Secondary battery and electrical device

Abstract

Disclosed in the present disclosure are a secondary battery and an electrical device. The secondary battery comprises a positive electrode sheet, the positive electrode sheet comprising a positive electrode active layer, and the positive electrode active layer comprising a first positive electrode active part, an intermediate positive electrode active part and a second positive electrode active part which are successively arranged. During a charging and discharging cycle process after an initial cycle of the positive electrode sheet, the areal capacity of the intermediate positive electrode active part is smaller than the areal capacity of the first positive electrode active part and / or smaller than the areal capacity of the second positive electrode active part. The present disclosure enables the areal capacity of the intermediate part to be lower than the areal capacity of an edge part, and the swelling ratio of the intermediate part to be lower than the swelling ratio of the edge part during the charging and discharging cycle process after the initial cycle of the positive electrode sheet, so as to reduce and / or alleviate lithium plating of a corresponding negative electrode sheet, thus reducing the risk of capacity plunge caused by lithium plating in late-stage cycling of secondary batteries, and improving safety performance of secondary batteries.

Description

Secondary batteries and electrical equipment

[0001] This disclosure claims priority to Chinese patent application No. 2023116330250 filed on November 29, 2023, entitled “A positive electrode sheet, electrode assembly, battery and electrical equipment”, which is incorporated herein by reference in its entirety. Technical Field

[0002] The present disclosure belongs to the technical field of batteries, and particularly relates to a secondary battery and an electrical device. Background Art

[0003] As a new generation of green energy storage and conversion devices, secondary batteries are widely used in portable electronic devices, electric vehicles, aerospace, and other fields. With the development and advancement of secondary battery technology, improving the safety performance of secondary batteries has become a key research topic in the industry.

[0004] The above statements are merely intended to provide background information related to the present disclosure and do not necessarily constitute prior art.

[0005] Summary of the Invention

[0006] In view of the technical problems existing in the background technology, the present disclosure provides a secondary battery and an electrical device, aiming to improve the safety performance of the secondary battery.

[0007] In a first aspect, an embodiment of the present disclosure provides a secondary battery, comprising a positive electrode sheet; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector, the positive electrode active layer comprising a positive electrode active material; along the TD direction of the positive electrode current collector, the positive electrode active layer comprises a first positive electrode active portion, an intermediate positive electrode active portion, and a second positive electrode active portion arranged in sequence;

[0008] In the positive electrode sheet, during the charge and discharge cycle after the first time, the surface capacity of the middle positive electrode active portion is smaller than the surface capacity of the first positive electrode active portion and / or smaller than the surface capacity of the second positive electrode active portion.

[0009] In the embodiments of the present disclosure, by ensuring that the surface capacity of the middle portion of the positive electrode plate is lower than the surface capacity of the edge portion during the first charge and discharge cycle, and the expansion degree of the middle portion is lower than the expansion degree of the edge portion, the lithium plating of the corresponding negative electrode plate is reduced and / or alleviated, thereby reducing the risk of secondary battery diving due to lithium plating in the later stage of the cycle and improving the safety performance of the secondary battery.

[0010] In some embodiments, the positive electrode active layer also includes a positive electrode lithium replenishing material. In the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the middle positive electrode active portion is different from the content of the positive electrode lithium replenishing material in the first positive electrode active portion.

[0011] In the embodiments disclosed herein, by regulating the content of the positive electrode lithium-replenishing material in the middle and edge portions of the positive electrode active layer, the areal capacity of the middle portion of the positive electrode plate is lower than that of the edge portion during the first charge-discharge cycle after the first charge-discharge cycle, thereby reducing and / or alleviating lithium plating in the corresponding negative electrode plate, reducing the risk of a battery charge drop due to lithium plating in the later stages of the secondary battery cycle, and improving the safety performance of the secondary battery. Furthermore, after the positive electrode lithium-replenishing material is consumed during the cycle, corresponding pores are generated in the positive electrode active layer, which facilitates electrolyte reflux during the cycle and reduces polarization of the secondary battery.

[0012] In some embodiments, the first positive electrode active portion, the middle positive electrode active portion, and the second positive electrode active portion are all single-layer structures mixed with a positive electrode active material and a positive electrode lithium supplementing material.

[0013] In the embodiment of the present disclosure, the above three parts of the positive electrode active layer are supplemented with lithium content by adding positive electrode lithium supplement materials, thereby improving the cycle performance of the secondary battery while reducing and / or alleviating lithium plating of the corresponding negative electrode sheet.

[0014] In some embodiments, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the intermediate positive electrode active part is greater than the content of the positive electrode lithium replenishing material in the first positive electrode active part and / or greater than the content of the positive electrode lithium replenishing material in the second positive electrode active part; in the initial state and during the first charge and discharge cycle of the positive electrode sheet, the surface capacity of the intermediate positive electrode active part is greater than the surface capacity of the first positive electrode active part and / or greater than the surface capacity of the second positive electrode active part.

[0015] In the embodiments of the present disclosure, by regulating the content of the positive electrode lithium-replenishing material in the middle part of the positive electrode active layer to be greater than the content of the positive electrode lithium-replenishing material in the edge part, the surface capacity of the middle part is higher than the surface capacity of the edge part in the initial state of the positive electrode plate and during the first charge and discharge cycle, thereby improving the energy density. In the charge and discharge cycle after the first time, due to the continuous consumption of the positive electrode lithium-replenishing material, the surface capacity of the part with a higher content of the positive electrode lithium-replenishing material in the initial state and during the first charge and discharge cycle is reduced instead, thereby achieving the surface capacity of the middle part being lower than the surface capacity of the edge part, thereby reducing and / or alleviating the lithium plating of the corresponding negative electrode plate.

[0016] In some embodiments, the first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion all have a double-layer structure; the first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion all include a bottom positive electrode active layer close to the positive electrode current collector and a top positive electrode active layer away from the positive electrode current collector; in the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of positive electrode active material in the bottom positive electrode active layer and the top positive electrode active layer is different.

[0017] In the embodiment of the present disclosure, by setting up a double-layer structure, only one layer of the structure can be changed, so that the content of the positive electrode active material in the single-layer structure is reduced, and the other layer of the structure can remain unchanged. This is beneficial for the positive electrode sheet as a whole to have better capacity, energy density and cycle performance while reducing the expansion degree of the positive electrode sheet and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0018] In some embodiments, the bottom positive electrode active layer and the top positive electrode active layer are both single-layer structures mixed with positive electrode active materials and positive electrode lithium replenishing materials; in the initial state of the positive electrode sheet and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer is greater than the content of the positive electrode lithium replenishing material in the top positive electrode active layer.

[0019] In the embodiments of the present disclosure, by setting up a double-layer structure, the content of positive electrode active materials and positive electrode lithium supplement materials in each layer and each region can be designed in layers according to actual conditions, thereby improving the flexibility of the design; it can also be coated in layers, and each single-layer structure can be formed as an integral whole, which is conducive to reducing production difficulty and improving production efficiency.

[0020] In some embodiments, the thickness of the underlying positive electrode active layer of the intermediate positive electrode active portion is greater than the thickness of the underlying positive electrode active layer of the first positive electrode active portion and the thickness of the underlying positive electrode active layer of the second positive electrode active portion.

[0021] In the embodiment of the present disclosure, the thickness of the underlying structure of the middle part is increased. Since the proportion of the positive electrode lithium replenishing material in the underlying structure is high, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, so that the surface capacity of the middle part during the first charge and discharge cycle is greater than the surface capacity of the first part and the second part. During the second charge and discharge cycle and multiple charge and discharge cycles thereafter, due to the consumption of the positive electrode lithium replenishing material, the capacity provided by the positive electrode lithium replenishing material decreases, so that the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet and reducing and / or slowing down the occurrence of lithium plating of the corresponding negative electrode sheet.

[0022] In some embodiments, in the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer of the intermediate positive electrode active portion is greater than the content of the positive electrode lithium replenishing material in the first positive electrode active portion and the content of the positive electrode lithium replenishing material in the second positive electrode active portion, respectively.

[0023] In the embodiments of the present disclosure, the surface capacity of the middle part during the first charge and discharge cycle is improved by increasing the content of the positive electrode lithium-supplementing material in the bottom structure of the middle part, and during the second charge and discharge cycle and multiple charge and discharge cycles thereafter, the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0024] In some embodiments, the bottom positive electrode active layer and the top positive electrode active layer are both single-layer structures mixed with positive electrode active materials and positive electrode lithium replenishing materials; in the initial state of the positive electrode sheet and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer is less than the content of the positive electrode lithium replenishing material in the top positive electrode active layer.

[0025] In the embodiments of the present disclosure, by setting up a double-layer structure, the content of positive electrode active materials and positive electrode lithium supplement materials in each layer and each region can be designed in layers according to actual conditions, thereby improving the flexibility of the design; it can also be coated in layers, and each single-layer structure can be formed as an integral whole, which is conducive to reducing production difficulty and improving production efficiency.

[0026] In some embodiments, the thickness of the underlying positive electrode active layer of the middle positive electrode active portion is smaller than the thickness of the underlying positive electrode active layer of the first positive electrode active portion and the thickness of the underlying positive electrode active layer of the second positive electrode active portion.

[0027] In the embodiment of the present disclosure, the thickness of the bottom structure of the middle part is reduced, so that the thickness of the top structure of the middle part is increased. Since the proportion of the positive electrode lithium replenishing material in the top structure is high, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, so that the surface capacity of the middle part during the first charge and discharge cycle is greater than the surface capacity of the first part and the second part. During the second charge and discharge cycle and multiple charge and discharge cycles thereafter, due to the consumption of the positive electrode lithium replenishing material, the capacity provided by the positive electrode lithium replenishing material decreases, and the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0028] In some embodiments, in the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the intermediate positive electrode active portion is greater than the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the first positive electrode active portion and the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the second positive electrode active portion.

[0029] In the embodiments of the present disclosure, by increasing the content of the positive electrode lithium-replenishing material in the top structure of the middle part, the surface capacity of the middle part during the first charge and discharge cycle is improved, and during the second charge and discharge cycle and multiple charge and discharge cycles thereafter, the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0030] In some embodiments, the first positive electrode active portion, the middle positive electrode active portion, and the second positive electrode active portion are flush with a surface away from the positive electrode current collector.

[0031] In the embodiments of the present disclosure, while reducing the surface capacity of the middle part of the positive electrode active layer, the three parts of the positive electrode active layer are aligned away from the surface of the positive electrode current collector, so that the lithium ion transmission paths of each part are close, further reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0032] In some embodiments, along the TD direction of the positive electrode current collector, the ratio of the width of the middle positive electrode active portion to the width of the positive electrode active layer is 0.02 to 0.20.

[0033] In the embodiments of the present disclosure, by regulating the width ratio of the middle part of the positive electrode active layer, the secondary battery has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode plate and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0034] In some embodiments, along the TD direction of the positive electrode current collector, the ratio of the width of the middle positive electrode active portion to the width of the positive electrode active layer is 0.05 to 0.1.

[0035] In the embodiments of the present disclosure, by further regulating the width ratio of the middle part of the positive electrode active layer, the secondary battery has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode sheet and reducing and / or alleviating lithium plating of the corresponding negative electrode sheet.

[0036] In some embodiments, the positive electrode current collector includes a main body portion and a pole ear portion. Along the TD direction of the positive electrode current collector, the main body portion includes a first positive electrode active portion, an intermediate positive electrode active portion and a second positive electrode active portion; the first positive electrode active portion is arranged on the side of the intermediate positive electrode active portion close to the pole ear portion, and the second positive electrode active portion is arranged on the side of the intermediate positive electrode active portion away from the pole ear portion; wherein the ratio of the width of the second positive electrode active portion to the width of the positive electrode active layer is 0.3 to 0.7.

[0037] In the embodiments of the present disclosure, by regulating the width ratio of each part of the positive electrode active layer, the secondary battery has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode plate and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0038] In some embodiments, the ratio of the width of the second positive electrode active portion to the width of the positive electrode active layer is 0.4 to 0.6.

[0039] In the embodiments of the present disclosure, by further regulating the width ratio of each part of the positive electrode active layer, the secondary battery has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode plate and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0040] In some embodiments, the gram capacity of the positive electrode lithium supplementing material is greater than the gram capacity of the positive electrode active material.

[0041] In the embodiments of the present disclosure, the gram capacity of the positive electrode lithium supplement material is greater than the gram capacity of the positive electrode active material, which is beneficial to improving the capacity and energy density of the positive electrode sheet and the cycle performance of the secondary battery.

[0042] In some embodiments, the positive electrode active material includes one or more of lithium iron phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium nickel oxide, and lithium iron manganese oxide.

[0043] In the embodiments of the present disclosure, by using one or more of the above materials as the active material of the positive electrode active layer, it is beneficial to make the positive electrode plate have good capacity and energy density.

[0044] In some embodiments, the positive electrode lithium supplement material includes: Li2C x O y 、Li2MO2、Li2MO3、Li5Fe x M 1-x O4 and Li6Mn y M 1-yOne or more of O4; wherein, M includes at least one of Ni, Mn, Cu, Fe, Cr, Mo, Zr, Si and Ru, x=0.5~1, y=0.5~1.

[0045] In the embodiments of the present disclosure, by using one or more of the above materials as the lithium supplement material of the positive electrode active layer, it is beneficial to make the positive electrode plate have good capacity and energy density.

[0046] In some embodiments, the positive electrode active layer includes only positive electrode active material, and based on the total mass of the positive electrode active layer, the lithium content of the positive electrode active material in the intermediate positive electrode active portion is less than the lithium content of the positive electrode active material in the first positive electrode active portion and / or less than the lithium content of the positive electrode active material in the second positive electrode active portion.

[0047] In the embodiments of the present disclosure, the lithium content of the positive electrode active material in the intermediate positive electrode active portion is less than the lithium content of the positive electrode active material in the first positive electrode active portion and / or less than the lithium content of the positive electrode active material in the second positive electrode active portion, so that the surface capacity of the intermediate positive electrode active portion is less than the surface capacity of the first positive electrode active portion and / or less than the surface capacity of the second positive electrode active portion, so that the surface capacity of the middle part of the positive electrode plate is lower than the surface capacity of the edge part, and the degree of expansion of the middle part is lower than the degree of expansion of the edge part, thereby reducing and / or alleviating lithium plating of the corresponding negative electrode plate, reducing the risk of water diving caused by lithium plating in the late stage of the secondary battery cycle, and improving the safety performance of the secondary battery.

[0048] In some embodiments, the positive active layer includes a mixture of at least two positive active materials.

[0049] In the embodiments of the present disclosure, a positive electrode active layer is formed by a mixture including at least two positive electrode active materials. By adjusting the mass ratio of each positive electrode active material in the mixture in each region, the surface capacity of the middle positive electrode active portion can be made less than / or less than the surface capacity of the first positive electrode active portion and the surface capacity of the second positive electrode active portion. The implementation method is simple and easy to operate, and has a good effect of alleviating lithium plating of the corresponding negative electrode sheet and improving the safety performance of the secondary battery.

[0050] In some embodiments, one of the at least two positive electrode active materials is a layered lithium-containing transition metal oxide; optionally, the positive electrode active material further includes one or more of lithium iron phosphate, lithium manganese oxide, lithium nickel manganese oxide, and lithium manganese iron phosphate.

[0051] In the embodiments of the present disclosure, one of the mixtures uses a layered lithium-containing transition metal oxide, which can reduce the content of the layered lithium-containing transition metal oxide in the intermediate positive electrode active portion so that the content of the layered lithium-containing transition metal oxide in the intermediate positive electrode active portion is less than the content of other positive electrode active materials, thereby reducing the surface capacity of the positive electrode active material in the intermediate positive electrode active portion, thereby alleviating lithium plating of the corresponding negative electrode sheet and improving the safety performance of the secondary battery.

[0052] In some embodiments, the positive electrode active material of the positive electrode active layer is the same positive electrode active material; based on the total mass of the positive electrode active layer, the content of the positive electrode active material in the intermediate positive electrode active portion is less than / or less than the content of the positive electrode active material in the first positive electrode active portion and the content of the positive electrode active material in the second positive electrode active portion.

[0053] In the embodiments of the present disclosure, by adopting the positive electrode active layer of the same positive electrode active material, the content of the positive electrode active material in the middle positive electrode active part is less than the content of the positive electrode active material in the first positive electrode active part and / or less than the content of the positive electrode active material in the second positive electrode active part, so that the expansion degree of the middle part is lower than the expansion degree of the edge part, reducing and / or alleviating the lithium plating of the corresponding negative electrode plate, reducing the risk of diving caused by lithium plating in the late stage of the secondary battery cycle, and improving the safety performance of the secondary battery.

[0054] In a second aspect, embodiments of the present disclosure provide an electrical device comprising the secondary battery provided in the first aspect, wherein the secondary battery is configured to provide electrical energy. The electrical device provided in embodiments of the present disclosure, because it comprises any of the secondary batteries provided in the first aspect, has at least the same advantages as the secondary battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] In order to more clearly illustrate the technical solutions in the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present disclosure, and other drawings can be obtained based on these drawings without creative work.

[0056] FIG1 is a schematic structural diagram of a vehicle provided in an embodiment of the present disclosure;

[0057] FIG2 is a schematic diagram of an exploded structure of a secondary battery provided by an embodiment of the present disclosure;

[0058] FIG3 is a schematic diagram of the exploded structure of a battery cell provided by an embodiment of the present disclosure;

[0059] FIG4 is a schematic structural diagram of a positive electrode sheet provided in some embodiments of the present disclosure;

[0060] FIG5 is a schematic diagram of a first structure of the positive electrode sheet shown in FIG4 along the thickness direction;

[0061] FIG6 is a schematic diagram of a second structure of the positive electrode sheet shown in FIG4 along the thickness direction;

[0062] FIG7 is a schematic diagram of a third structure of the positive electrode sheet shown in FIG4 along the thickness direction;

[0063] FIG8 is a schematic diagram of a fourth structure of the positive electrode sheet shown in FIG4 along the thickness direction;

[0064] FIG9 is a schematic diagram of a fifth structure of the positive electrode sheet shown in FIG4 along the thickness direction;

[0065] FIG10 is a sixth structural schematic diagram of the positive electrode sheet shown in FIG4 along the thickness direction.

[0066] Explanation of the accompanying drawings: 1000-vehicle, 100-secondary battery, 200-controller, 300-motor, 10-casing, 20-battery cell, 11-first part, 12-second part, 21-end cover, 22-shell, 23-electrode assembly, 21a-electrode terminal, 23a-ear, 30-positive electrode sheet, 31-positive electrode current collector, 32-positive electrode active layer, 311-main body, 312-ear, 321-first positive electrode active part, 323-intermediate positive electrode active part, 322-second positive electrode active part, 32a-bottom positive electrode active layer, 32b-top positive electrode active layer, W-second width, W1-first width, W2-third width. DETAILED DESCRIPTION

[0067] The following will clearly and completely describe the technical solutions in the embodiments of the present disclosure in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present disclosure, not all of the embodiments. All other embodiments obtained based on the embodiments of the present disclosure without creative work are within the scope of protection of the present disclosure.

[0068] The terms "first", "second" and "third" in this disclosure are only used for descriptive purposes and should not be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first", "second" and "third" may explicitly or implicitly include at least one such feature. In the description of this disclosure, "multiple" means at least two, for example, two, three, etc., unless otherwise clearly and specifically defined. All directional indications in the embodiments of this disclosure (such as up, down, left, right, front, back...) are only used to explain the relative positional relationship, movement, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication also changes accordingly. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but optionally also includes steps or units that are not listed, or optionally also includes other steps or units inherent to these processes, methods, products or devices.

[0069] References herein to an "embodiment" mean that a particular feature, structure, or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor do they constitute separate or alternative embodiments that are mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, that the embodiments described herein may be combined with other embodiments.

[0070] Currently, most positive electrode plates are coated in an integrated manner, with the same material applied to all locations on the plate. During the cycle of a secondary battery, the central portion of the positive electrode plate is subject to significant expansion forces, resulting in delayed electrolyte reflux. This can easily lead to lithium deposition in the corresponding negative electrode plate, causing a potential battery surge and even safety risks, worsening cycle life. To increase the capacity of secondary batteries, some solutions also incorporate a lithium supplement into the positive electrode active layer. However, due to the high gram capacity of the lithium supplement, which can deviate more than conventional positive electrode active materials, this can easily lead to unstable cell balance (CB) in the central portion of the plate, which refers to the excess of the negative electrode capacity over the positive electrode capacity in the same stage and under the same conditions, further exacerbating lithium deposition in the negative electrode plate.

[0071] In the embodiments of the present disclosure, by ensuring that the surface capacity of the middle portion of the positive electrode plate is lower than the surface capacity of the edge portion during the first charge and discharge cycle, and the expansion degree of the middle portion is lower than the expansion degree of the edge portion, the lithium plating of the corresponding negative electrode plate is reduced and / or alleviated, thereby reducing the risk of secondary battery diving due to lithium plating in the later stage of the cycle and improving the safety performance of the secondary battery.

[0072] The present disclosure is described in detail below with reference to the accompanying drawings and embodiments.

[0073] The secondary battery disclosed in the embodiments of the present disclosure can be used in electrical devices that use the secondary battery as a power source or various energy storage systems that use the secondary battery as an energy storage element. The electrical device can be, but is not limited to, a mobile phone, a tablet, a laptop computer, an electric toy, an electric tool, a battery car, an electric car, a ship, a spacecraft, and the like. Among them, the electric toy can include fixed or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, and the like, and the spacecraft can include airplanes, rockets, space shuttles, and spacecraft, and the like.

[0074] 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 disclosure.

[0075] Please refer to Figure 1, which is a schematic structural diagram of a vehicle provided in an embodiment of the present disclosure. The vehicle 1000 can be a fuel vehicle, a gas vehicle or a new energy vehicle. The new energy vehicle can 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 can be provided at the bottom, head or tail of the vehicle 1000. The secondary battery 100 can be used to power the vehicle 1000. For example, the secondary battery 100 can serve as an operating power source for the vehicle 1000. The vehicle 1000 can also include a controller 200 and a motor 300. The controller 200 is used to control the secondary battery 100 to power the motor 300, for example, for starting, navigating and driving the vehicle 1000.

[0076] In some embodiments of the present disclosure, the secondary battery 100 can serve 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 .

[0077] Please refer to Figure 2, which is a schematic diagram of the exploded structure of a secondary battery provided in an embodiment of the present disclosure. The secondary battery 100 includes a housing 10 and a battery cell 20, with the battery cell 20 housed within the housing 10. The housing 10 is used to provide a storage space for the battery cell 20 and can have various structures. In some embodiments, the housing 10 can include a first portion 11 and a second portion 12, which overlap each other and together define a storage space for the battery cell 20. The second portion 12 can be a hollow structure with one end open. The first portion 11 can be a plate-like structure, with the first portion 11 overlapping the open side of the second portion 12, so that the first portion 11 and the second portion 12 together define a storage space. Alternatively, the first portion 11 and the second portion 12 can each be a hollow structure with one end open, with the open side of the first portion 11 overlapping the open side of the second portion 12. Of course, the housing 10 formed by the first portion 11 and the second portion 12 can have various shapes, such as a cylinder, a rectangular parallelepiped, etc.

[0078] In the secondary battery 100, there may be multiple battery cells 20, which may be connected in series, in parallel, or in a hybrid configuration. A hybrid configuration refers to a combination of series and parallel configurations within the battery cells 20. Multiple battery cells 20 may be directly connected in series, in parallel, or in a hybrid configuration, and then the entire battery cell 20 may be housed within the housing 10. Alternatively, the secondary battery 100 may be constructed by first connecting multiple battery cells 20 in series, in parallel, or in a hybrid configuration to form a battery module, which is then connected in series, in parallel, or in a hybrid configuration to form a single unit housed within the housing 10. The secondary battery 100 may also include other structures, such as a busbar assembly for electrically connecting the multiple battery cells 20.

[0079] Each battery cell 20 may be a lithium-sulfur secondary battery, a sodium-ion secondary battery, or a magnesium-ion secondary battery, but is not limited thereto. The battery cell 20 may be cylindrical, flat, rectangular, or in other shapes.

[0080] Please refer to Figure 3, which is a schematic diagram of the exploded structure of a battery cell provided by an embodiment of the present disclosure. A battery cell 20 is the smallest unit that makes up a battery. As shown in Figure 3, a battery cell 20 includes an end cap 21, a housing 22, an electrode assembly 23, and other functional components.

[0081] The end cap 21 is a component that covers the opening of the housing 22 to isolate the internal environment of the battery cell 20 from the external environment. The shape of the end cap 21 can be adapted to the shape of the housing 22 to fit the housing 22. Optionally, the end cap 21 can be made of a material with a certain hardness and strength (such as an aluminum alloy). This prevents the end cap 21 from deforming when subjected to compression or collision, thereby providing the battery cell 20 with greater structural strength and improved safety. Functional components such as electrode terminals 21a can be provided on the end cap 21. The electrode terminals 21a can be used to electrically connect to the electrode assembly 23 to output or input electrical energy to the battery cell 20. In some embodiments, the end cap 21 can also be provided with a pressure relief mechanism for relieving internal pressure when the internal pressure or temperature of the battery cell 20 reaches a threshold. The end cap 21 can also be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the present disclosure does not impose any particular limitations on this. In some embodiments, an insulating member may be provided inside the end cap 21 to isolate the electrical connection components in the housing 22 from the end cap 21 to reduce the risk of short circuit. For example, the insulating member may be made of plastic, rubber, or the like.

[0082] The housing 22 is a component that cooperates with the end cap 21 to form the internal environment of the battery cell 20. This internal environment can be used to accommodate the electrode assembly 23, electrolyte, and other components. The housing 22 and end cap 21 can be separate components. An opening can be provided in the housing 22, and the end cap 21 is placed over the opening to form the internal environment of the battery cell 20. Alternatively, the end cap 21 and housing 22 can be integrated. Specifically, the end cap 21 and housing 22 can form a common connection surface before other components are inserted into the housing. When the interior of the housing 22 needs to be enclosed, the end cap 21 is placed over the housing 22. The housing 22 can have a variety of shapes and sizes, such as a rectangular parallelepiped, a cylindrical shape, or a hexagonal prism. Specifically, the shape of the housing 22 can be determined based on the specific shape and size of the electrode assembly 23. The housing 22 can be made of a variety of materials, such as copper, iron, aluminum, stainless steel, aluminum alloy, plastic, etc., and the disclosed embodiments are not particularly limited in this regard.

[0083] The electrode assembly 23 is a component in the secondary battery 100 where electrochemical reactions occur. One or more electrode assemblies 23 may be contained in the housing 22. The electrode assembly 23 is mainly formed by winding or stacking the positive electrode sheet and the negative electrode sheet, and a separator is usually provided between the positive electrode sheet and the negative electrode sheet. The parts of the positive electrode sheet and the negative electrode sheet with active materials constitute the main body of the battery cell assembly, and the parts of the positive electrode sheet and the negative electrode sheet without active materials each constitute the tab. The positive electrode tab and the negative electrode tab may be located together at one end of the main body or respectively at both ends of the main body. During the charge and discharge process of the secondary battery 100, the positive electrode active material and the negative electrode active material react with the electrolyte, and the tabs connect the electrode terminals to form a current loop.

[0084] Please refer to Figures 4 to 6. Figure 4 is a structural schematic diagram of the positive electrode sheet provided in some embodiments of the present disclosure. Figure 5 is a first structural schematic diagram of the positive electrode sheet shown in Figure 4 along the thickness direction. Figure 6 is a second structural schematic diagram of the positive electrode sheet shown in Figure 4 along the thickness direction.

[0085] 4 to 6 , an embodiment of the present disclosure provides a secondary battery 100 including a positive electrode sheet 30. The positive electrode sheet 30 includes a positive electrode current collector 31 and a positive electrode active layer 32 disposed on at least one side of the positive electrode current collector 31. The positive electrode active layer 32 includes a positive electrode active material. Along the TD direction of the positive electrode current collector 31, the positive electrode active layer 32 includes a first positive electrode active portion 321, an intermediate positive electrode active portion 323, and a second positive electrode active portion 322 arranged in sequence. During the charge and discharge cycle of the positive electrode sheet 30 after the first charge and discharge cycle, the surface capacity of the intermediate positive electrode active portion 323 is less than the surface capacity of the first positive electrode active portion 321 and / or less than the surface capacity of the second positive electrode active portion 322.

[0086] The positive electrode sheet 30, a crucial component of the secondary battery 100, contains active materials that undergo a reduction reaction during discharge. The positive current collector 31 represents the metal substrate within the positive electrode sheet 30 to which the active materials are attached. In lithium-ion secondary batteries, for example, the positive current collector 31 can be made of aluminum. The positive active layer 32 represents a layered structure primarily composed of positive active materials. In some embodiments, the positive current collector 31 has a first surface and a second surface disposed opposite each other along its thickness. In some embodiments, the positive electrode sheet 30 may have the positive active layer 32 disposed only on one side of the first or second surface of the positive current collector 31, or may have a layer of positive active layer 32 disposed on both the first and second surfaces of the positive current collector 31. In some embodiments, the positive active layer 32 may be disposed directly on the surface of the positive current collector 31. In other embodiments, other functional layers may be disposed between the positive active layer 32 and the positive current collector 31. The positive active material represents an active material that undergoes a reduction reaction during discharge. The positive electrode active material can be any positive electrode active material known in the art for the secondary battery 100. In some embodiments, in addition to the positive electrode active material, the positive electrode active layer 32 may also contain other additives, such as a binder, a conductive agent, etc., which are specifically configured as needed. The TD direction (Transverse Direction) represents the width direction of the positive electrode current collector 31. The surface capacity represents the discharge capacity of the active material layer per unit area, which is equal to the discharge capacity of the positive electrode active material layer divided by the area of ​​the positive electrode active material layer. In some embodiments, the numerical unit of the surface capacity is mAh / cm 2In some embodiments, the lithium content can be obtained by ICP (Inductive Coupled Plasma Emission Spectrometer) testing. In some embodiments, the surface of the positive electrode current collector 31 is flat, and the surfaces of the first positive electrode active portion 321, the intermediate positive electrode active portion 323, and the second positive electrode active portion 322 away from the positive electrode current collector 31 are also flat, which helps to simplify the process and reduce the risk of performance deterioration of the secondary battery 100. It is understood that the surface of the positive electrode current collector 31 and at least one of the surfaces of the first positive electrode active portion 321, the intermediate positive electrode active portion 323, and the second positive electrode active portion 322 away from the positive electrode current collector 31 can be undulating, depending on the actual specific configuration and is not limited by this disclosure. In some embodiments, as shown in Figure 5, by making the surface of the intermediate positive electrode active portion 323 away from the positive electrode current collector 31 concave relative to the surface of the first positive electrode active portion 321 away from the positive electrode current collector 31 and the surface of the second positive electrode active portion 322 away from the positive electrode current collector 31, the surface capacity of the intermediate positive electrode active portion 323 can be reduced. In some embodiments, the proportion of other additives in the intermediate positive electrode active portion 323 can be increased so that the first positive electrode active portion 321, the intermediate positive electrode active portion 323, and the second positive electrode active portion 322 are aligned away from the surface of the positive electrode current collector 31 (as shown in FIG6 ). This helps reduce the active ion transmission path and reduces lithium plating of the corresponding negative electrode. In some embodiments, the additive can be a positive electrode lithium supplement material, or a binder, a conductive agent, etc., as required. The lithium element in the positive electrode sheet participates in the formation of a solid interface film on the negative electrode sheet during the first charge and discharge cycle, resulting in irreversible lithium consumption.

[0087] In the embodiments of the present disclosure, by ensuring that the surface capacity of the middle portion of the positive electrode plate 30 is lower than the surface capacity of the edge portion during the first charge and discharge cycle after the first time, and the expansion degree of the middle portion is lower than the expansion degree of the edge portion, lithium plating of the corresponding negative electrode plate is reduced and / or alleviated, thereby reducing the risk of water diving caused by lithium plating in the later stage of the charge and discharge cycle of the secondary battery 100, and improving the safety performance of the secondary battery 100.

[0088] In some embodiments, the positive electrode active layer 32 also includes a positive electrode lithium replenishing material. In the initial state and during the first charge and discharge cycle of the positive electrode plate 30, based on the total mass of the positive electrode active layer 32, the content of the positive electrode lithium replenishing material in the middle positive electrode active portion 323 is different from the content of the positive electrode lithium replenishing material in the first positive electrode active portion 321.

[0089] Among them, the positive electrode lithium supplement material refers to a lithium-containing high-gram capacity material added to the positive electrode active layer 32. In some embodiments, the positive electrode lithium supplement material can be mixed with the positive electrode active material, wherein the positive electrode lithium supplement material can be uniformly mixed with the positive electrode active material. In some embodiments, the positive electrode lithium supplement material can be arranged in layers with the positive electrode active material. In some embodiments, the positive electrode lithium supplement material can be arranged in a layer close to the positive electrode current collector 31, and the positive electrode active material is arranged in a layer away from the positive electrode current collector 31. This is beneficial to shorten the lithium ion transmission path and improve the performance of the secondary battery 100. Because the positive electrode lithium supplement material has a high gram capacity, it participates in the formation of a solid interface film of the negative electrode plate during the first charge and discharge cycle of the secondary battery 100, reducing the loss of the positive electrode active material and facilitating the improvement of the electrode plate capacity. The positive electrode lithium supplement material is continuously consumed during the charge and discharge cycles after the first one, causing the surface capacity of the part with a high content of positive electrode lithium supplement material in the initial state and during the first charge and discharge cycle to decrease.

[0090] In the embodiments of the present disclosure, by regulating the content of the positive electrode lithium-replenishing material in the middle and edge portions of the positive electrode active layer 32, the areal capacity of the middle portion of the positive electrode plate 30 is lower than that of the edge portion during the first charge-discharge cycle after the first charge-discharge cycle. This reduces and / or alleviates lithium plating in the corresponding negative electrode plate, reduces the risk of a voltage drop due to lithium plating in the later stages of the charge-discharge cycle of the secondary battery 100, and improves the safety performance of the secondary battery 100. Furthermore, after the positive electrode lithium-replenishing material is consumed during the charge-discharge cycle, corresponding pores are generated in the positive electrode active layer 32, which facilitates electrolyte reflux during the cycle and reduces polarization of the secondary battery 100.

[0091] In some embodiments, referring to FIG. 5 and FIG. 6 , the first positive electrode active portion 321 , the middle positive electrode active portion 323 and the second positive electrode active portion 322 are all single-layer structures mixed with positive electrode active materials and positive electrode lithium supplementing materials.

[0092] The thickness of the three parts can be the same. By adjusting the content of the positive electrode active material in the middle part, the surface capacity of the middle part can be reduced, which is beneficial to shortening the lithium ion transmission path in the middle part and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet. In some cases, the positive electrode active material and the positive electrode lithium supplement material in the middle part can be reduced in the same proportion. In some cases, it is also possible to increase the content of the positive electrode lithium supplement material while reducing the content of the positive electrode active material in the middle part.

[0093] In the embodiment of the present disclosure, the three parts of the positive electrode active layer 32 are supplemented with lithium content by adding positive electrode lithium supplement materials, thereby reducing and / or alleviating lithium plating of the corresponding negative electrode sheets and improving the cycle performance of the secondary battery 100.

[0094] In some embodiments, based on the total mass of the positive electrode active layer 32, the content of the positive electrode lithium replenishing material in the intermediate positive electrode active portion 323 is greater than the content of the positive electrode lithium replenishing material in the first positive electrode active portion 321 and / or greater than the content of the positive electrode lithium replenishing material in the second positive electrode active portion 322; in the initial state and during the first charge and discharge cycle of the positive electrode sheet 30, the surface capacity of the intermediate positive electrode active portion 323 is greater than the surface capacity of the first positive electrode active portion 321 and / or greater than the surface capacity of the second positive electrode active portion 322.

[0095] In the embodiment of the present disclosure, by regulating the content of the positive electrode lithium-replenishing material in the middle part of the positive electrode active layer 32 to be greater than the content of the positive electrode lithium-replenishing material in the edge part, the surface capacity of the middle part of the positive electrode plate 30 in the initial state and during the first charge and discharge cycle is higher than the surface capacity of the edge part, thereby improving the energy density. In the charge and discharge cycle after the first time, due to the continuous consumption of the positive electrode lithium-replenishing material, the surface capacity of the part with a higher content of the positive electrode lithium-replenishing material in the initial state and during the first charge and discharge cycle is reduced instead, thereby achieving the surface capacity of the middle part being lower than the surface capacity of the edge part, thereby reducing and / or alleviating the lithium plating of the corresponding negative electrode plate.

[0096] Please refer to FIG. 7 , which is a third structural schematic diagram of the positive electrode sheet shown in FIG. 4 along the thickness direction.

[0097] In some embodiments, referring to FIG7 , the first positive electrode active portion 321 , the intermediate positive electrode active portion 323 , and the second positive electrode active portion 322 all have a double-layer structure; the first positive electrode active portion 321 , the intermediate positive electrode active portion 323 , and the second positive electrode active portion 322 all include a bottom positive electrode active layer 32 a close to the positive electrode current collector 31 and a top positive electrode active layer 32 b away from the positive electrode current collector 31 ; in the initial state and during the first charge and discharge cycle of the positive electrode sheet 30 , based on the total mass of the positive electrode active layer 32 , the contents of positive electrode active materials in the bottom positive electrode active layer 32 a and the top positive electrode active layer 32 b are different.

[0098] The positive electrode active material in each single layer of the double-layer structure can be evenly distributed, and the content of the positive electrode active material in the two-layer structure can be different. The content of the positive electrode active material in the three-part bottom structure can be the same or different, and the content of the positive electrode active material in the three-part top structure can be the same or different.

[0099] In the embodiment of the present disclosure, by setting up a double-layer structure, only one layer of the structure can be changed, so that the content of the positive electrode active material in the single-layer structure is reduced, and the other layer of the structure can remain unchanged. This is beneficial for the positive electrode plate 30 as a whole to have better capacity, energy density and cycle performance while reducing the expansion degree of the positive electrode plate 30 and reducing and / or slowing down the lithium plating of the corresponding negative electrode plate.

[0100] In some embodiments, the bottom positive electrode active layer 32a and the top positive electrode active layer 32b are both single-layer structures mixed with positive electrode active material and positive electrode lithium replenishing material; in the initial state of the positive electrode plate 30 and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer 32, the content of positive electrode lithium replenishing material in the bottom positive electrode active layer 32a is greater than the content of positive electrode lithium replenishing material in the top positive electrode active layer 32b.

[0101] In each single layer structure of the double-layer structure, the positive electrode active material and the positive electrode lithium replenishing material are uniformly mixed, and the contents of the positive electrode active material and the positive electrode lithium replenishing material in the two layers are different. In the bottom structure of the three parts, the content of the positive electrode lithium replenishing material can be the same, or the content of the positive electrode lithium replenishing material in the bottom structure of the middle part can be greater than the content of the positive electrode lithium replenishing material in the bottom structure of the first part and the second part, respectively. In the top structure of the three parts, the content of the positive electrode lithium replenishing material can be the same, or the content of the positive electrode lithium replenishing material in the bottom structure of the middle part can be greater than the content of the positive electrode lithium replenishing material in the bottom structure of the first part and the second part, respectively. In the bottom structure, a positive electrode lithium replenishing material layer can be used, and the top structure can be used as a positive electrode active material layer. Since the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer 32a is greater than the content of the positive electrode lithium replenishing material in the top positive electrode active layer 32b, during the first charge and discharge cycle of the secondary battery 100, the surface capacity of the bottom positive electrode active layer 32a is greater than the surface capacity of the top positive electrode active layer 32b. During the charge and discharge cycles after the first time, due to the continuous consumption of the positive electrode lithium replenishing material, the surface capacity of the bottom positive electrode active layer 32a is actually smaller than the surface capacity of the top positive electrode active layer 32b.

[0102] In the embodiments of the present disclosure, by setting up a double-layer structure, the content of positive electrode active materials and positive electrode lithium supplement materials in each layer and each region can be designed in layers according to actual conditions, thereby improving the flexibility of the design; it can also be coated in layers, and each single-layer structure can be formed as an integral whole, which is conducive to reducing production difficulty and improving production efficiency.

[0103] In some embodiments, please continue to refer to Figure 7, the thickness of the bottom positive electrode active layer 32a of the middle positive electrode active portion 323 is greater than the thickness of the bottom positive electrode active layer 32a of the first positive electrode active portion 321 and the thickness of the bottom positive electrode active layer 32a of the second positive electrode active portion 322.

[0104] Among them, the surface of the positive electrode current collector 31 can be flat, and the thickness of the bottom structure of the middle part is greater than the thickness of the bottom structure on both sides thereof, which increases the surface capacity of the middle part, is conducive to improving the capacity and energy density of the middle part, and improving the overall cycle performance of the secondary battery 100. In addition, since the positive electrode lithium supplement material does not embed lithium after consumption, the capacity of the middle part is lower than the parts on both sides thereof, further reducing and / or slowing down the lithium plating of the corresponding negative electrode pole piece. In some embodiments, during the process of coating to form a double-layer structure, the nut in the lower center area of ​​the double-layer coating machine can be appropriately raised so that the thickness of the bottom structure of the middle part is greater than the thickness of the bottom structure on both sides thereof. After the double-layer coating, the top structure of the three parts is flush away from the surface of the positive electrode current collector 31. Since each single-layer structure can be coated at one time, the production difficulty is reduced. In some embodiments, the thickness of the bottom structure of the first part and the second part can be the same or different, as long as the thickness of both is less than the thickness of the bottom structure of the middle part.

[0105] In the embodiment of the present disclosure, the thickness of the underlying structure of the middle part is increased. Since the proportion of the positive electrode lithium replenishing material in the underlying structure is high, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, so that the surface capacity of the middle part during the first charge and discharge cycle is greater than the surface capacity of the first part and the second part. During the second charge and discharge cycle and multiple charge and discharge cycles thereafter, due to the consumption of the positive electrode lithium replenishing material, the capacity provided by the positive electrode lithium replenishing material decreases, so that the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet 30 and reducing and / or slowing down the occurrence of lithium plating of the corresponding negative electrode sheet.

[0106] Please refer to FIG8 , which is a fourth structural diagram of the positive electrode sheet shown in FIG4 along the thickness direction.

[0107] In some embodiments, in the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer 32, the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer 32a of the intermediate positive electrode active portion 323 is greater than the content of the positive electrode lithium replenishing material in the first positive electrode active portion 321 and the content of the positive electrode lithium replenishing material in the second positive electrode active portion 322.

[0108] In some embodiments, referring to FIG. 8 , the surface of each single-layer structure of the double-layer structure away from the positive electrode current collector 31 is flush.

[0109] In the embodiment of the present disclosure, the surface capacity of the middle portion during the first charge and discharge cycle is increased, and during the second charge and discharge cycle and multiple charge and discharge cycles thereafter, the surface capacity of the middle portion is lower than the surface capacity of the first portion and the second portion, thereby reducing the degree of expansion of the middle portion of the positive electrode plate 30 and reducing and / or slowing down the lithium plating of the corresponding negative electrode plate.

[0110] In some embodiments, the bottom positive electrode active layer 32a and the top positive electrode active layer 32b are both single-layer structures mixed with positive electrode active material and positive electrode lithium replenishing material; in the initial state of the positive electrode plate 30 and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer 32, the content of positive electrode lithium replenishing material in the bottom positive electrode active layer 32a is less than the content of positive electrode lithium replenishing material in the top positive electrode active layer 32b.

[0111] The positive electrode active material and the positive electrode lithium replenishing material are uniformly mixed in each single layer structure of the double-layer structure, and the contents of the positive electrode active material and the positive electrode lithium replenishing material in the two layers are different. The contents of the positive electrode lithium replenishing material in the bottom structure of the three parts can be the same or different, and it is necessary to meet the requirement that the content of the positive electrode lithium replenishing material in the bottom structure of each part is less than the content of the positive electrode lithium replenishing material in its top structure. The contents of the positive electrode lithium replenishing material in the top structures of the three parts can be the same, or the content of the positive electrode lithium replenishing material in the bottom structure of the middle part can be greater than the content of the positive electrode lithium replenishing material in the bottom structures of the first part and the second part, respectively.

[0112] In the embodiments of the present disclosure, by setting up a double-layer structure, the content of positive electrode active materials and positive electrode lithium supplement materials in each layer and each region can be designed in layers according to actual conditions, thereby improving the flexibility of the design; it can also be coated in layers, and each single-layer structure can be formed as an integral whole, which is conducive to reducing production difficulty and improving production efficiency.

[0113] Please refer to FIG. 9 , which is a fifth structural diagram of the positive electrode sheet shown in FIG. 4 along the thickness direction.

[0114] 9 , the thickness of the bottom positive electrode active layer 32a of the middle positive electrode active portion 323 is smaller than the thickness of the bottom positive electrode active layer 32a of the first positive electrode active portion 321 and the thickness of the bottom positive electrode active layer 32a of the second positive electrode active portion 322 .

[0115] Among them, the surface of the positive electrode current collector 31 can be flat, and the surface of the top structure of the three parts away from the positive electrode current collector 31 can also be flat. The content of the positive electrode lithium replenishing material in the bottom structure of each part is less than the content of the positive electrode lithium replenishing material in its top structure, and the thickness of the bottom structure of the middle part is less than the thickness of the bottom structure on both sides thereof, that is, the thickness of the top structure of the middle part accounts for a larger proportion than the thickness of the top structure of the first part and the second part, that is, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, which is beneficial to improving the initial charge and discharge capacity and energy density of the middle part, and improving the overall cycle performance of the secondary battery 100. In addition, since the positive electrode lithium replenishing material does not embed lithium after consumption, the capacity of the middle part is lower than the capacity on both sides thereof, further reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet. In some embodiments, during the coating process to form a double-layer structure, the nut in the center area of ​​the lower layer of the double-layer coater can be appropriately lowered so that the thickness of the bottom structure in the middle portion is less than the thickness of the bottom structures on both sides. After the double-layer coating, the top surface of the three-part structure away from the positive electrode current collector 31 is flush. Since each single-layer structure can be coated in one go, the production difficulty is reduced. In some embodiments, the thickness of the bottom structure of the first and second parts can be the same or different, as long as the thickness of both parts is greater than the thickness of the bottom structure of the middle portion.

[0116] In the embodiment of the present disclosure, the thickness of the bottom structure of the middle part is reduced, so that the thickness of the top structure of the middle part is increased. Since the proportion of the positive electrode lithium replenishing material in the top structure is high, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, so that the surface capacity of the middle part during the first charge and discharge cycle is greater than the surface capacity of the first part and the second part. During the second charge and discharge cycle and multiple charge and discharge cycles thereafter, due to the consumption of the positive electrode lithium replenishing material, the capacity provided by the positive electrode lithium replenishing material decreases, and the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the expansion degree of the middle part of the positive electrode plate 30 and reducing and / or slowing down the lithium plating of the corresponding negative electrode plate.

[0117] In some embodiments, referring to FIG8 , in the initial state and during the first charge and discharge cycle of the positive electrode sheet 30, based on the total mass of the positive electrode active layer 32, the content of the positive electrode lithium replenishing material in the top positive electrode active layer 32b of the intermediate positive electrode active portion 323 is greater than the content of the positive electrode lithium replenishing material in the top positive electrode active layer 32b of the first positive electrode active portion 321 and the content of the positive electrode lithium replenishing material in the top positive electrode active layer 32b of the second positive electrode active portion 322, respectively.

[0118] In some embodiments, the surface of each single-layer structure of the double-layer structure away from the positive electrode current collector 31 may be flush.

[0119] In the embodiments of the present disclosure, by increasing the content of the positive electrode lithium-replenishing material in the top structure of the middle part, the surface capacity of the middle part during the first charge and discharge cycle is improved, and during the second charge and discharge cycle and multiple charge and discharge cycles thereafter, the surface capacity of the middle part is lower than the surface capacity of the first part and the second part, thereby reducing the degree of expansion of the middle part of the positive electrode sheet 30 and reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0120] Please refer to FIG. 10 , which is a sixth structural diagram of the positive electrode sheet shown in FIG. 4 along the thickness direction.

[0121] In some embodiments, referring to FIG10 , the first positive electrode active portion 321 and the second positive electrode active portion 322 are both single-layer structures; the intermediate positive electrode active portion 323 is a double-layer structure, and the intermediate positive electrode active portion 323 includes a bottom positive electrode active layer 32a close to the positive electrode current collector 31 and a top positive electrode active layer 32b away from the positive electrode current collector 31; based on the total mass of the positive electrode active layer 32, the contents of positive electrode active materials in the bottom positive electrode active layer 32a and the top positive electrode active layer 32b are different.

[0122] In the embodiment of the present disclosure, by only modifying the middle portion of the positive electrode active layer 32 , the formulas of the first portion and the second portion remain unchanged, which is beneficial for the secondary battery 100 to have a good capacity.

[0123] In some embodiments, the bottom positive electrode active layer 32a and the top positive electrode active layer 32b are both single-layer structures mixed with positive electrode active material and positive electrode lithium replenishing material; in the initial state of the positive electrode sheet 30 and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer 32, the content of positive electrode active material in the top positive electrode active layer 32b is less than the content of positive electrode active material in the bottom positive electrode active layer 32a; the content of positive electrode lithium replenishing material in the top positive electrode active layer 32b is greater than the content of positive electrode lithium replenishing material in the bottom positive electrode active layer 32a.

[0124] In the embodiment of the present disclosure, by only changing the top structure of the middle part of the positive electrode active layer 32, the first part, the bottom structure of the middle part, and the second part can be of the same formula and can be formed as one piece, which is conducive to simplifying the process and reducing the difficulty of the process.

[0125] In some embodiments, the first positive electrode active portion 321 , the middle positive electrode active portion 323 , and the second positive electrode active portion 322 are away from and flush with the surface of the positive electrode current collector 31 .

[0126] In some embodiments, the surface of the positive electrode current collector 31 is flat, and the surfaces of the three portions of the positive electrode active layer 32 away from the positive electrode current collector 31 are also flat, so that the three portions have the same thickness.

[0127] In the embodiment of the present disclosure, while reducing the surface capacity of the middle part of the positive electrode active layer 32, the three parts of the positive electrode active layer 32 are aligned away from the surface of the positive electrode current collector 31, so that the lithium ion transmission paths of each part are close, further reducing and / or slowing down the lithium plating of the corresponding negative electrode sheet.

[0128] In some embodiments, referring to FIG. 4 , along the TD direction of the positive electrode current collector 31 , the width of the middle positive electrode active portion is defined as a first width W1 , the width of the positive electrode active layer is defined as a second width W, and the ratio of the first width W1 to the second width W is 0.02 to 0.20.

[0129] In which, along the TD direction of the positive electrode current collector 31, the ratio of the first width W1 to the second width W can be 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, or a range consisting of any two of the above values. For example, along the TD direction of the positive electrode current collector 31, the ratio of the first width W1 to the second width W can be 0.02-0.05, 0.05-0.10, 0.10-0.15, or 0.15-0.20, etc.

[0130] In the embodiment of the present disclosure, by regulating the width ratio of the middle part of the positive electrode active layer 32, the secondary battery 100 has good capacity, charge and discharge efficiency, cycle life and safety while reducing the expansion degree of the middle part of the positive electrode plate 30 and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0131] In some embodiments, along the TD direction of the positive electrode current collector, a ratio of the first width W1 to the second width W is 0.05 to 0.1.

[0132] Among them, along the TD direction of the positive electrode current collector 31, the ratio of the first width W1 to the second width W can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, or a range consisting of any two of the above values. For example, along the TD direction of the positive electrode current collector 31, the ratio of the first width W1 to the second width W can be 0.05-0.08, 0.06-0.1, etc.

[0133] In the embodiment of the present disclosure, by further regulating the width ratio of the middle part of the positive electrode active layer 32, the secondary battery 100 has good capacity, charge and discharge efficiency, cycle life and safety while reducing the expansion degree of the middle part of the positive electrode sheet 30 and reducing and / or alleviating lithium plating of the corresponding negative electrode sheet.

[0134] In some embodiments, the positive electrode current collector 31 includes a main body portion 311 and a pole ear portion 312. Along the TD direction of the positive electrode current collector 31, the main body portion 311 includes a first positive electrode active portion 321, an intermediate positive electrode active portion 323 and a second positive electrode active portion 322; the first positive electrode active portion 321 is arranged on a side of the intermediate positive electrode active portion 323 close to the pole ear portion 312, and the second positive electrode active portion 322 is arranged on a side of the intermediate positive electrode active portion 323 away from the pole ear portion 312; the width of the second positive electrode active portion is defined as a third width W2, and the ratio of the third width W2 to the second width W is 0.3 to 0.7.

[0135] In which, along the TD direction of the positive electrode current collector 31, the ratio of the third width W2 to the second width W can be 0.3, 0.32, 0.34, 0.36, 0.38, 0.4, 0.42, 0.44, 0.46, 0.48, 0.5, 0.52, 0.54, 0.56, 0.58, 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, or a range consisting of any two of the above values. For example, along the TD direction of the positive electrode current collector 31, the ratio of the third width W2 to the second width W can be 0.3-0.4, 0.4-0.6, or 0.6-0.7, etc. It is understood that the width of the first positive electrode active portion 321 along the TD direction of the positive electrode current collector 31 can be calculated by subtracting the width of the intermediate positive electrode active portion (i.e., the first width W1) and the width of the second positive electrode active portion (i.e., the third width W2) from the width of the positive electrode active layer (i.e., the second width W). In some embodiments, the width of the main body 311 in the TD direction of the positive electrode current collector 31 is greater than the width of the positive electrode active layer (i.e., the second width W). In some embodiments, an insulating layer is provided on one edge of the main body 311, and the insulating layer is provided on the side of the positive electrode active layer near the tab 312.

[0136] In the embodiment of the present disclosure, by regulating the width ratio of each part of the positive electrode active layer 32, the secondary battery 100 has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode plate 30 and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0137] In some embodiments, the ratio of the third width W2 to the second width W is 0.4-0.6.

[0138] The ratio of the third width W2 to the second width W may be 0.4, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.51, 0.52, 0.53, 0.54, 0.55, 0.56, 0.57, 0.58, 0.59, or 0.6, or a range consisting of any two of the above values. For example, the ratio of the third width W2 to the second width W may be 0.4-0.5, 0.45-0.55, or 0.5-0.6, etc.

[0139] In the embodiment of the present disclosure, by further regulating the width ratio of each part of the positive electrode active layer 32, the secondary battery 100 has good capacity, charge and discharge efficiency, cycle life and safety performance while reducing the expansion degree of the middle part of the positive electrode plate 30 and reducing and / or alleviating lithium plating of the corresponding negative electrode plate.

[0140] In some embodiments, the gram capacity of the positive electrode lithium supplementing material is greater than the gram capacity of the positive electrode active material.

[0141] The gram capacity represents the ratio of the capacity that can be released by the active material inside the secondary battery 100 to the mass of the active material.

[0142] In the embodiment of the present disclosure, the gram capacity of the positive electrode lithium supplementing material is greater than the gram capacity of the positive electrode active material, which is beneficial to improving the capacity and energy density of the positive electrode plate 30 and the cycle performance of the secondary battery 100.

[0143] In some embodiments, the positive electrode active material includes lithium iron phosphate (LiFePO4), lithium manganate (LiMn2O4), lithium nickel cobalt manganate (LiNi x Co y Mn 1-x-y O2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganate (LiMn2O4) or one or more thereof.

[0144] In the embodiments of the present disclosure, by using one or more of the above materials as the active material of the positive electrode active layer 32 , it is beneficial for the positive electrode plate 30 to have good capacity and energy density.

[0145] In some embodiments, the positive electrode lithium supplement material includes: Li2C x O y 、Li2MO2、Li2MO3、Li5Fe x M 1-x O4 and Li6Mn y M 1-y One or more of O4; wherein, M includes at least one of Ni, Mn, Cu, Fe, Cr, Mo, Zr, Si and Ru, x=0.5~1, y=0.5~1.

[0146] Among them, M of Li2MO2 can be at least one of Ni, Mn, Cu, Fe, Cr and Mo; M of Li2MO3 can be at least one of Ni, Mn, Fe, Mo, Zr, Si, Cu, Cr and Ru; Li5Fe x M 1-x M of O4 can be at least one of Al, Nb, Co, Mn, Ni, Mo, Ru and Cr; Li6Mn y M 1-y M of O4 may be at least one of Ni, Fe, Cu and Ru.

[0147] In the embodiments of the present disclosure, by using one or more of the above materials as the lithium supplement material of the positive electrode active layer 32 , it is beneficial for the positive electrode plate 30 to have good capacity and energy density.

[0148] In some embodiments, the positive electrode active layer 32 includes only positive electrode active material, and based on the total mass of the positive electrode active layer 32, the lithium content of the positive electrode active material of the intermediate positive electrode active portion 323 is less than the lithium content of the positive electrode active material of the first positive electrode active portion 321 and / or less than the lithium content of the positive electrode active material of the second positive electrode active portion 322.

[0149] In the embodiments of the present disclosure, the lithium content of the positive electrode active material of the intermediate positive electrode active portion 323 is less than the lithium content of the positive electrode active material of the first positive electrode active portion 321 and / or less than the lithium content of the positive electrode active material of the second positive electrode active portion 322, so that the surface capacity of the intermediate positive electrode active portion 323 is less than the surface capacity of the first positive electrode active portion 321 and / or less than the surface capacity of the second positive electrode active portion 322, and the degree of expansion of the middle portion is lower than the degree of expansion of the edge portion, thereby reducing and / or alleviating lithium plating of the corresponding negative electrode sheet, reducing the risk of water diving caused by lithium plating in the late charge and discharge cycle of the secondary battery 100, and improving the safety performance of the secondary battery 100.

[0150] In some embodiments, the positive active layer 32 includes a mixture of at least two positive active materials.

[0151] The mixture refers to the positive electrode active material particles being uniformly mixed together and uniformly distributed in the positive electrode active material arrangement region of the positive electrode current collector 31 .

[0152] In the embodiment of the present disclosure, the positive electrode active layer 32 is formed by a mixture including at least two positive electrode active materials. By adjusting the mass ratio of each positive electrode active material in the mixture in each region, the surface capacity of the middle positive electrode active portion can be made smaller than the surface capacity of the first positive electrode active portion and / or smaller than the surface capacity of the second positive electrode active portion. The implementation method is simple and easy to operate, and it has a good effect of alleviating lithium plating of the corresponding negative electrode sheet and improving the safety performance of the secondary battery 100.

[0153] In some embodiments, one of the at least two positive electrode active materials is a layered lithium-containing transition metal oxide; optionally, the positive electrode active material further includes any one or more of lithium iron phosphate, lithium manganese oxide, lithium nickel manganese oxide, and lithium manganese iron phosphate.

[0154] The gram capacity of the layered lithium-containing transition metal oxide is higher than the gram capacity of other positive electrode active materials (one or more of lithium iron phosphate, lithium manganate, lithium nickel manganate, and lithium manganese iron phosphate). In some embodiments, the layered lithium-containing transition metal oxide can be lithium nickel cobalt manganate.

[0155] In the embodiments of the present disclosure, one of the mixtures uses a layered lithium-containing transition metal oxide, which can reduce the content of the layered lithium-containing transition metal oxide in the intermediate positive electrode active portion 323 so that the content of the layered lithium-containing transition metal oxide in the intermediate positive electrode active portion 323 is less than the content of other positive electrode active materials, thereby reducing the surface capacity of the intermediate positive electrode active portion 323, thereby alleviating lithium plating of the corresponding negative electrode sheet and improving the safety performance of the secondary battery 100.

[0156] For example, the positive electrode active layer 32 includes a mixture of lithium nickel cobalt manganese oxide and lithium iron phosphate, and the first positive electrode active portion 321, the intermediate positive electrode active portion 323 and the second positive electrode active portion 322 are uniformly coated with a mixture of layered lithium-containing transition metal oxide and lithium iron phosphate, and the layered lithium-containing transition metal oxide and lithium iron phosphate are uniformly mixed in the mixture. Among them, the content of layered lithium-containing transition metal oxide in the intermediate positive electrode active part 323 is lower than the content of lithium iron phosphate, the content of layered lithium-containing transition metal oxide in the first positive electrode active part 321 is higher than the content of lithium iron phosphate, and the content of layered lithium-containing transition metal oxide in the second positive electrode active part 322 is higher than the content of lithium iron phosphate, so that the surface capacity of the intermediate positive electrode active part 323 is smaller than the surface capacity of the first positive electrode active part 321 and / or smaller than the surface capacity of the second positive electrode active part 322, so that the expansion degree of the middle part of the positive electrode plate 30 is lower than the expansion degree of the edge part, reducing and / or alleviating the lithium plating of the corresponding negative electrode plate, reducing the risk of diving caused by lithium plating in the late charge and discharge cycle of the secondary battery 100, and improving the safety performance of the secondary battery 100.

[0157] In some embodiments, the positive electrode active material of the positive electrode active layer 32 is the same positive electrode active material; based on the total mass of the positive electrode active layer 32, the content of the positive electrode active material in the intermediate positive electrode active portion 323 is less than the content of the positive electrode active material in the first positive electrode active portion 321 and / or less than the content of the positive electrode active material in the second positive electrode active portion 322.

[0158] In the embodiment of the present disclosure, by adopting the positive electrode active layer 32 of the same positive electrode active material, the content of the positive electrode active material in the middle positive electrode active part 323 is less than the content of the positive electrode active material in the first positive electrode active part 321 and / or less than the content of the positive electrode active material in the second positive electrode active part 322, so that the expansion degree of the middle part is lower than the expansion degree of the edge part, reducing and / or alleviating the lithium plating of the corresponding negative electrode plate, reducing the risk of diving caused by lithium plating in the late cycle of the secondary battery 100, and improving the safety performance of the secondary battery 100.

[0159] The beneficial effects of the present disclosure are further illustrated below with reference to the examples.

[0160] In order to make the technical problems, technical solutions and beneficial effects solved by the embodiments of the present disclosure clearer, the following will be further described in detail with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present disclosure, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is in no way intended to limit the present disclosure and its applications. Based on the embodiments in the present disclosure, all other embodiments obtained without creative work are within the scope of protection of the present disclosure.

[0161] Example 1:

[0162] The positive electrode active material (lithium iron phosphate), conductive agent (CNT), and binder (PVDF) are mixed uniformly in a mass ratio of 96.8:1:2.2, and then NMP (N-methylpyrrolidone) is added as a solvent. The mixture is stirred in a vacuum mixer until the system becomes uniform to obtain a positive electrode slurry. The positive electrode slurry is directly coated on the surface of the positive electrode collector 31 and cold pressed to obtain a positive electrode plate 30. During the forming process, the middle portion of the positive electrode plate 30 is sunken (relative to the first and second portions). The configuration is shown in Figure 5. In the TD direction, the ratio of the first width W1 to the second width W is 0.1; the ratio of the third width W2 to the second width W is 0.45; the surface capacity of the middle portion is 4.17 mAh / cm 2 The surface capacity of the other parts (the first and second parts) is 4.22 mAh / cm 2 The positive electrode sheet 30, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive electrode sheet 30 and the negative electrode sheet to provide isolation. The sheets are then wound to form a bare cell. The bare cell is placed in an outer package, and electrolyte is injected and packaged to form a secondary battery 100.

[0163] Example 2:

[0164] The difference between this embodiment and Example 1 is that the positive electrode sheet 30 further includes a positive electrode lithium supplement material (Li5FeO4), and the mass ratio of the remaining positive electrode active materials (lithium iron phosphate) is 5.1:94.9. The rest is the same as in Example 1. It should be noted that the amount of positive electrode active material used in this embodiment is the same as that used in Example 1.

[0165] Example 3:

[0166] The difference between this embodiment and embodiment 2 is that the surface capacity of the middle part is 4.05 mAh / cm 2 , the rest are the same as in Example 2.

[0167] Example 4:

[0168] The difference between this embodiment and embodiment 2 is that the surface capacity of the middle part is 4.19 mAh / cm 2 , the rest are the same as in Example 2.

[0169] Example 5:

[0170] The difference between this embodiment and embodiment 2 is that the ratio of the first width W1 to the second width W is 0.2, and the ratio of the third width W2 to the second width W is 0.4. The rest is the same as embodiment 2.

[0171] Example 6:

[0172] The difference between this embodiment and Example 5 is that the surfaces of the three parts (the first part, the middle part, and the second part) of the positive electrode plate 30 away from the positive electrode current collector 31 are flush, and the content of the positive electrode active material in the middle part of the positive electrode plate 30 is less than the content of the positive electrode active material in the first part and the second part, respectively. During the first charge and discharge cycle, the content of the positive electrode lithium replenishing material in the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part, respectively. The configuration is shown in Figure 6, and the rest is the same as Example 5.

[0173] Example 7:

[0174] The difference between this embodiment and embodiment 5 is that: as shown in Figure 7, the three parts of the positive electrode plate 30 (the first part, the middle part, and the second part) are all double-layer structures, and the formula of the bottom structure of the three parts is the same, the formula of the top structure of the three parts is the same, and the thickness of the bottom structure of the middle part is greater than the thickness of the bottom structure of the first part and the second part, respectively, and the surface capacity of the positive electrode lithium replenishing material of the bottom structure is greater than the surface capacity of the positive electrode lithium replenishing material of each top structure. The rest is the same as embodiment 5.

[0175] Example 8:

[0176] The difference between this embodiment and embodiment 5 is that: as shown in Figure 9, the three parts of the positive electrode plate 30 (the first part, the middle part, and the second part) are all double-layer structures, and the formula of the bottom structure of the three parts is the same, the formula of the top structure of the three parts is the same, and the thickness of the bottom structure of the middle part is smaller than the thickness of the bottom structure of the first part and the second part, respectively, and the surface capacity of the positive electrode lithium replenishing material of the bottom structure is smaller than the surface capacity of the positive electrode lithium replenishing material of each top structure. The rest is the same as embodiment 5.

[0177] Example 9:

[0178] The difference between this embodiment and Example 5 is that: as shown in Figure 8, the three parts of the positive electrode plate 30 (the first part, the middle part and the second part) are all double-layer structures, and the formula of the bottom structure of the three parts is the same, the content of the positive electrode active material in the top structure of the middle part is less than the content of the positive electrode active material in the first part and the second part, and during the first charge and discharge cycle, the content of the positive electrode lithium replenishing material in the top structure of the middle part is greater than the content of the positive electrode lithium replenishing material in the first part and the second part. The rest is the same as Example 5.

[0179] Comparative Example 1:

[0180] The difference between this comparative example and Example 1 is that the positive electrode active layer is not partitioned in this comparative example, the formulation of each part is the same, the thickness of each part is the same, and the rest is the same as Example 1.

[0181] The secondary batteries 100 prepared in each embodiment and comparative example were tested:

[0182] Test of the surface capacity of the positive electrode sheet 30:

[0183] Take the positive electrode sheet 30 in each embodiment and comparative example, cut the positive electrode sheet along the boundaries of the middle and edge parts of the positive electrode sheet 30, and punch out small discs of the same size in the corresponding areas of the middle and edge parts of the positive electrode sheet 30, respectively. Measure the mass, area, and thickness of the corresponding areas. Then assemble the small discs punched out from the middle and edge parts into batteries. Under a constant temperature environment of 25°C, discharge at 1 / 3C to 2.5V; let it stand for 5 minutes, charge at 1 / 3C to 4.35V, then charge at constant voltage at 4.35V to a current of ≤0.05C; let it stand for 5 minutes, and then discharge at 1 / 3C to 2.5V. The amount of electricity during this discharge process is the discharge capacity. The compaction density is obtained by dividing the mass of the corresponding area by the product of the area and thickness; the areal capacity is obtained by dividing the discharge capacity by the area of ​​the corresponding area.

[0184] Testing of device performance of the secondary battery 100:

[0185] Take the secondary battery 100 in each embodiment and comparative example, and each example has at least 2 parallel samples. First, place the secondary battery 100 in a constant temperature environment at 25°C and let it sit for 1 hour. Then, perform constant current charging on the secondary battery 100 at a charging rate of 2C. After charging to 3.65V, switch to constant voltage charging. Stop charging when the charging current is lower than 0.05C and let it sit for 5 minutes. Then, use a discharge rate of 1C to discharge the secondary battery 100 to 2.5V at a constant current and let it sit for 5 minutes. The above process is repeated for 1000 cycles. The cycle retention rate is calculated as follows: the capacity retention rate of the 1000th cycle = the discharge capacity of the 1000th cycle / the discharge capacity of the second cycle * 100%. After the secondary battery 100 undergoes 1000 cycles of charge and discharge tests as described above, disassemble the secondary battery 100, remove the negative electrode plate, and observe whether lithium deposition occurs in the middle of the negative electrode plate.

[0186] Table 1 Performance test results of secondary batteries of various embodiments and comparative examples

[0187] Note: The surface capacity of the middle part of the electrode and the surface capacity of the edge part of the electrode in Table 1 refer to the surface capacity of the second circle.

[0188] According to the results:

[0189] Analysis of the test data of Comparative Example 1 and Example 1 shows that the areal capacity of the middle portion of the positive electrode is lower than that of the edge portion, and the corresponding negative electrode does not exhibit central lithium deposition, and the cycle performance of the secondary battery 100 is significantly improved.

[0190] Analysis of the test data of Examples 1 and 2 shows that the cycle performance of the secondary battery 100 is significantly improved by adding lithium-supplementing materials to the positive electrode.

[0191] Analysis of the test data of Examples 2 to 4 shows that as the surface capacity in the middle of the positive electrode plate decreases, the capacity of the secondary battery 100 decreases accordingly, and the cycle performance first increases and then decreases. Therefore, when processing the positive electrode plate, regulating the surface capacity in the middle of the positive electrode plate is beneficial to balancing the capacity and cycle performance of the secondary battery 100.

[0192] Analysis of the test data of Example 2 and Example 5 shows that within the set range, the width of the middle portion of the positive electrode sheet has little effect on the capacity and cycle performance of the secondary battery 100 .

[0193] Analysis of the test data of Examples 5 and 6 shows that the surface of the positive electrode sheet away from the positive electrode current collector is flat, and compared with the solution in which the middle part of the positive electrode sheet is sunken, the cycle performance of the secondary battery 100 is significantly improved.

[0194] Analysis of the test data of Example 5 and Example 7 shows that the thickness of the underlying structure in the middle of the positive electrode plate is greater than the thickness of the underlying structure on both sides thereof. Compared with the solution in which the middle part of the positive electrode plate is sunken, the cycle performance of the secondary battery 100 is significantly improved.

[0195] Analysis of the test data of Example 5 and Example 8 shows that the thickness of the underlying structure in the middle part of the positive electrode plate is smaller than the thickness of the underlying structure on both sides thereof. Compared with the solution in which the middle part of the positive electrode plate is sunken, the cycle performance of the secondary battery 100 is significantly improved.

[0196] Analysis of the test data of Example 5 and Example 9 shows that the surface capacity of the top structure in the middle part is smaller than the surface capacity on both sides thereof. During the first charge and discharge cycle, the content of the positive electrode lithium replenishing material is greater than the content of the positive electrode lithium replenishing material on both sides thereof. Compared with the solution in which the middle part of the positive electrode plate is sunken, the cycle performance of the secondary battery 100 is significantly improved.

[0197] In the several embodiments provided in the present disclosure, it should be understood that the disclosed systems, devices and methods can be implemented in other ways. For example, the device embodiments described above are merely schematic. For example, the division of units is only a logical function division. In actual implementation, there may be other division methods, such as multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the mutual coupling or direct coupling or communication connection shown or discussed can be an indirect coupling or communication connection through some interfaces, devices or units, which can be electrical, mechanical or other forms.

[0198] In addition, the functional units in the various embodiments of the present disclosure may be integrated into a single processing unit, or each unit may exist physically separately, or two or more units may be integrated into a single unit. The aforementioned integrated units may be implemented in the form of hardware or software functional units.

[0199] The above description is merely an embodiment of the present disclosure and does not limit the patent scope of the present disclosure. Any equivalent structure or equivalent process transformation made using the contents of the present disclosure specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present disclosure.

Claims

1. A secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode active layer disposed on at least one side of the positive electrode current collector; the positive electrode active layer comprises a positive electrode active material; along the TD direction of the positive electrode current collector, the positive electrode active layer comprises a first positive electrode active portion, an intermediate positive electrode active portion and a second positive electrode active portion arranged in sequence; in, During the charge and discharge cycle of the positive electrode sheet after the first time, the surface capacity of the intermediate positive electrode active portion is smaller than the surface capacity of the first positive electrode active portion and / or smaller than the surface capacity of the second positive electrode active portion.

2. The secondary battery according to claim 1, wherein The positive electrode active layer also includes positive electrode lithium replenishing material; in the initial state and during the first charge and discharge cycle of the positive electrode plate, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the intermediate positive electrode active part is different from the content of the positive electrode lithium replenishing material in the first positive electrode active part.

3. The secondary battery according to claim 1 or 2, wherein: The first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion are all single-layer structures in which positive electrode active materials and positive electrode lithium supplementing materials are mixed.

4. The secondary battery according to any one of claims 1 to 3, wherein Based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the intermediate positive electrode active part is greater than the content of the positive electrode lithium replenishing material in the first positive electrode active part and / or greater than the content of the positive electrode lithium replenishing material in the second positive electrode active part; in the initial state of the positive electrode sheet and during the first charge and discharge cycle, the surface capacity of the intermediate positive electrode active part is greater than the surface capacity of the first positive electrode active part and / or greater than the surface capacity of the second positive electrode active part.

5. The secondary battery according to any one of claims 1 to 4, wherein The first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion all have a double-layer structure; the first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion all include a bottom positive electrode active layer close to the positive electrode current collector and a top positive electrode active layer away from the positive electrode current collector; in the initial state of the positive electrode sheet and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer, the content of positive electrode lithium replenishing material in the bottom positive electrode active layer and the top positive electrode active layer is different.

6. The secondary battery according to claim 5, wherein The bottom positive electrode active layer and the top positive electrode active layer are both single-layer structures mixed with positive electrode active materials and positive electrode lithium replenishing materials; in the initial state of the positive electrode plate and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer, the content of positive electrode lithium replenishing material in the bottom positive electrode active layer is greater than the content of positive electrode lithium replenishing material in the top positive electrode active layer.

7. The secondary battery according to claim 5 or 6, wherein: The thickness of the bottom positive electrode active layer of the intermediate positive electrode active portion is greater than the thickness of the bottom positive electrode active layer of the first positive electrode active portion and the thickness of the bottom positive electrode active layer of the second positive electrode active portion.

8. The secondary battery according to claim 5 or 6, wherein: In the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the bottom positive electrode active layer of the intermediate positive electrode active portion is respectively greater than the content of the positive electrode lithium replenishing material in the first positive electrode active portion and the content of the positive electrode lithium replenishing material in the second positive electrode active portion.

9. The secondary battery according to claim 5, wherein The bottom positive electrode active layer and the top positive electrode active layer are both single-layer structures mixed with positive electrode active materials and positive electrode lithium replenishing materials; in the initial state of the positive electrode plate and during the first charge and discharge cycle, based on the total mass of the positive electrode active layer, the content of positive electrode lithium replenishing material in the bottom positive electrode active layer is less than the content of positive electrode lithium replenishing material in the top positive electrode active layer.

10. The secondary battery according to any one of claims 5 to 9, wherein The thickness of the bottom positive electrode active layer of the intermediate positive electrode active portion is smaller than the thickness of the bottom positive electrode active layer of the first positive electrode active portion and the thickness of the bottom positive electrode active layer of the second positive electrode active portion.

11. The secondary battery according to any one of claims 5 to 9, wherein In the initial state and during the first charge and discharge cycle of the positive electrode sheet, based on the total mass of the positive electrode active layer, the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the intermediate positive electrode active part is respectively greater than the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the first positive electrode active part and the content of the positive electrode lithium replenishing material in the top positive electrode active layer of the second positive electrode active part.

12. The secondary battery according to any one of claims 1 to 11, wherein The surfaces of the first positive electrode active portion, the middle positive electrode active portion and the second positive electrode active portion away from the positive electrode current collector are flush.

13. The secondary battery according to any one of claims 1 to 12, wherein: Along the TD direction of the positive electrode current collector, the ratio of the width of the middle positive electrode active portion to the width of the positive electrode active layer is 0.02 to 0.

20.

14. The secondary battery according to any one of claims 1 to 13, wherein Along the TD direction of the positive electrode current collector, the ratio of the width of the middle positive electrode active portion to the width of the positive electrode active layer is 0.05 to 0.

1.

15. The secondary battery according to any one of claims 1 to 14, wherein The positive electrode current collector includes a main body portion and a pole ear portion. Along the TD direction of the positive electrode current collector, the main body portion includes the first positive electrode active portion, the intermediate positive electrode active portion and the second positive electrode active portion; the first positive electrode active portion is arranged on a side of the intermediate positive electrode active portion close to the pole ear portion, and the second positive electrode active portion is arranged on a side of the intermediate positive electrode active portion away from the pole ear portion; wherein the ratio of the width of the second positive electrode active portion to the width of the positive electrode active layer is 0.3 to 0.

7.

16. The secondary battery according to any one of claims 1 to 15, wherein The ratio of the width of the second positive electrode active portion to the width of the positive electrode active layer is 0.4 to 0.

6.

17. The secondary battery according to any one of claims 2 to 16, wherein The gram capacity of the positive electrode lithium supplement material is greater than the gram capacity of the positive electrode active material.

18. The secondary battery according to any one of claims 1 to 17, wherein The positive electrode active material includes one or more of lithium iron phosphate, lithium manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium cobalt oxide, lithium nickel oxide, and lithium iron manganese oxide.

19. The secondary battery according to any one of claims 2 to 18, wherein The positive electrode lithium supplement material includes: Li2C x O y 、Li2MO2、Li2MO3、Li5Fe x M 1-x O4 and Li6Mn y M 1-y O4; wherein, M includes at least one of Ni, Mn, Cu, Fe, Cr, Mo, Zr, Si and Ru, x=0.5~1, y=0.5~1.

20. The secondary battery according to any one of claims 1 to 19, wherein The positive electrode active layer only includes positive electrode active material, and based on the total mass of the positive electrode active layer, the lithium content of the positive electrode active material of the intermediate positive electrode active portion is less than the lithium content of the positive electrode active material of the first positive electrode active portion and / or less than the lithium content of the positive electrode active material of the second positive electrode active portion.

21. The secondary battery according to any one of claims 1 to 20, wherein The positive electrode active layer includes a mixture of at least two positive electrode active materials.

22. The secondary battery according to claim 21, wherein One of the at least two positive electrode active materials is a layered lithium-containing transition metal oxide; optionally, the positive electrode active material also includes one or more of lithium iron phosphate, lithium manganese oxide, lithium nickel manganese oxide, and lithium manganese iron phosphate.

23. The secondary battery according to any one of claims 1 to 20, wherein The positive electrode active materials are the same positive electrode active materials; based on the total mass of the positive electrode active layer, the content of the positive electrode active material in the intermediate positive electrode active portion is less than the content of the positive electrode active material in the first positive electrode active portion and / or less than the content of the positive electrode active material in the second positive electrode active portion.

24. An electrical device, wherein: The invention comprises the secondary battery according to any one of claims 1 to 23, wherein the secondary battery is used for providing electric energy.

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