheet.

[0102] The positive electrode sheet and the negative electrode sheet were placed in a press for pressing, and then a puncher was used to cut out Φ15mm positive electrode discs and Φ18mm negative electrode discs, respectively. The positive electrode discs and the negative electrode discs were placed in a glove box filled with argon protective atmosphere for battery assembly. A solution obtained by dissolving 1 mol / L lithium hexafluorophosphate in a mixed solvent of ethylene carbonate and diethyl carbonate with a molar ratio of 1:1 was used as an electrolyte. The positive electrode disc, the negative electrode disc, a polyethylene separator and a battery casing were assembled, and then the electrolyte was injected to finally produce a button-type lithium-ion battery.

[0103] The positive electrode includes:

[0104] A current collector, wherein the current collector is a series 1 aluminum foil, and the thickness of the current collector is 15 μm;

[0105] An active layer, the active layer is stacked on the current collector, the active layer includes a first outer surface, the first outer surface is the outer surface of the active layer away from the current collector, and the active layer includes a conductive layer, a transition layer, and a functional layer stacked in sequence;

[0106] A conductive layer is provided on the current collector, the conductive layer comprises acetylene black and polyacrylate in a mass percentage of 95%:5%, and the thickness of the conductive layer is 2 μm;

[0107] The transition layer is provided on the side of the conductive layer away from the current collector. The transition layer includes a second active particle, a second conductive agent and a second binder, the mass percentage of which is 90%:5%:5%. The second active particle is lithium manganese iron phosphate with an average particle size Dv50 of 0.56 μm, and its molecular formula is LiMn 0.5 Fe 0.5 PO4, the second conductive agent is conductive carbon black, the second binder is polyvinylidene fluoride, and the thickness of the transition layer is 40 μm; and,

[0108] The functional layer is located on the side of the transition layer facing away from the current collector. It comprises first active particles, second active particles, a first conductive agent, and a first binder in a mass ratio of 90%:5%:2%:3%. The first active particles are lithium-rich manganese-based materials with an average particle size (Dv50) of 7 μm and a molecular formula of 0.1Li2MnO30.9LiMnO2. The first conductive agent is conductive carbon black, and the first binder is polyvinylidene fluoride. The thickness of the functional layer is 30 μm, and the ratio of the average particle size (Dv50) of the second active particles to the average particle size (Dv50) of the first active particles is 0.08.

[0109] The preparation method of the positive electrode sheet comprises the following steps:

[0110] providing a current collector;

[0111] Acetylene black and polyacrylate were weighed according to the corresponding mass percentages, and deionized water was mechanically stirred for 2 hours to obtain a first slurry. The first slurry was evenly coated on the current collector, and the mixture was placed in a vacuum oven and dried at 150° C. for 10 hours to prepare a conductive layer on the current collector.

[0112] A second slurry obtained by mixing the second active particles, the second conductive agent, the second binder, and an appropriate amount of methyl pyrrolidone in corresponding mass percentages is mechanically stirred for 6 hours to obtain a second slurry. The second slurry is evenly coated on the conductive layer and placed in a vacuum oven and dried at 150° C. for 10 hours to prepare a transition layer on the current collector;

[0113] A third slurry is obtained by mixing the first active particles, the second active particles, the first conductive agent and the first binder according to corresponding mass percentages and an appropriate amount of methyl pyrrolidone, and then mechanically stirring for 6 hours to obtain a third slurry. The third slurry is evenly coated on the transition layer and placed in a vacuum oven and dried at 150°C for 10 hours to prepare a functional layer on the transition layer.

[0114] Example 2

[0115] The functional layer in Example 3 includes first active particles, second active particles, a first conductive agent, and a first binder, and their mass percentages are controlled to be 85%: 10%: 2%: 3%. The rest are consistent with Example 1.

[0116] Example 3

[0117] The functional layer in Example 3 includes first active particles, second active particles, a first conductive agent, and a first binder, and their mass percentages are controlled to be 80%:15%:2%:3%. The rest are consistent with Example 1.

[0118] Example 4

[0119] The functional layer in the fourth embodiment includes first active particles, second active particles, a first conductive agent, and a first binder in a mass percentage of 75%: 20%: 2%: 3%. The rest is the same as in the first embodiment.

[0120] Example 5

[0121] The thickness of the transition layer in the fifth embodiment is 10 μm, and the rest is the same as that in the first embodiment.

[0122] Example 6

[0123] The thickness of the transition layer in the sixth embodiment is 20 μm, and the rest is the same as that in the first embodiment.

[0124] Example 7

[0125] The thickness of the transition layer in the seventh embodiment is 30 μm, and the rest is the same as that in the first embodiment.

[0126] Example 8

[0127] The thickness of the transition layer in the eighth embodiment is 50 μm, and the rest is consistent with the first embodiment.

[0128] Embodiment 9

[0129] The ninth embodiment does not provide a transition layer, and the rest is consistent with the first embodiment.

[0130] Example 10

[0131] In the tenth embodiment, no conductive layer is provided, and the rest is consistent with the first embodiment.

[0132] Example 11

[0133] In Example 11, the average particle size Dv50 of the first active particles is 10 μm, the average particle size Dv50 of the second active particles is 0.8 μm, and the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.08. The rest remains the same as in Example 1.

[0134] Example 12

[0135] In Example 12, the average particle size Dv50 of the first active particles is 8 μm, the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.07, and the rest is consistent with Example 1.

[0136] Example 13

[0137] In Example 13, the average particle size Dv50 of the second active particles is 0.7 μm, the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.1, and the rest remains the same as in Example 1.

[0138] Comparative Example 1

[0139] In Comparative Example 1, the average particle size Dv50 of the first active particles of the functional layer is 3 μm, the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.186, and the rest is consistent with Example 1.

[0140] Comparative Example 2

[0141] In Comparative Example 2, the average particle size Dv50 of the first active particles of the functional layer is 3 μm, the Dv50 of the second active particles is 0.24 μm, and the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.08. The rest is consistent with Example 1.

[0142] Comparative Example 3

[0143] In Comparative Example 3, the average particle size Dv50 of the first active particles of the functional layer is 14 μm, the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.04, and the rest is consistent with Example 1.

[0144] Comparative Example 4

[0145] In Comparative Example 4, the average particle size Dv50 of the first active particles of the functional layer is 14 μm, the average particle size Dv50 of the second active particles is 1.12 μm, and the ratio of Dv50 of the second active particles to Dv50 of the first active particles is 0.08. The rest is consistent with Example 1.

[0146] Comparative Example 5

[0147] The functional layer in Comparative Example 5 includes the first active particles, the first conductive agent and the first binder, but does not include the second active particles. The mass percentages thereof are: 95%:2%:3%, and the rest are consistent with Example 1.

[0148] Performance Testing

[0149] 1. Micromorphology Characterization

[0150] First, the first active particles, second active particles and functional layer used in this application were subjected to SEM testing, and the characterization results were as follows: Figures 5 to 7 shown.

[0151] Figure 5 Figures (a) and (b) show the particle morphology of the second active particles (lithium iron manganese phosphate material) and the first active particles (lithium-rich manganese-based material). Figure (a) clearly shows that the lithium iron manganese phosphate particles prepared in this application have a small particle size and uniform particle distribution. Figure (b) clearly shows that the lithium-rich manganese-based particles have clear boundaries, are spherical, and have a complete and stable particle size. Figure 6 This is the surface morphology of the functional layer. It can be clearly seen from the figure that small-sized lithium manganese iron phosphate has been successfully and evenly coated on the surface of larger lithium-rich manganese-based particles to form a coating layer.

[0152] from Figure 7 It can be seen that the positive electrode sheet prepared in this application has an obvious layered structure, with the functional layer at the top and a relatively thick thickness; the transition layer is below the functional layer; the conductive layer is below the transition layer and is relatively thin, and below the conductive layer is the current collector (black part) with a clear dividing line.

[0153] Using an EDS spectrometer, an electron beam is bombarded on the surface of the positive electrode in a vacuum chamber, exciting the material to emit characteristic x-rays. Elemental analysis is performed based on the wavelength of the characteristic x-rays. The elemental analysis results of the functional layer, transition layer, and current collector are shown in Table 1:

[0154] Table 1 EDS element detection

[0155]

[0156] 2. Macro performance test

[0157] All lithium-ion batteries from Examples 1 to 12 and Comparative Examples 1 to 3 were subjected to electrochemical performance testing using a battery tester (Neware CT4000, Newwell Electronics Co., Ltd.). A 100% SOC needle penetration test was also performed under the same test conditions to compare safety. The needle penetration test involves penetrating the battery from the center with a high-temperature steel needle and leaving it for 1 hour to observe whether it would ignite.

[0158] Capacity retention rate calculation method: In the 1C charge and discharge cycle test, the capacity of the first cycle is taken as the initial capacity, and the capacity of the 300th cycle is divided by the initial capacity to obtain the retention rate value.

[0159] The peel force test involves cutting a 20mm x 70mm section of the positive electrode sheet, securing it with 3M double-sided tape, and then performing a 180° peel test on a high-speed rail tensile tester at a strain rate of 10mm / min. The average peel force along the length is reported in N / cm. Table 2 shows the comparative results of the electrochemical performance test, peel force, and 100% SOC needle penetration test.

[0160] Table 2 Comparison results of lithium-ion battery electrochemical test, peel force and 100% SOC needle penetration test

[0161]

[0162]

[0163] Compared with Example 1 and Comparative Example 1, the average particle size of the first active particles in Comparative Example 1 is less than the range of 7μm to 10μm, and the particle size ratio is greater than the range of 0.07 to 0.1. The discharge capacity and peeling force of Comparative Example 1 are significantly lower than those of Example 1. The particle size of the first active particles in Comparative Example 1 is too small to maintain the high capacity of the lithium-rich manganese-based material itself. Moreover, because the particle size ratio is out of range, the second active particles cannot form a coating layer on the surface of the first active particles, unable to protect the structure of the lithium-rich manganese-based material. This causes the dissolution of manganese elements in the first active particles and irreversible capacity decay, which is not conducive to maintaining cycle life and improving adhesion.

[0164] Compared with Example 1 and Comparative Example 3, the average particle size of the first active particles in Comparative Example 3 is larger than the range of 7 μm to 10 μm, and the particle size ratio is less than the range of 0.07 to 0.1. The excessive size of the first active particles in Comparative Example 3 significantly reduces the bonding strength and compaction density of the positive electrode sheet, resulting in a significant decrease in the peel strength of the positive electrode sheet and impairing capacity retention.

[0165] Compared with Comparative Examples 2 and 4, Example 1 shows that the average particle size of the first active particles in Comparative Example 2 is less than the range of 7 μm to 10 μm, while the average particle size of the first active particles in Comparative Example 4 is greater than the range of 7 μm to 10 μm. However, the particle size ratios of Comparative Examples 2 and 4 are both within the range of 0.07 to 0.1. The capacity and peel force of Comparative Examples 2 and 4 are both inferior to those of Comparative Example 1. This shows that simply meeting the average particle size ratio without meeting the average particle size range of the first active material is still unable to effectively improve the capacity and cycle life of lithium-ion batteries.

[0166] Compared with Example 1 and Comparative Example 5, the functional layer of Comparative Example 5 does not contain active particles, and the capacity of Comparative Example 5 is significantly reduced. The reason is that the first active particles in the functional layer are not protected and are corroded by the electrolyte, resulting in serious voltage decay and capacity loss.

[0167] Compared with Examples 1 to 4, when the mass ratio of the first active particles to the second active particles in Example 3 is maintained at 80 wt %:15 wt %, the film has better capacity and optimal peeling force.

[0168] Compared with Examples 5 to 9, the presence of the transition layer in Example 1 can reduce interfacial contact resistance and enhance the bonding strength of the entire positive electrode material, directly improving the electrochemical performance of the material and increasing the capacity of the lithium-ion battery. In Example 6, the thickness of the transition layer is controlled at 20 μm, achieving optimal discharge capacity and peel strength.

[0169] Compared with Example 10, the presence of the conductive layer can not only reduce the internal resistance of the positive electrode material, but also increase its capacity.

[0170] The above is a detailed introduction to the positive electrode sheet, its preparation method, and the lithium-ion battery disclosed in the embodiments of the present invention. Specific examples are used herein to illustrate the principles and implementation methods of the present invention. The description of the above embodiments is only used to help understand the technical solution and core ideas of the present invention: At the same time, for general technical personnel in this field, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scopes. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A positive electrode sheet, characterized in that: include: current collector; an active layer, the active layer being stacked on the current collector, the active layer comprising a first outer surface, the first outer surface being the outer surface of the active layer facing away from the current collector, and the active layer comprising a functional layer, the functional layer being a portion of the active layer extending from the first outer surface into the active layer along the thickness direction of the active layer to a fixed thickness; The ratio of the thickness of the functional layer to the thickness of the active layer is less than or equal to 1; wherein the active layer includes or does not include a transition layer. When the active layer includes the transition layer, the ratio of the thickness of the functional layer to the thickness of the transition layer is (1.5-2.5):1; The active material of the functional layer comprises a plurality of first active particles containing manganese and a plurality of second active particles containing iron, wherein at least a portion of the second active particles are attached to the surface of the first active particles, the average particle size Dv50 of the first active particles is 7 μm to 10 μm, and the ratio of the average particle size Dv50 of the second active particles to the average particle size Dv50 of the first active particles is 0.07 to 0.1; The molecular formula of the first active particle is xLi2MnO3·(1 - x)LiMO2, where M = Ni and / or Mn, 0 < x < 1, and the molecular formula of the second active particle is LiMn y Fe 1-y PO4, where 0 < y < 1; The mass percentage of the first active particles in the functional layer is 80 wt % to 90 wt %, and the mass percentage of the second active particles in the functional layer is 5 wt % to 15 wt %.

2. The positive electrode sheet according to claim 1, characterized in that The average particle size Dv50 of the second active particles is 0.5 μm to 0.8 μm.

3. The positive electrode sheet according to claim 1, characterized in that The functional layer further includes a first binder, and the mass percentage of the first binder in the functional layer is 2 wt % to 5 wt %.

4. The positive electrode sheet according to claim 1, characterized in that The transition layer includes a plurality of second active particles, and the mass percentage of the second active particles in the transition layer is 90wt% to 95wt%. The transition layer is stacked on the current collector and is located between the current collector and the functional layer.

5. The positive electrode sheet according to claim 4, characterized in that: The thickness of the functional layer is 30 μm to 100 μm, and / or the thickness of the transition layer is 10 μm to 40 μm.

6. The positive electrode sheet according to any one of claims 4 to 5, characterized in that: The functional layer has a first main portion and a first thinned portion connected to each other, and the first main portion and the first thinned portion cover the surface of the transition layer.

7. The positive electrode sheet according to any one of claims 4 to 5, characterized in that: The functional layer has a first main body and a first thinned portion connected to each other, and the transition layer has a second main body and a second thinned portion connected to each other, the first main body and the first thinned portion cover the surface of the second main body, and the second thinned portion is exposed outside the functional layer.

8. A method for preparing a positive electrode sheet, characterized in that: The positive electrode sheet is the positive electrode sheet according to any one of claims 1 to 7, and the preparation method of the positive electrode sheet comprises the following steps: providing the current collector; A plurality of the first active particles, a plurality of the second active particles, a first conductive agent and a first binder are mixed and coated on the current collector, and then dried to prepare the active layer.

9. The preparation method according to claim 8, characterized in that The method for preparing the positive electrode sheet comprises the following steps: providing the current collector; disposing a conductive layer on the current collector; Disposing a transition layer on a side of the conductive layer away from the current collector; A plurality of the first active particles, a plurality of the second active particles, a first conductive agent and a first binder are mixed and coated on a side of the transition layer away from the current collector, and then dried to prepare the active layer.

10. The method for preparing a positive electrode sheet according to claim 9, characterized in that: The step of disposing the conductive layer on the current collector comprises: coating a first slurry obtained by mixing 90 wt% to 95 wt% of acetylene black and 5 wt% to 10 wt% of polyacrylate on the current collector, and drying the slurry to obtain the conductive layer; and / or, The step of setting the transition layer on the side of the conductive layer away from the current collector is: mixing 90wt% to 95wt% of the second active particles, 2wt% to 5wt% of the second conductive agent and 3wt% to 5wt% of the second binder to obtain a second slurry, coating it on the conductive layer, and drying it to prepare the transition layer.

11. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the positive electrode sheet according to any one of claims 1 to 7.

12. A battery pack, characterized in that: The battery pack includes a box and the lithium-ion battery according to claim 11 disposed in the box.

13. An electrical device, characterized in that: The electric device includes an electric device body and a lithium-ion battery as claimed in claim 11 disposed in the electric device body, wherein the lithium-ion battery is used to supply power to the electric device body.

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