Positive plate, battery monomer, battery and electric device

Abstract

The invention discloses a positive plate, a battery monomer, a battery and an electric device and belongs to the technical field of batteries. The positive plate comprises a positive electrode active material layer, the positive electrode active material layer comprises at least three sub-layers, the difference value m of the electronic conductivity multiples of the positive electrode active materials of the two sub-layers with the farthest distance in the positive electrode active material layer is not less than 1000, and the difference value n of the electronic conductivity multiples of the positive electrode active materials of the two adjacent sub-layers is less than 1000; a sub-layer formed by a positive electrode active material with the electronic conductivity between two sub-layers formed by two positive electrode active materials with the difference value of the electronic conductivity multiples not less than 1000 is arranged between the two sub-layers formed by the two positive electrode active materials, so that the difference value of the electronic conductivity multiples between the sub-layers is less than 1000; and the probability of over-discharge of the positive electrode active material with relatively small electronic conductivity between the contact interfaces of the sub-layers is reduced, so that the electrical performance of the whole positive plate is improved.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode sheet, a battery cell, a battery and an electrical device. Background Art

[0002] In a positive electrode sheet using multiple positive electrode active materials, in order to reduce the possibility of current unevenness between the positive electrode active materials, the positive electrode active materials are usually arranged in layers. However, the positive electrode sheet after the layered arrangement still has the problem of poor electrical performance. Summary of the invention

[0003] In view of the above problems, the present application provides a positive electrode sheet, a battery cell, a battery and an electrical device, which can reduce the conductivity difference between each positive electrode active material sublayer and improve the electrical performance.

[0004] In a first aspect, the present application provides a positive electrode sheet, comprising a positive electrode active material layer, the positive electrode active material layer comprising at least three sublayers, the electronic conductivity multiple difference m of the positive electrode active materials of the two sublayers farthest apart in the positive electrode active material layer is not less than 1000, and the electronic conductivity multiple difference n of the positive electrode active materials of the two adjacent sublayers is less than 1000.

[0005] In the technical solution of the embodiment of the present application, a sublayer composed of positive electrode active materials with an electronic conductivity between the two sublayers composed of positive electrode active materials with an electronic conductivity multiple difference value of not less than 1000 is arranged, so that the electronic conductivity multiple difference value between each sublayer is less than 1000, and the probability of over-discharge of positive electrode active materials with smaller electronic conductivity between the interfaces of the sublayers is reduced, thereby improving the electrical performance of the entire positive electrode sheet. At the same time, arranging each positive electrode active material in layers can reduce the possibility of current unevenness between the positive electrode active materials, thereby making the positive electrode sheet have a higher capacity retention rate in the early stage of the cycle.

[0006] In some embodiments, the ratio of the electronic conductivity multiple difference value n between any one of the middle sub-layers in the positive electrode active material layer and its two adjacent sub-layers is no more than 10.

[0007] In the above implementation process, by controlling the ratio of the electronic conductivity multiple difference value n between a sublayer and its two adjacent sublayers to not exceed 10, the probability of over-discharge of the positive electrode active material with smaller electronic conductivity at the interface between the sublayers is further reduced, which is more beneficial to the electrical performance of the entire positive electrode sheet.

[0008] In some embodiments, the total thickness of the two sub-layers that are farthest apart in the positive electrode active material layer accounts for at least 80% of the positive electrode active material layer.

[0009] In the above implementation process, by controlling the total thickness of the two farthest sublayers to account for at least 80% of the positive electrode active material layer, the performance of the positive electrode active materials in these two sublayers can be more fully utilized, reducing the impact of the addition of the middle sublayer on the performance of the entire electrode.

[0010] In some embodiments, the positive electrode sheet further includes a positive electrode current collector, the positive electrode active material layer is disposed on at least one side surface of the positive electrode current collector, and the electronic conductivity of the positive electrode active material in the sublayer closer to the positive electrode current collector is greater.

[0011] In the above implementation process, by placing the positive electrode active material with greater electronic conductivity closer to the positive electrode current collector, the probability of polarization of the positive electrode sheet can be reduced, which is also beneficial to the rate performance and cycle performance of the positive electrode sheet.

[0012] In some embodiments, the positive electrode active material layer includes a first positive electrode active material sublayer, a second positive electrode active material sublayer and a transition positive electrode active material sublayer arranged between the first positive electrode active material sublayer and the second positive electrode active material sublayer, the first positive electrode active material sublayer includes a first positive electrode active material, the second positive electrode active material sublayer includes a second positive electrode active material, the transition positive electrode active material sublayer includes a transition positive electrode active material, and the relationship between the electronic conductivity a of the first positive electrode active material, the electronic conductivity b of the transition positive electrode active material and the electronic conductivity c of the second positive electrode active material satisfies: 30<a:b<1000, 30<b:c<1000.

[0013] In the above implementation process, usually only one transition positive electrode active material sublayer is needed to reduce the difference in the electronic conductivity multiple of the positive electrode active materials in adjacent sublayers to less than 1000, which can achieve better improvement in the electrical performance of the positive electrode sheet under a relatively simple structure.

[0014] In some embodiments, the electronic conductivity a of the first positive electrode active material is 10 -2 ~10 -1 S / cm; and / or

[0015] The electronic conductivity b of the transition cathode active material is 10 -4 ~10 -2 S / cm; and / or

[0016] The electronic conductivity c of the second positive electrode active material is 10 -6 ~10 -4 S / cm.

[0017] In some embodiments, the first cathode active material comprises: at least one of a spinel structure cathode material, a layered structure cathode material and an olivine structure phosphate cathode material; and / or

[0018] The second positive electrode active material includes at least one of a spinel structure positive electrode material, a layered structure positive electrode material and an olivine structure phosphate positive electrode material.

[0019] In some embodiments, the first cathode active material includes Li a Ni b Mn c M1 2-b-c At least one of O4 and LiM2PO4, wherein 0.9≤a≤1.1, 0≤b≤2, 0≤c≤2, b+c=2, M1 includes at least one of Mg, Zn, Ti, Zr, W, Nb, Al, B, P, Mo, V, and Cr, M2 includes Mn and non-Mn elements, and the non-Mn elements include one or both of a first doping element and a second doping element, the first doping element is manganese-doped, and the second doping element is phosphorus-doped; and / or

[0020] The second positive electrode active material includes Li d Ni x Co y M3 1-x-y O 2+e and at least one of zLi2MnO3·(1-z)LiM4O2, wherein 0.2≤d<1.2, 0≤x≤1.0, 0≤y≤1.0, x+y≤1, -0.02≤e<0.02, M3 includes at least one of Mn, Zr, Al, B, Ta, Mo, W, Nb, Sb, La, and M4 includes at least one of Co, Ni, and Mn.

[0021] In the above implementation process, Li a Ni b Mn c M1 2-b-c O4 and LiM2PO4 generally show good safety performance. d Ni x Co y M3 1-x-y O 2+e and zLi2MnO3·(1-z)LiM4O2 usually show higher energy density. a Ni b Mn c M1 2-b-c Any one of O4 and LiM2PO4 as the second positive electrode active material, Li d Ni x Co y M3 1-x-y O 2+eUsing any one of zLi2MnO3·(1-z)LiM4O2 as the first positive electrode active material enables the positive electrode sheet to have both higher safety performance and higher energy density.

[0022] In the second aspect, the present application provides a positive electrode sheet, which includes a positive electrode active material layer, the positive electrode active material layer includes a first positive electrode active material sublayer, a second positive electrode active material sublayer and a transition positive electrode active material sublayer arranged between the first positive electrode active material sublayer and the second positive electrode active material sublayer, and the thickness of the transition positive electrode active material sublayer is less than 20% of the positive electrode active material layer; the first positive electrode active material sublayer includes a first positive electrode active material, the second positive electrode active material sublayer includes a second positive electrode active material, the transition positive electrode active material sublayer includes a transition positive electrode active material, and the relationship between the electronic conductivity a of the first positive electrode active material, the electronic conductivity b of the transition positive electrode active material and the electronic conductivity c of the second positive electrode active material satisfies: a:c≥1000, 30<a:b<1000, 30<b:c<1000.

[0023] In the technical solution of the embodiment of the present application, a transitional positive electrode active material sublayer composed of a transitional positive electrode active material with an electronic conductivity between the first positive electrode active material sublayer composed of a first positive electrode active material with an electronic conductivity multiple difference value not less than 1000 and the second positive electrode active material sublayer composed of a second positive electrode active material is arranged, so that the electronic conductivity multiple difference value between each sublayer is less than 1000, and the probability of over-discharge of the positive electrode active material with a smaller electronic conductivity between the interfaces of the sublayers is reduced, thereby improving the electrical performance of the entire positive electrode sheet. In addition, by controlling the thickness of the transitional positive electrode active material sublayer to account for less than 20% of the positive electrode active material layer, its influence on the performance of the first positive electrode active material and the second positive electrode active material is reduced, which is more conducive to the performance of the positive electrode sheet. At the same time, arranging each positive electrode active material in layers can reduce the possibility of current unevenness between the positive electrode active materials, thereby enabling the positive electrode sheet to have a higher capacity retention rate in the early stage of the cycle.

[0024] In a third aspect, the present application provides a battery cell, the battery cell comprising the positive electrode sheet provided in the first aspect or the second aspect.

[0025] In a fourth aspect, the present application provides a battery, the battery comprising the battery cell provided in the third aspect.

[0026] In a fifth aspect, the present application provides an electrical device, which includes the battery cell provided in the third aspect or the battery provided in the fourth aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0028] Figure 1 A schematic diagram of the structure of a vehicle provided in some embodiments of the present application;

[0029] Figure 2 A schematic diagram of the exploded structure of a secondary battery provided in some embodiments of the present application;

[0030] Figure 3 A schematic diagram of the structure of a battery cell provided in some embodiments of the present application;

[0031] Figure 4 An exploded view of a battery cell provided for some embodiments of the present application;

[0032] Figure 5 A first structural schematic diagram of a positive electrode sheet provided in some embodiments of the present application;

[0033] Figure 6 A second structural schematic diagram of a positive electrode sheet provided in some embodiments of the present application;

[0034] Figure 7 A flow chart of a method for preparing a positive electrode sheet provided in some embodiments of the present application.

[0035] The reference numerals in the specific implementation manner are as follows:

[0036] 1000-vehicle; 100-secondary battery; 200-motor; 300-controller; 10-housing; 11-accommodating space; 12-first part; 13-second part; 20-battery cell; 21-housing; 211-opening; 22-end cover assembly; 221-end cover; 222-electrode terminal; 23-electrode assembly; 231-positive electrode sheet; 2311-positive current collector; 2312-positive active material layer; 2312a-first positive active material sublayer; 2312b-transition positive active material sublayer; 2312c-second positive active material sublayer; 24-current collecting member; 25-insulating protection member. DETAILED DESCRIPTION

[0037] The following embodiments of the technical solution of the present application are described in detail in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present application, and are therefore only used as examples, and cannot be used to limit the scope of protection of the present application.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by technicians in the technical field to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" in the specification and claims of this application and the above-mentioned figure descriptions and any variations thereof are intended to cover non-exclusive inclusions.

[0039] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0040] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0041] In the description of the embodiments of the present application, the term "and / or" is only a description of the association relationship of the associated objects, indicating that there may be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship.

[0042] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces).

[0043] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, which are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the embodiments of the present application.

[0044] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0045] At present, from the perspective of market development, the application of power batteries is becoming more and more extensive. Power batteries are not only used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, but also widely used in electric vehicles such as electric bicycles, electric motorcycles, electric cars, as well as military equipment and aerospace and other fields. With the continuous expansion of the application field of power batteries, the market demand is also constantly expanding.

[0046] The power battery can be a lithium-ion battery, which is widely used in portable electronic devices, electric vehicles and other fields. In the production process of lithium-ion secondary battery electrode plates, a single-layer coating is usually used to coat the required active material on the current collector. With the increase in energy density and safety performance requirements, some people have proposed the use of positive electrode active materials with high energy density characteristics such as ternary system materials and positive electrode active materials with high safety performance characteristics such as polyanion positive electrode materials to balance energy density and safety performance.

[0047] However, when a positive electrode active material with high energy density characteristics and a positive electrode active material with high safety performance characteristics are directly incorporated into a positive electrode active material layer, current unevenness is likely to occur between the two positive electrode active materials in the positive electrode active material layer. This phenomenon will cause the positive electrode sheet to decay rapidly in the early stage of the cycle, which will have a great impact on the cycle retention rate of the positive electrode.

[0048] In order to further improve the problem of rapid attenuation of the electrode in the early stage of the cycle, the two active materials can be layered. However, after the layered arrangement, the conductivity difference between the two layers of positive electrode active material is large, so that the positive electrode 231 still has the problem of poor electrical performance.

[0049] Based on the above considerations, in order to reduce the conductivity difference between each positive electrode active material layer and improve the electrical performance, the present application proposes a positive electrode sheet, the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes at least three sub-layers, the electronic conductivity multiple difference value m of the positive electrode active materials of the two sub-layers d farthest apart in the positive electrode active material layer is not less than 1000, and the electronic conductivity multiple difference value n of the positive electrode active materials of the adjacent two sub-layers is less than 1000.

[0050] In such a positive electrode sheet, a sublayer composed of positive electrode active materials with an electronic conductivity between the two sublayers composed of positive electrode active materials with an electronic conductivity multiple difference value of not less than 1000 is arranged, so that the electronic conductivity multiple difference value between each sublayer is less than 1000, and the probability of over-discharge of positive electrode active materials with smaller electronic conductivity at the interface between each sublayer is reduced, thereby improving the electrical performance of the entire positive electrode sheet. At the same time, arranging each positive electrode active material in layers can reduce the possibility of current unevenness between positive electrode active materials, thereby making the positive electrode sheet have a higher capacity retention rate in the early stage of the cycle.

[0051] The positive electrode sheet can be used to prepare an electrode assembly, which can be used in, but not limited to, electrical devices such as vehicles, ships or aircraft. A power supply system comprising the battery monomers and secondary batteries disclosed in the present application can be used to form the electrical device.

[0052] The embodiment of the present application provides an electric device using a battery as a power source, and the electric device may be, but is not limited to, a mobile phone, a tablet, a laptop, an electric toy, an electric tool, a battery car, an electric car, a ship, a spacecraft, etc. Among them, the electric toy may include a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy, and an electric airplane toy, etc., and the spacecraft may include an airplane, a rocket, a space shuttle, and a spacecraft, etc.

[0053] For the convenience of description, the following embodiments are described by taking a vehicle 1000 as an example of an electrical device according to an embodiment of the present application.

[0054] Please refer to Figure 1 , Figure 1A schematic diagram of the structure of a vehicle 1000 provided for some embodiments of the present application. The vehicle 1000 may be a fuel vehicle, a gas vehicle or a new energy vehicle, and the new energy vehicle may be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc. A secondary battery 100 is provided inside the vehicle 1000, and the secondary battery 100 may be provided at the bottom, head or tail of the vehicle 1000. The secondary battery 100 may be used to power the vehicle 1000, for example, the secondary battery 100 may be used as an operating power source for the vehicle 1000. The vehicle 1000 may also include a controller 300 and a motor 200, and the controller 300 is used to control the secondary battery 100 to power the motor 200, for example, for starting, navigating and driving the vehicle 1000.

[0055] In some embodiments of the present application, the secondary battery 100 can be used not only as an operating power source for the vehicle 1000 , but also as a driving power source for the vehicle 1000 , replacing or partially replacing fuel or natural gas to provide driving power for the vehicle 1000 .

[0056] In the present application, the secondary battery 100 may refer to a single battery cell 20, or it may refer to a single physical module including multiple battery cells 20 to provide higher voltage and capacity, which may be in the form of a battery pack, a battery module, etc. The secondary battery 100 may include a case 10 for encapsulating multiple battery cells 20, and the case 10 may prevent liquid or other foreign matter from affecting the charging or discharging of the battery cells 20.

[0057] Figure 2 This is a schematic diagram of the exploded structure of a secondary battery 100 provided in some embodiments of the present application. Figure 2 The secondary battery 100 includes a housing 10 and a battery cell 20 , and the battery cell 20 is accommodated in the housing 10 .

[0058] The box body 10 is used to provide a storage space 11 for the battery cell 20. In some embodiments, the box body 10 may include a first portion 12 and a second portion 13, and the first portion 12 and the second portion 13 cover each other to define the storage space 11 for accommodating the battery cell 20. Of course, the connection between the first portion 12 and the second portion 13 can be sealed by a sealing member (not shown in the figure), and the sealing member can be a sealing ring, a sealant, etc.

[0059] The first part 12 and the second part 13 can be in various shapes, such as a cuboid, a cylinder, etc. The first part 12 can be a hollow structure with one side open to form a receiving cavity for receiving the battery cell 20, and the second part 13 can also be a hollow structure with one side open to form a receiving cavity for receiving the battery cell 20. The opening side of the second part 13 covers the opening side of the first part 12, thereby forming a box body 10 with a receiving space 11. Of course, if Figure 2As shown, the first part 12 may be a hollow structure with one side open, and the second part 13 may be a plate-like structure. The second part 13 covers the open side of the first part 12 to form a box body 10 with an accommodating space 11.

[0060] In the secondary battery 100, there are multiple battery cells 20, and the multiple battery cells 20 can be connected in series, in parallel, or in mixed connection. Mixed connection means that the multiple battery cells 20 are both connected in series and in parallel. The multiple battery cells 20 can be directly connected in series, in parallel, or in mixed connection, and then the whole body formed by the multiple battery cells 20 is accommodated in the box 10; of course, multiple battery cells 20 can also be connected in series, in parallel, or in mixed connection to form a battery module, and then the multiple battery modules are connected in series, in parallel, or in mixed connection to form a whole body, and accommodated in the box 10. The battery cell 20 can be cylindrical, flat, rectangular, or other shapes. Figure 2 The example shows that the battery cell 20 is in a square shape.

[0061] In some embodiments, the secondary battery 100 may further include a busbar component (not shown), and the multiple battery cells 20 may be electrically connected via the busbar component to achieve series connection, parallel connection, or mixed connection of the multiple battery cells 20 .

[0062] Figure 3 This is a schematic diagram of the structure of a battery cell 20 provided in some embodiments of the present application. Figure 4 This is an exploded view of a battery cell 20 provided in some embodiments of the present application. Figure 3 and Figure 4 The battery cell 20 may include a housing 21 , an end cap assembly 22 and an electrode assembly 23 . The housing 21 has an opening 211 , the electrode assembly 23 is accommodated in the housing 21 , and the end cap assembly 22 is used to cover the opening 211 .

[0063] The shape of the housing 21 can be determined according to the specific shape of the electrode assembly 23. For example, if the electrode assembly 23 is a rectangular parallelepiped structure, the housing 21 can be a rectangular parallelepiped structure. Figure 3 and Figure 4 The case where the housing 21 and the electrode assembly 23 are square is exemplarily shown.

[0064] The shell 21 may also be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, etc., and the embodiment of the present application does not impose any special restrictions on this.

[0065] The end cap assembly 22 includes an end cap 221 and an electrode terminal 222. The end cap assembly 22 is used to seal the opening 211 of the housing 21 to form a closed installation space (not shown), and the installation space is used to accommodate the electrode assembly 23. The installation space is also used to accommodate an electrolyte, such as an electrolyte. The end cap assembly 22 is a component for outputting the electrical energy of the electrode assembly 23. The electrode terminal 222 in the end cap assembly 22 is used to be electrically connected to the electrode assembly 23, that is, the electrode terminal 222 is electrically connected to the tab of the electrode assembly 23. For example, the electrode terminal 222 is connected to the tab through the current collecting member 24 to achieve electrical connection between the electrode terminal 222 and the tab.

[0066] It should be noted that the opening 211 of the shell 21 can be one or two. If the opening 211 of the shell 21 is one, the end cap assembly 22 can also be one, and two electrode terminals 222 can be provided in the end cap assembly 22, and the two electrode terminals 222 are respectively used to electrically connect to the positive pole ear and the negative pole ear of the electrode assembly 23. If the opening 211 of the shell 21 is two, for example, the two openings 211 are provided on opposite sides of the shell 21, and the end cap assembly 22 can also be two, and the two end cap assemblies 22 are respectively covered at the two openings 211 of the shell 21. In this case, the electrode terminal 222 in one end cap assembly 22 can be a positive electrode terminal, which is used to electrically connect to the positive pole ear of the electrode assembly 23; the electrode terminal 222 in the other end cap assembly 22 can be a negative electrode terminal, which is used to electrically connect to the negative electrode sheet of the electrode assembly 23.

[0067] In some embodiments, Figure 4 As shown, the battery cell 20 may further include an insulating protective member 25 fixed to the periphery of the electrode assembly 23, and the insulating protective member 25 is used to insulate and isolate the electrode assembly 23 from the housing 21. Exemplarily, the insulating protective member 25 is a tape bonded to the periphery of the electrode assembly 23. In some embodiments, there are multiple electrode assemblies 23, and the insulating protective member 25 is arranged around the periphery of the multiple electrode assemblies 23, and the multiple electrode assemblies 23 are formed into an integral structure to keep the electrode assembly 23 structurally stable.

[0068] The electrode assembly 23 includes a positive electrode sheet 231, a negative electrode sheet and a separator. The positive electrode sheet 231 includes a positive electrode collector 2311 and a positive electrode active material layer 2312. The positive electrode active material layer 2312 is coated on the surface of the positive electrode collector 2311. The positive electrode collector 2311 not coated with the positive electrode active material layer 2312 protrudes from the positive electrode collector 2311 coated with the positive electrode active material layer 2312. The positive electrode collector 2311 not coated with the positive electrode active material layer 2312 serves as a positive electrode tab.

[0069] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer, the negative electrode active material layer is coated on the surface of the negative electrode current collector, the negative electrode current collector not coated with the negative electrode active material layer protrudes from the negative electrode current collector coated with the negative electrode active material layer, and the negative electrode current collector not coated with the negative electrode active material layer serves as a negative electrode tab. The material of the negative electrode current collector may be copper, and the negative electrode active material may be carbon or silicon, etc. In order to ensure that a large current passes without melting, the number of positive electrode tabs is multiple and stacked together, and the number of negative electrode tabs is multiple and stacked together. The material of the isolation film may be PP (polypropylene) or PE (polyethylene), etc. In addition, the electrode assembly 23 may be a wound electrode assembly or a laminated electrode assembly, and the embodiments of the present application are not limited thereto.

[0070] Figure 5 This is a first structural schematic diagram of the positive electrode sheet 231 provided in some embodiments of the present application. Figure 6 A second structural schematic diagram of the positive electrode sheet 231 provided in some embodiments of the present application; see Figure 5 and Figure 6 The embodiment of the present application provides a positive electrode sheet 231, which includes a positive electrode active material layer 2312, and the positive electrode active material layer 2312 includes at least three sub-layers. The electronic conductivity multiple difference value m of the positive electrode active materials of the two sub-layers farthest apart in the positive electrode active material layer 2312 is not less than 1000, and the electronic conductivity multiple difference value n of the positive electrode active materials of the two adjacent sub-layers is less than 1000.

[0071] The positive electrode active material layer 2312 is attached to at least part of the surface of the positive electrode current collector 2311; the two sublayers with the farthest interval refer to the two sublayers closest to the positive electrode current collector 2311 and the two sublayers farthest from the positive electrode current collector 2311; electronic conductivity refers to the conductivity of electrons in a substance under the action of an electric field; the electronic conductivity multiple difference value m of the positive electrode active materials of the two sublayers with the farthest interval is obtained by dividing the electronic conductivity of the larger positive electrode active material by the electronic conductivity of the smaller positive electrode active material; wherein, the sublayer composed of the positive electrode active material with the larger electronic conductivity may be in direct contact with the positive electrode current collector 2311, or the sublayer composed of the positive electrode active material with the smaller electronic conductivity may be in direct contact with the positive electrode current collector 2311. The electronic conductivity multiple difference value n of the positive electrode active materials of the adjacent two sublayers is obtained by dividing the electronic conductivity of the larger positive electrode active material by the electronic conductivity of the smaller positive electrode active material.

[0072] The material of the positive electrode current collector 2311 may be one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. Figure 5In one embodiment, a positive electrode active material layer 2312 is disposed on one surface of the positive electrode current collector 2311; please continue to refer to Figure 6 In another embodiment, positive electrode active material layers 2312 are disposed on both surfaces of the positive electrode current collector 2311 .

[0073] For lithium-ion batteries, the positive electrode active material in each sublayer refers to a substance that can intercalate and deintercalate lithium ions.

[0074] The positive electrode sheet 231 is provided with a sublayer composed of positive electrode active materials with an electronic conductivity between the two sublayers composed of positive electrode active materials with an electronic conductivity multiple difference value of not less than 1000, so that the electronic conductivity multiple difference value between each sublayer is less than 1000, thereby reducing the probability of over-discharge of positive electrode active materials with smaller electronic conductivity at the interface between each sublayer, thereby improving the electrical performance of the entire positive electrode sheet 231. At the same time, arranging each positive electrode active material in layers can reduce the possibility of current non-uniformity between the positive electrode active materials, thereby making the positive electrode sheet 231 have a higher capacity retention rate in the early stage of the cycle.

[0075] In the technical solutions of some embodiments of the present application, the ratio of the electronic conductivity multiple difference value n between any middle sublayer in the positive electrode active material layer 2312 and its two adjacent sublayers does not exceed 10.

[0076] The middle sublayer in the positive electrode active material layer 2312 refers to the sublayer located between the two sublayers farthest from each other in the positive electrode active material layer 2312. The ratio of the electronic conductivity multiple difference value n between any middle sublayer in the positive electrode active material layer 2312 and its two adjacent sublayers is no more than 10, which means that the ratio of the electronic conductivity multiple difference value n1 of the positive electrode active material between the middle sublayer and its adjacent sublayer and the electronic conductivity multiple difference value n2 of the positive electrode active material between the middle sublayer and its other adjacent sublayer is no more than 10, and the ratio is obtained by dividing the larger value of n1 and n2 by the smaller value, that is, when n1 is greater than n2, the ratio is calculated as n1 / n2, and when n2 is greater than n2, the ratio is calculated as n2 / n1.

[0077] By controlling the ratio of the difference values ​​n of two adjacent electronic conductivity multiples to not exceed 10, the probability of over-discharge of the positive electrode active material with smaller electronic conductivity at the interface between the sub-layers is further reduced, which is more beneficial to the electrical performance of the entire positive electrode sheet 231.

[0078] For example, the ratio of the difference values ​​n of two adjacent electronic conductivity multiples may be 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10, etc., and may also be any value not exceeding 10.

[0079] In the technical solutions of some embodiments of the present application, the total thickness of the two sub-layers that are farthest apart in the positive electrode active material layer 2312 accounts for at least 80% of the positive electrode active material layer 2312 .

[0080] The total thickness of the two farthest sublayers accounts for at least 80% of the positive electrode active material layer 2312, which means that the total thickness of the sublayers closest to the positive electrode current collector 2311 and farthest from the positive electrode current collector 2311 in the positive electrode active material layer 2312 divided by the thickness of the positive electrode active material layer 2312 is greater than 80%.

[0081] By controlling the total thickness of the two furthest sublayers to account for at least 80% of the positive electrode active material layer 2312, the performance of the positive electrode active materials in the two sublayers can be more fully utilized, reducing the impact of the addition of the middle sublayer on the performance of the entire electrode.

[0082] Exemplarily, the total thickness of the two farthest sublayers may account for 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98% or 99% of the positive electrode active material layer 2312, etc., and it may also be any value above 85%.

[0083] In the technical solutions of some embodiments of the present application, the positive electrode sheet 231 also includes a positive electrode current collector 2311, and the positive electrode active material layer 2312 is disposed on at least one side surface of the positive electrode current collector 2311, and the electronic conductivity of the positive electrode active material in the sublayer closer to the positive electrode current collector 2311 is greater. By disposing the positive electrode active material with greater electronic conductivity at a position closer to the positive electrode current collector 2311, the probability of polarization of the positive electrode sheet 231 can be reduced, and it is also beneficial to the rate performance and cycle performance of the positive electrode sheet 231.

[0084] In the technical solutions of some embodiments of the present application, the positive electrode active material layer 2312 includes a first positive electrode active material sublayer 2312a, a second positive electrode active material sublayer 2312c and a transition positive electrode active material sublayer 2312b arranged between the first positive electrode active material sublayer 2312a and the second positive electrode active material sublayer 2312c, the first positive electrode active material sublayer 2312a includes a first positive electrode active material, the second positive electrode active material sublayer 2312c includes a second positive electrode active material, the transition positive electrode active material sublayer 2312b includes a transition positive electrode active material, and the relationship between the electronic conductivity a of the first positive electrode active material, the electronic conductivity b of the transition positive electrode active material and the electronic conductivity c of the second positive electrode active material satisfies: 30<a:b<1000, 30<b:c<1000. Usually, only one transition positive electrode active material sublayer 2312b is needed to reduce the difference in the multiple of the electronic conductivity of the positive electrode active materials of adjacent sublayers to less than 1000, which can achieve a better improvement in the electrical performance of the positive electrode sheet 231 with a relatively simple structure.

[0085] Exemplarily, the ratio of the electronic conductivity a of the first positive electrode active material to the electronic conductivity b of the transition positive electrode active material can be 30, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000, etc., and it can also be any value in the range of 30 to 1000. The ratio of the electronic conductivity b of the transition positive electrode active material to the electronic conductivity c of the second positive electrode active material can be 30, 100, 150, 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950 or 1000, etc., and it can also be any value in the range of 30 to 1000.

[0086] In the technical solutions of some embodiments of the present application, the electronic conductivity a of the first positive electrode active material is 10 -2 ~10 - 1 S / cm; the electronic conductivity b of the transition cathode active material is 10 -4 ~10 -2 S / cm; the electronic conductivity c of the second positive electrode active material is 10 -6 ~10 -4 S / cm.

[0087] For example, the electronic conductivity a of the first positive electrode active material may be 0.01 S / cm, 0.02 S / cm, 0.03 S / cm, 0.04 S / cm, 0.05 S / cm, 0.06 S / cm, 0.07 S / cm, 0.08 S / cm, 0.09 S / cm or 0.1 S / cm, etc., and may also be 10 -2 ~10 -1 The electronic conductivity b of the transition cathode active material may be 0.0001 S / cm, 0.0005 S / cm, 0.001 S / cm, 0.0015 S / cm, 0.002 S / cm, 0.0025 S / cm, 0.003 S / cm, 0.0035 S / cm, 0.004 S / cm, 0.0045 S / cm, 0.005 S / cm, 0.0055 S / cm, 0.006 S / cm, 0.0065 S / cm, 0.007 S / cm, 0.0075 S / cm, 0.008 S / cm, 0.0085 S / cm, 0.009 S / cm, 0.0095 S / cm or 0.01 S / cm, etc., and it may also be 10 -4 ~10 -2 The electronic conductivity c of the second positive electrode active material may be 0.000001 S / cm, 0.000005 S / cm, 0.00001 S / cm, 0.000015 S / cm, 0.00002 S / cm, 0.000025 S / cm, 0.00003 S / cm, 0.000035 S / cm, 0.00004 S / cm, 0.000045 S / cm, 0.00005S / cm, 0.000055S / cm, 0.00006S / cm, 0.000065S / cm, 0.00007S / cm, 0.000075S / cm, 0.00008S / cm, 0.000085S / cm, 0.00009S / cm, 0.000095S / cm or 0.0001S / cm, etc., which may also be 10 -6 ~10 -4 Any value within the range of S / cm.

[0088] In the technical solutions of some embodiments of the present application, the first positive electrode active material includes: at least one of a spinel structure positive electrode material, a layered structure positive electrode material and an olivine structure phosphate positive electrode material; the second positive electrode active material includes: at least one of a spinel structure positive electrode material, a layered structure positive electrode material and an olivine structure phosphate positive electrode material.

[0089] Spinel structure positive electrode materials refer to positive electrode active materials with spinel structure, usually including lithium manganese oxide, lithium nickel oxide, etc. Layered structure positive electrode materials refer to positive electrode active materials with layered structure, usually including ternary system materials, lithium-rich manganese-based materials, etc. Olivine structure phosphate positive electrode materials refer to positive electrode active materials with olivine structure, usually including lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate and other polyanion positive electrode materials. Among them, polyanion positive electrode materials are a general term for a series of compounds containing tetrahedral or octahedral anion structural units (XOm)n-, which have the advantages of high charge and discharge voltage, large energy storage capacity, fast charge and discharge capability and good cycle stability. Its synthesis methods mainly include: high temperature solid phase method, sol-gel method, hydrothermal method, electrospinning method, etc. Ternary system materials usually include two types: NCA and NCM. Among them, NCA is widely used due to its long life, large capacity, and high energy density, but its specific heat capacity is relatively low; NCM combines the advantages of three types of positive electrode materials: lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide, and has an obvious ternary synergistic effect. NCM can usually be expressed as: LiNi x Co y Mn z O2.

[0090] In the technical solutions of some embodiments of the present application, the first positive electrode active material includes Li a Ni b Mn c M1 2-b-c At least one of O4 and LiM2PO4, wherein 0.9≤a≤1.1, 0≤b≤2, 0≤c≤2, b+c=2, M1 includes at least one of Mg, Zn, Ti, Zr, W, Nb, Al, B, P, Mo, V, and Cr, M2 includes Mn and non-Mn elements, and the non-Mn elements include one or both of a first doping element and a second doping element, the first doping element is manganese doped, and the second doping element is phosphorus doped; the second positive electrode active material includes Li d Ni x Co y M3 1-x-y O 2+e and at least one of zLi2MnO3·(1-z)LiM4O2, wherein 0.2≤d<1.2, 0≤x≤1.0, 0≤y≤1.0, x+y≤1, -0.02≤e<0.02, M3 includes at least one of Mn, Zr, Al, B, Ta, Mo, W, Nb, Sb, La, and M4 includes at least one of Co, Ni, and Mn.

[0091] Lithium manganese oxide, lithium nickel oxide and polyanion positive electrode materials generally exhibit good safety performance, lithium cobalt oxide, lithium-rich manganese-based materials and ternary system materials generally exhibit higher energy density. Using any one of lithium manganese oxide, lithium nickel oxide and polyanion positive electrode materials as the second positive electrode active material and any one of lithium cobalt oxide, lithium-rich manganese-based materials and ternary system materials as the first positive electrode active material can enable the positive electrode sheet 231 to have both higher safety performance and higher energy density.

[0092] It should be noted that the above-mentioned LiM2PO4 is not a specific molecular structure formula, but a general expression of lithium manganese-containing phosphate.

[0093] In the technical solutions of some embodiments of the present application, the non-Mn element includes one or both of a first doping element and a second doping element, the first doping element is manganese-doped, and the second doping element is phosphorus-doped.

[0094] In the technical solutions of some embodiments of the present application, the first doping element includes one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge.

[0095] In the technical solutions of some embodiments of the present application, the first doping element includes at least two of Fe, Ti, V, Ni, Co and Mg.

[0096] In the technical solutions of some embodiments of the present application, the second doping element includes one or more elements selected from the group consisting of B, S, Si and N.

[0097] In the technical solutions of some embodiments of the present application, the second positive electrode active material includes Li 1+x Mn 1-y A y P 1-z R z O4, Li 1+x Mn 1-y A y P 1-z R z In O4, x is any value in the range of -0.100 to 0.100, y is any value in the range of 0.001 to 0.500, z is any value in the range of 0.001 to 0.100, A includes one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge, and R includes one or more elements selected from the group consisting of B, S, Si and N.

[0098] In the technical solutions of some embodiments of the present application, the compound Li 1+x Mn 1-y AHr1-z The preparation method of RzO4 may include the following steps:

[0099] (1) dissolving a manganese source, a source of an element A to be doped at the manganese position, and an acid in a solvent and stirring to generate a suspension of a manganese salt doped with the element A, filtering the suspension and drying the filter cake to obtain a manganese salt doped with the element A;

[0100] (2) adding a lithium source, a phosphorus source, an element R source, a solvent, and the manganese salt doped with element A obtained in step (1) into a reaction container, grinding and mixing to obtain a slurry;

[0101] (3) transferring the slurry obtained in step (2) to a spray drying device for spray drying and granulation to obtain granules;

[0102] (4) Sintering the particles obtained in step (3) to obtain a positive electrode active material.

[0103] In any embodiment, the manganese source may be a manganese-containing substance known in the art that can be used to prepare lithium manganese phosphate. For example, the manganese source may be selected from elemental manganese, manganese dioxide, manganese phosphate, manganese oxalate, manganese carbonate, or a combination thereof.

[0104] The acid is selected from one or more of hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, organic acid such as oxalic acid, etc., for example, oxalic acid. The source of element R is selected from at least one of sulfate, borate, nitrate and silicate of element R. The source of element A is selected from at least one of simple substance, oxide, phosphate, oxalate, carbonate and sulfate of A.

[0105] In the technical solutions of some embodiments of the present application, the polyanion positive electrode material includes Li h A i Mn 1-j B j P 1-k C k O 4-l D l , wherein A includes one or more elements selected from the group consisting of Zn, Al, Na, K, Mg, Nb, Mo and W; B includes one or more elements selected from the group consisting of Ti, V, Zr, Fe, Ni, Mg, Co, Ga, Sn, Sb, Nb and Ge; C includes one or more elements selected from the group consisting of B, S, Si and N; D includes one or more elements selected from the group consisting of S, F, Cl and Br; h is selected from the range of 0.9 to 1.1, i is selected from the range of 0.001 to 0.1, j is selected from the range of 0.001 to 0.5, k is selected from the range of 0.001 to 0.1, l is selected from the range of 0.001 to 0.1, and the polyanion positive electrode material is electrically neutral.

[0106] It should be noted that Li h A iMn 1-j B j P 1-k C k O 4-l D l The compound is actually a specific LiMPO4 material. Its preparation method can refer to Li 1+x Mn 1-y A y P 1-z R z O4, no limitation is made here.

[0107] Below is Li 0.994 Mo 0.001 Mn 0.65 Fe 0.35 P 0.999 Si 0.001 O 3.999 F 0.001 The preparation process is further explained: 1. Preparation of doped manganese oxalate: 1.3 mol of MnSO4﹒H2O and 0.7 mol of FeSO4﹒H2O are fully mixed in a mixer for 6 hours. The mixture is transferred to a reactor, and 10L of deionized water and 2 mol of dihydrated oxalic acid (calculated as oxalic acid) are added. The reactor is heated to 80°C and stirred at 600 rpm for 6 hours. The reaction is terminated (no bubbles are generated) to obtain a Fe-doped manganese oxalate suspension. The suspension is then filtered, the filter cake is dried at 120°C, and then ground to obtain a median particle size Dv 50 Fe-doped manganese oxalate particles of about 100 nm. 2. Preparation of doped lithium manganese phosphate: Take 1 mol of the above manganese oxalate particles, 0.497 mol of lithium carbonate, 0.001 mol of Mo(SO4)3, an aqueous phosphoric acid solution containing 0.999 mol of phosphoric acid at a concentration of 85%, 0.001 mol of H4SiO4, 0.0005 mol of NH4HF2 and 0.005 mol of sucrose and add them to 20L of deionized water. Transfer the mixture to a sand mill and grind and stir it for 10 hours to obtain a slurry. Transfer the slurry to a spray drying equipment for spray drying and granulation, set the drying temperature to 250°C, dry for 4 hours, and obtain particles. In a protective atmosphere of nitrogen (90% volume) + hydrogen (10% volume), sinter the above powder at 700°C for 10 hours to obtain carbon-coated Li 0.994 Mo 0.001 Mn 0.65 Fe 0.35 P 0.999 Si 0.001 O 3.999 F 0.001 .

[0108] In the technical solutions of some embodiments of the present application, the first positive electrode active material also has a carbon-containing coating layer.

[0109] The introduction of the carbon-containing coating layer improves the conductivity of the first positive electrode active material. At this time, the structure of the first positive electrode active material is actually a core-shell structure with LiMPO4 as the core and the surface of the core covered with the coating layer.

[0110] In the list of positive electrode materials in this application, the molar content of Li is the initial state of the material, that is, the state before adding the materials. The positive electrode material is used in the battery system, and the molar content of Li will change after charge and discharge cycles.

[0111] In addition, due to the different preparation processes and conditions of the materials, the molar content of the O element is usually not strictly the coefficient of the O element in the chemical formula, and there will be fluctuations, such as Li 1+x Mn 1-y A y P 1-z R z The molar content of the O element in O4 is not strictly 4.

[0112] For example, the first positive electrode active material may be a ternary system material or LiFePO4, 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 0.3 PO4、LiMn 0.8 Fe 0.2 PO4、LiMn 0.9 Fe 0.1 PO4, LiMnPO4, LiMn 0.5 Al 0.5 P 0.5 B 0.5 O4、LiMn 0.5 Mg 0.5 P 0.5 S 0.5O4, etc., M2 in LiM2PO4 includes Mn and non-Mn elements, the non-Mn elements include one or two of the first doping element and the second doping element, the first doping element is manganese doping, and the second doping element is phosphorus doping. The first doping element includes one or more elements of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Fe, Ni, Co, Ga, Sn, Sb, Nb and Ge; the second doping element includes one or more elements of B, S, Si and N.

[0113] In the technical solutions of some embodiments of the present application, the ternary system materials include nickel-cobalt-manganese ternary materials and modified materials thereof and nickel-cobalt-aluminum ternary materials and modified materials thereof. The modified materials of nickel-cobalt-manganese ternary materials and the modified materials of nickel-cobalt-aluminum ternary materials refer to materials obtained by doping or coating nickel-cobalt-manganese ternary materials or nickel-cobalt-aluminum ternary materials, respectively. The coated coating layer can be selected as a combination of one or more oxides, nitrates, phosphates, and carbonates containing one or more elements of Al, Ba, Zn, Ti, Co, W, Y, Si, Sn, B, and P, such as Al2O3, B2O3, TiO2, etc. For example, the chemical formula of the ternary system material is LiNi x Co y Mn z O2, where 0≤x≤1, 0≤y≤1, 0≤z≤1, x+y+z=1. Specifically, it can be LiNi 0.4 Co 0.2 Mn 0.4 O2、LiNi 0.5 Co 0.2 Mn 0.3 O2、LiNi 0.3 Co 0.3 Mn 0.3 O2 and LiNi 0.7 Co 0.15 Mn 0.15 O2, etc.

[0114] In the technical solutions of some embodiments of the present application, the lithium-rich manganese-based material can specifically be 0.5Li2MnO3·0.5LiNi 0.5 Mn 0.3 Na 0.2 O2、0.5Li2MnO3·0.5LiNi 0.5 Mn 0.3 Mg 0.2 O2、0.5Li2MnO3·0.5LiNi 0.5 Mn 0.3 Al 0.2 O2、0.5Li2MnO3·0.5LiNi 0.5 Mn 0.3 Ca<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Ba<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> V<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Zn<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Ti<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Fe<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Co<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Cr<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Nb<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> W<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Mo<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Zr<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Ta<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Hf<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.1Li2MnO3·0.9LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.2Li2MnO3·0.8LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.3Li2MnO3·0.7LiNi<h2 style=";text-align:left;direction:ltr"> 0.5<h2 style=";text-align:left;direction:ltr">Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.4Li2MnO3·0.6LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.6Li2MnO3·0.4LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.7Li2MnO3·0.3LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.8Li2MnO3·0.2LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.9Li2MnO3·0.1LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.7 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.6 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.4 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.4 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.6 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.7 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.2 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Na<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> Mg<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Cr<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> W<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi<h2 style=";text-align:left;direction:ltr"> 0.5 <h2 style=";text-align:left;direction:ltr"> Mn<h2 style=";text-align:left;direction:ltr"> 0.3 <h2 style=";text-align:left;direction:ltr"> Fe<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> Co<h2 style=";text-align:left;direction:ltr"> 0.1 <h2 style=";text-align:left;direction:ltr"> O2, 0.5Li2MnO3·0.5LiNi0.5 Mn 0.3 Zn 0.1 Ta 0.1 O2、0.5Li2MnO3·0.5LiNi 0.5 Mn 0.4 Na 0.1 O2、0.5Li2MnO3·0.5LiNi 0.5 Mn 0.5 O2.

[0115] It should be noted that the above enumeration of the first positive electrode active material and the second positive electrode active material is only used as an enumeration to illustrate that this solution can be implemented, and is not intended to limit this solution. The implementation of this solution only requires that the first positive electrode active material and the second positive electrode active material meet the corresponding electronic conductivity requirements. In other embodiments, those skilled in the art can select specific materials of the first positive electrode active material and the second positive electrode active material according to actual needs, such as the materials listed above and their modified materials, the modification includes doping or coating, etc., or select other materials that meet the electronic conductivity requirements of this application.

[0116] In the technical solutions of some embodiments of the present application, the transition positive electrode active material may be the same as the first positive electrode active material or the second positive electrode active material. When the transition positive electrode active material is the same as the first positive electrode active material, that is, the transition positive electrode active material is selected from lithium manganese oxide, lithium nickel oxide or polyanion positive electrode material, the electronic conductivity can be reduced by reducing the amount of carbon coating, thereby realizing the transition of electronic conductivity between the first positive electrode active material sublayer 2312a and the second positive electrode active material sublayer 2312c; when the transition positive electrode active material and the second positive electrode active material are the same, that is, the transition positive electrode active material is selected from lithium cobalt oxide, lithium manganese-rich based material or ternary system material, the electronic conductivity can be increased by increasing the amount of carbon coating, thereby realizing the transition of electronic conductivity between the first positive electrode active material sublayer 2312a and the second positive electrode active material sublayer 2312c.

[0117] After the above introduction to the material and structure of the positive electrode sheet 231 , the preparation method of the positive electrode sheet 231 will be specifically introduced below.

[0118] The preparation method of the positive electrode sheet 231 includes the following steps: preparing sublayers layer by layer on the positive electrode current collector 2311 to form a positive electrode active material layer 2312, the positive electrode active material layer 2312 includes at least three sublayers, the electronic conductivity multiple difference m of the positive electrode active materials of the two sublayers farthest apart in the at least three sublayers is not less than 1000, and the electronic conductivity multiple difference n of the positive electrode active materials of two adjacent sublayers is less than 1000.

[0119] The method arranges a sublayer composed of positive electrode active materials with an electronic conductivity between the two sublayers composed of positive electrode active materials with an electronic conductivity multiple difference value of not less than 1000, so that the electronic conductivity multiple difference value between each sublayer is less than 1000, and reduces the probability of over-discharge of positive electrode active materials with smaller electronic conductivity at the interface between each sublayer, thereby improving the electrical performance of the entire positive electrode sheet 231. At the same time, arranging each positive electrode active material in layers can reduce the possibility of current unevenness between the positive electrode active materials, thereby making the positive electrode sheet 231 have a higher capacity retention rate in the early stage of the cycle.

[0120] The following is an example of preparing a positive electrode sheet 231 in which the positive electrode active material layer 2312 includes a first positive electrode material active layer, a transition positive electrode material active layer and a second positive electrode active material layer 2312 .

[0121] Figure 7 For a flow chart of a method for preparing the positive electrode sheet 231 provided in some embodiments of the present application, please refer to Figure 7 The present application embodiment provides a method for preparing a positive electrode sheet 231, the method comprising:

[0122] S110, preparing a first positive electrode active slurry: dispersing a first positive electrode active material, a binder and a conductive agent in a solvent to form a first positive electrode active slurry. The first positive electrode active material may be the first positive electrode active material described above, for example, a material having a chemical formula of LiMn a Fe 1-a PO4, (0≤a≤1) polyanion positive electrode material, optionally, a small amount of other positive electrode active materials may be added.

[0123] For the specific selection of the polyanion positive electrode material, reference may be made to the selection of the polyanion positive electrode material in the first positive electrode active material sublayer 2312a in the aforementioned positive electrode sheet 231, which will not be described in detail here.

[0124] The binder may be one or more of styrene-butadiene rubber, water-based acrylic resin, carboxymethyl cellulose, polyvinylidene fluoride, polytetrafluoroethylene, ethylene-vinyl acetate copolymer, polyvinyl alcohol and polyvinyl butyral. The conductive agent may be at least one of conductive carbon black, carbon fiber, carbon nanotube, Ketjen black, graphene or acetylene black. The solvent may be one or more of dimethyl glutarate and N-methyl pyrrolidone. A leveling agent, a dispersant, etc. may also be added to the first positive electrode active slurry.

[0125] S120, preparing a transition positive electrode active slurry: dispersing a transition positive electrode active material, a binder and a conductive agent in a solvent to form a transition positive electrode active slurry, wherein the electronic conductivity of the transition positive electrode active material is between that of the first positive electrode active material and the second positive electrode active material.

[0126] S130, preparing a second positive electrode active slurry: dispersing a second positive electrode active material, a binder and a conductive agent in a solvent to form a second positive electrode active slurry. The second positive electrode active material may be the second positive electrode active material described above, for example: a material having a chemical formula of LiNi x Co y Mn z O2, (0≤x≤1, 0≤y≤1, 0≤z≤1, x+y+z=1) ternary system material, optionally, a small amount of other positive electrode active materials may be added.

[0127] The binder, the conductive agent and the solvent may be the binder, the conductive agent and the solvent in the first positive electrode active slurry, etc. The binder in the first positive electrode active slurry may be the same as or different from the binder in the transition positive electrode active material; the conductive agent in the first positive electrode active slurry may be the same as or different from the conductive agent in the transition positive electrode active material; the solvent in the first positive electrode active slurry may be the same as or different from the solvent in the transition positive electrode active material. At the same time, a leveling agent, a dispersant, etc. may also be added to the transition positive electrode active slurry, which is not limited in this application.

[0128] For the specific selection of the ternary system material, reference may be made to the selection of the ternary system material in the second positive electrode active material sub-layer 2312 c in the aforementioned positive electrode sheet 231 , which will not be described in detail here.

[0129] The binder, the conductive agent and the solvent may be the binder, the conductive agent and the solvent in the first positive electrode active slurry, etc. The binder in the first positive electrode active slurry may be the same as or different from the binder in the second positive electrode active material; the conductive agent in the first positive electrode active slurry may be the same as or different from the conductive agent in the second positive electrode active material; the solvent in the first positive electrode active slurry may be the same as or different from the solvent in the second positive electrode active material. At the same time, a leveling agent, a dispersant, etc. may also be added to the second positive electrode active slurry, which is not limited in this application.

[0130] It should be noted that step S120 and step S130 can be interchanged or performed simultaneously, and this application does not limit this.

[0131] S140, preparing the first positive electrode active material sublayer 2312a: coating the first positive electrode active slurry on the surface of the positive electrode current collector 2311, and then drying to form the first positive electrode active material sublayer 2312a. The coating can be applied to one surface or both surfaces of the positive electrode current collector 2311 according to requirements.

[0132] The coating method may be: scraping, roller coating, slit coating, etc., which is not limited in this application. It should be noted that step S120, step S130 and step S140 can be interchanged or performed simultaneously, which is not limited in this application.

[0133] S150, preparing the transition positive electrode active material sublayer 2312b: coating the transition positive electrode active slurry on the surface of the first positive electrode active material sublayer 2312a, and then drying to form the transition positive electrode active material sublayer 2312b. During coating, the transition positive electrode active material sublayer 2312b can be formed on the surface of the first positive electrode active material sublayer 2312a according to the condition of the first positive electrode active material sublayer 2312a.

[0134] It should be noted that step S130 and step S150 can be interchanged or performed simultaneously, and this application does not limit this.

[0135] S160, preparing the second positive electrode active material sublayer 2312c: coating the second positive electrode active slurry on the surface of the transition positive electrode active material sublayer 2312b, and then drying to form the second positive electrode active material sublayer 2312c. During coating, the second positive electrode active material sublayer 2312c can be formed on the surface of the transition positive electrode active material sublayer 2312b according to the condition of the transition positive electrode active material sublayer 2312b.

[0136] S170 , rolling the second positive electrode active material sublayer 2312 c to obtain the positive electrode sheet 231 .

[0137] It should be noted that the above is only an example of sequentially arranging the first positive electrode active material sublayer 2312a, the transition positive electrode active material sublayer 2312b and the second positive electrode active material sublayer 2312c on the positive electrode current collector 2311. In other embodiments, the first positive electrode active material sublayer 2312a and the second positive electrode active material sublayer 2312c can exchange positions.

[0138] After the positive electrode sheet 231 is prepared, the first isolation film, the positive electrode sheet 231, the second isolation film and the negative electrode sheet are stacked in sequence, wound to form a wound flat structure, and then hot pressed to obtain a wound electrode assembly; or, after the positive electrode sheet 231 is prepared, the positive electrode sheet 231, the isolation film, the negative electrode sheet, the isolation film, and so on, are stacked in sequence to form a stacked electrode assembly.

[0139] The electrode assembly 23 can be used to prepare a battery cell 20 , and the battery cell 20 can be used to prepare a secondary battery 100 and provide electric energy to an electric device.

[0140] Next, one or more embodiments are described in more detail with reference to the following examples. Of course, these examples do not limit the scope of one or more embodiments.

[0141] Examples and Comparative Examples

[0142]

Preparation of positive electrode

[0143] Preparation of the first positive electrode active material sublayer: add the first positive electrode active material, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 96.5:1:2.5 into N-methylpyrrolidone (NMP), stir and mix evenly to obtain a first coating layer slurry; then evenly coat the slurry on the positive electrode collector, and dry to obtain the first positive electrode active material sublayer.

[0144] Preparation of transition positive electrode active material sublayer: Add transition positive electrode active material, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) into N-methylpyrrolidone (NMP) in a mass ratio of 96.5:1:2.5, stir and mix evenly to obtain transition coating layer slurry; then evenly coat the slurry on the first positive electrode active material sublayer, and dry to obtain the transition positive electrode active material sublayer.

[0145] Preparation of the second positive electrode active material sublayer: add the second positive electrode active material, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) in a mass ratio of 96.5:1:2.5 into N-methylpyrrolidone (NMP), stir and mix evenly to obtain a second coating layer slurry; then evenly coat the slurry on the transition positive electrode active material sublayer, the weight ratio of the first positive electrode active material to the second positive electrode active material is 1:1, and after drying, roll pressing and die cutting are performed to obtain a lithium-ion battery positive electrode sheet.

[0146]

Preparation of negative electrode sheet

[0147] The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium hydroxymethyl cellulose (CMC) are dissolved in the solvent deionized water in a mass ratio of 90:5:3:2, and mixed evenly to prepare a negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil once or multiple times, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0148]

Preparation of electrolyte

[0149] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), the organic solvents ethylene carbonate (EC) / diethyl carbonate (DEC) / ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1 / 1 / 1, 1 mol / L LiPF6 lithium salt was added and dispersed evenly, and then 5% mass concentration of fluoroethylene carbonate was dissolved in the above organic solvents and stirred evenly to obtain an electrolyte.

[0150]

Isolation film

[0151] A polyethylene film is used as the isolation film.

[0152]

Preparation of lithium-ion batteries

[0153] The prepared positive electrode sheet, negative electrode sheet and separator (polyethylene (PE) porous polymer film) are made into corresponding battery cells according to the Z-shaped stacking structure. The battery cells are vacuum dried at 90°C for 12 hours, and then the positive and negative pole tabs are ultrasonically welded. The positive pole tabs are aluminum tabs, and the negative pole tabs are nickel tabs. The positive and negative pole tabs are located on the same side of the battery cell. The battery cells after the tabs are welded are placed in aluminum-plastic films of appropriate sizes for top and side sealing. The top and side sealing temperature is 145°C, and then the electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery then undergoes static, hot and cold pressing, formation, shaping, capacity testing and other processes in sequence to obtain a lithium-ion battery product.

[0154] The main parameter control of Examples 1 to 11 and Comparative Examples 1 to 4 is shown in the following table:

[0155]

[0156]

[0157] It should be noted that, in Comparative Examples 1 to 3, no transition positive electrode active material sublayer is provided.

[0158] The first positive electrode active material in Comparative Example 1 is (LiFePO4), whose electronic conductivity is (0.081), and the second positive electrode active material is (LiNi 0.5 Co 0.2 Mn 0.3 ), its electronic conductivity is (0.00009);

[0159] The first positive electrode active material in Comparative Example 2 is (LiFePO4), whose electronic conductivity is (0.081), and the second positive electrode active material is (LiNi 0.5 Co 0.25 Mn 0.25 ), its electronic conductivity is (0.000081);

[0160] The first positive electrode active material in Comparative Example 3 is (LiFePO4), whose electronic conductivity is (0.081), and the second positive electrode active material is (LiNi 0.5 Co 0.2 Mn 0.3 ), its electronic conductivity is (0.0000081);

[0161] In Comparative Example 4, the first positive electrode active material is (LiFePO4), whose electronic conductivity is (0.081), the transition positive electrode active material is (LiMn2O4), whose electronic conductivity is (0.0405), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0162] The first positive electrode active material in Example 1 is (LiFePO4), whose electronic conductivity is (0.081), the transition positive electrode active material is (LiCoO2), whose electronic conductivity is (0.00256), and the second positive electrode active material is (LiNi 0.5 Co 0.2 Mn 0.3 ), its electronic conductivity is (0.00009);

[0163] The first positive electrode active material in Example 2 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.9 Mn 0.1 ), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0164] The first positive electrode active material in Example 3 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.6 Co 0.3 Mg 0.1 ), whose electronic conductivity is (0.001013), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0165] The first positive electrode active material in Example 4 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.6 Co 0.2 Mg 0.2 ), whose electronic conductivity is (0.00162), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0166] The first positive electrode active material in Example 5 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.6 Co 0.3 Al 0.1 ), whose electronic conductivity is (0.002025), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0167] The first positive electrode active material in Example 6 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Mg 0.1 ), whose electronic conductivity is (0.002562), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0168] The first positive electrode active material in Example 7 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.6 Co 0.2 Al 0.2 ), whose electronic conductivity is (0.00324), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0169] The first positive electrode active material in Example 8 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Al 0.1 ), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0170] The first positive electrode active material in Example 9 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Al 0.1), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0171] The first positive electrode active material in Example 10 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Al 0.1 ), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0172] The first positive electrode active material in Example 11 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Al 0.1 ), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), its electronic conductivity is (0.0000081);

[0173] The first positive electrode active material in Example 12 is (LiFePO4), whose electronic conductivity is (0.081), and the transition positive electrode active material is (LiNi 0.8 Co 0.1 Al 0.1 ), whose electronic conductivity is (0.00081), and the second positive electrode active material is (LiNi 0.6 Co 0.2 Mn 0.2 ), and its electronic conductivity is (0.0000081).

[0174] The performance test was performed on the batteries provided in each embodiment and comparative example, and the performance test specifically included:

[0175] Capacity retention rate test: At room temperature 25°C, first charge the lithium-ion secondary battery to 4.35V at a constant current of 1C, then charge it to a current of 0.05C at a constant voltage of 4.35V, and then discharge it to 2V at a constant current of 1C. This is a charge and discharge cycle process, and the discharge capacity this time is the discharge capacity of the first cycle. The battery is tested for multiple cycles of charge and discharge in the above manner, and the discharge capacity of the 1000th cycle is detected, and the capacity retention rate of the battery after the cycle is calculated by dividing the discharge capacity of the 1000th cycle by the discharge capacity of the first cycle.

[0176] The test results are shown in the following table:

[0177] Capacity retention rate after 1000 cycles / % Comparative Example 1 88.23 Comparative Example 2 86.55 Example 1 94.11 Comparative Example 3 82.59 Comparative Example 4 84.23 Example 2 92.15 Example 3 91.56 Example 4 90.98 Example 5 90.43 Example 6 90.11 Example 7 89.35 Example 8 88.55 Example 9 90.73 Example 10 91.67 Embodiment 11 90.37 Example 12 89.11 Example 13 90.34

[0178] It can be seen from the above table that the battery made using the positive electrode sheet provided in the embodiment of the present application has a good capacity retention rate.

[0179] By comparing the data of Examples 1 to 3, it can be seen that as the difference in the electronic conductivity multiples between the two sublayers of the positive electrode active material layer becomes larger and larger, the capacity retention rate of the battery shows a trend of getting worse and worse. When the difference in the electronic conductivity multiples between the two sublayers reaches more than 1000, the capacity retention rate of the battery drops below 85%.

[0180] By comparing the data of Example 1 and Comparative Example 2, it can be obtained that by providing a transition positive electrode active material layer with an electronic conductivity located in the middle between two sublayers with an electronic conductivity multiple difference of 1000, the capacity retention rate of the battery can be significantly improved, and the capacity retention rate is increased to about 94%, an increase of about 9%.

[0181] By comparing the data of Examples 2 to 7 and Comparative Examples 3 to 4, it can be seen that the battery capacity retention rate can be significantly improved by providing a transitional positive electrode active material layer with an electronic conductivity located between two sublayers with a large difference in electronic conductivity multiples. The smaller the ratio of the difference values ​​n of two adjacent electronic conductivity multiples is, the higher the battery cycle capacity retention rate is. In particular, when the ratio of the difference values ​​n of two adjacent electronic conductivity multiples is less than 10, the battery cycle capacity retention rate can reach more than 90%.

[0182] By comparing the data of Example 2 and Examples 8 to 12, it can be obtained that as the thickness ratio of the transition positive electrode active material layer increases, the cycle capacity retention rate of the battery shows a trend of first increasing and then decreasing. When the thickness ratio of the transition positive electrode active material layer is 5% to 20%, the cycle capacity retention rate of the battery is above 90%.

[0183] By comparing the data of Example 2 and Example 13, it can be obtained that when the positive electrode active material with greater electronic conductivity is closer to the positive electrode current collector, the battery has better cycle capacity retention rate and energy density. The reason may be that the positive electrode active material with greater electronic conductivity is arranged at a position closer to the positive electrode current collector, which can reduce the probability of polarization of the positive electrode sheet, and thus is beneficial to the cycle capacity retention rate and energy density of the battery.

[0184] The above are only specific embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes at least three sub-layers, the electronic conductivity multiple difference value m of the positive electrode active materials of the two sub-layers farthest apart in the positive electrode active material layer is not less than 1000, and the electronic conductivity multiple difference value n of the positive electrode active materials of the two adjacent sub-layers is less than 1000.

2. The positive electrode sheet according to claim 1, characterized in that: The ratio of the electronic conductivity multiple difference value n between any one of the middle sub-layers in the positive electrode active material layer and its two adjacent sub-layers is no more than 10.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The total thickness of the two sub-layers which are farthest apart from each other in the positive electrode active material layer accounts for at least 80% of the positive electrode active material layer.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The positive electrode sheet further includes a positive electrode current collector. The positive electrode active material layer is disposed on at least one side surface of the positive electrode current collector. The closer the sublayer is to the positive electrode current collector, the greater the electronic conductivity of the positive electrode active material is.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: The positive electrode active material layer includes a first positive electrode active material sublayer, a second positive electrode active material sublayer and a transition positive electrode active material sublayer arranged between the first positive electrode active material sublayer and the second positive electrode active material sublayer, the first positive electrode active material sublayer includes a first positive electrode active material, the second positive electrode active material sublayer includes a second positive electrode active material, the transition positive electrode active material sublayer includes a transition positive electrode active material, and the relationship between the electronic conductivity a of the first positive electrode active material, the electronic conductivity b of the transition positive electrode active material and the electronic conductivity c of the second positive electrode active material satisfies: 30<a:b<1000, 30<b:c<1000.

6. The positive electrode sheet according to claim 5, characterized in that: The electronic conductivity a of the first positive electrode active material is 10 -2 ~10 -1 S / cm; and / or The electronic conductivity b of the transition positive electrode active material is 10 -4 ~10 -2 S / cm; and / or The electronic conductivity c of the second positive electrode active material is 10 -6 ~10 -4 S / cm.

7. The positive electrode sheet according to any one of claims 5 to 6, characterized in that: The first positive electrode active material comprises: at least one of a spinel structure positive electrode material, a layered structure positive electrode material and an olivine structure phosphate positive electrode material; and / or The second positive electrode active material includes at least one of a spinel structure positive electrode material, a layered structure positive electrode material and an olivine structure phosphate positive electrode material.

8. The positive electrode sheet according to any one of claims 5 to 7, characterized in that: The first positive electrode active material includes Li a Ni b Mn c M1 2-b-c At least one of O4 and LiM2PO4, wherein 0.9≤a≤1.1, 0≤b≤2, 0≤c≤2, b+c=2, M1 includes at least one of Mg, Zn, Ti, Zr, W, Nb, Al, B, P, Mo, V, and Cr, M2 includes Mn and non-Mn elements, and the non-Mn elements include one or both of a first doping element and a second doping element, the first doping element is manganese-doped, and the second doping element is phosphorus-doped; and / or The second positive electrode active material includes Li d Ni x Co y M3 1-x-y O 2+e and at least one of zLi2MnO3·(1-z)LiM4O2, wherein 0.2≤d<1.2, 0≤x≤1.0, 0≤y≤1.0, x+y≤1, -0.02≤e<0.02, M3 includes at least one of Mn, Zr, Al, B, Ta, Mo, W, Nb, Sb, La, and M4 includes at least one of Co, Ni, and Mn.

9. A positive electrode sheet, characterized in that: The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a first positive electrode active material sublayer, a second positive electrode active material sublayer and a transition positive electrode active material sublayer arranged between the first positive electrode active material sublayer and the second positive electrode active material sublayer, the thickness of the transition positive electrode active material sublayer accounts for less than 20% of the positive electrode active material layer; the first positive electrode active material sublayer includes a first positive electrode active material, the second positive electrode active material sublayer includes a second positive electrode active material, the transition positive electrode active material sublayer includes a transition positive electrode active material, and the relationship between the electronic conductivity a of the first positive electrode active material, the electronic conductivity b of the second positive electrode active material and the electronic conductivity c of the transition positive electrode active material satisfies: a:c≥1000, 30<a:b<1000, 30<b:c<1000.

10. A battery cell, characterized in that: The battery cell comprises the positive electrode sheet according to any one of claims 1 to 9.

11. A battery, characterized in that: The battery comprises the battery cell according to claim 10 .

12. An electrical device, characterized in that: The electrical device comprises the battery cell according to claim 10 or the battery according to claim 11.

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