Positive electrode sheet, secondary battery, and electric device

Abstract

A positive electrode sheet, a secondary battery, and an electric device. The positive electrode sheet comprises a positive electrode active material layer, the positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material, the first positive electrode active material and the second positive electrode active material each independently comprise a lithium-containing transition metal oxide, and the outer surface of the second positive electrode active material comprises a passivation layer; and / or the average particle size Dv50 of the second positive electrode active material is greater than the average particle size Dv50 of the first positive electrode active material. The positive electrode sheet can enable the secondary battery to have high cycle life.

Description

Positive electrode sheet, secondary battery and electrical device Technical Field

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

[0002] The statements herein merely provide background information related to the present application and do not necessarily constitute prior art.

[0003] In recent years, as the application scope of secondary batteries has become increasingly wider, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in many fields such as power tools, electric bicycles, electric motorcycles and electric vehicles.

[0004] As secondary batteries have made great progress, higher requirements have been placed on their cycle life. Therefore, seeking a secondary battery with a longer cycle life has become one of the key areas of focus for those skilled in the art.

[0005] Summary of the Invention

[0006] The present application is made in view of the above-mentioned problems, and one of its purposes is to provide a positive electrode sheet that can enable a secondary battery to have a longer cycle life.

[0007] To achieve the above-mentioned object, the first aspect of the present application provides a positive electrode sheet, comprising a positive electrode active material layer, wherein the positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material, wherein the first positive electrode active material and the second positive electrode active material each independently comprise a lithium-containing transition metal oxide;

[0008] The outer surface of the second positive active material comprises a passivation layer; and / or

[0009] The average particle size Dv50 of the second positive electrode active material is greater than the average particle size Dv50 of the first positive electrode active material.

[0010] The above-mentioned positive electrode plate of the present application makes the first positive electrode active material and the second positive electrode active material independently include a lithium-containing transition metal oxide, and the outer surface of the second positive electrode active material includes a passivation layer, and / or the average particle size Dv50 of the second positive electrode active material is greater than the average particle size Dv50 of the first positive electrode active material; the passivation layer on the outer surface of the second positive electrode active material can passivate the second positive electrode active material, so that the first discharge gram capacity of the second positive electrode active material is reduced, and during the battery cycle process, the passivation layer on the surface of the second positive electrode active material will gradually break, so that the capacity of the second positive electrode active material is gradually restored and released, forming a self-lithium replenishment effect, so that the cycle attenuation of the secondary battery is slowed down and the cycle life is improved.

[0011] Similarly, the average particle size Dv50 of the second positive electrode active material is larger than that of the first positive electrode active material. Due to its larger particle size, the specific surface area is correspondingly smaller, which reduces the active sites exposed to the electrolyte. This can also passivate the second positive electrode active material and reduce the initial discharge capacity of the second positive electrode active material. During the battery cycle, the second positive electrode active material will gradually crack, gradually exposing more active sites, allowing the capacity of the second positive electrode active material to gradually recover and release, and also forming a self-lithium replenishment effect, which can slow the cycle decay of the secondary battery and increase the cycle life.

[0012] In any embodiment, the first positive electrode active material and the second positive electrode active material each independently include one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich lithium manganese oxide or lithium cobalt oxide.

[0013] In any embodiment, the first positive electrode active material and the second positive electrode active material are simultaneously selected from one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganate, lithium nickel cobalt aluminum oxide, lithium manganate, lithium nickel manganate, lithium-rich lithium manganate, or lithium cobalt oxide. Thus, the first positive electrode active material and the second positive electrode active material are of the same type, which can reduce the adverse effects of mixing different types of positive electrode active materials on other performance of the secondary battery besides cycle life, such as safety, reliability, storage performance, and energy density.

[0014] In any embodiment, the first positive electrode active material and the second positive electrode active material are both lithium iron phosphate.

[0015] In any embodiment, the first positive electrode active material and the second positive electrode active material have the same molecular formula, which can further alleviate the adverse effects on other battery properties except cycle life.

[0016] In any embodiment, the passivation layer includes one or more of an oxide passivation layer and a nitride passivation layer. The passivation layer can effectively passivate the surface of the positive electrode active material.

[0017] In any embodiment, the passivation layer includes one or more of an aluminum oxide layer, a zirconium oxide layer, a titanium oxide layer, or a titanium nitride layer.

[0018] In any embodiment, the thickness of the passivation layer is 1 nm to 50 nm. In this way, while achieving a good passivation effect, the capacity of the active material inside the passivation layer can be well released during subsequent cycles.

[0019] In any embodiment, the thickness of the passivation layer is 3 nm to 10 nm.

[0020] In any embodiment, the ratio of the average particle size Dv50 of the second positive electrode active material to the average particle size Dv50 of the first positive electrode active material is 2 to 18. Thus, the second positive electrode active material can be effectively passivated, and its capacity can be effectively released during the cycling process.

[0021] In any embodiment, both the first positive electrode active material and the second positive electrode active material are lithium iron phosphate, the average particle size Dv50 of the first positive electrode active material is 0.5 μm to 3 μm, and the average particle size Dv50 of the second positive electrode active material is 1.5 μm to 9 μm.

[0022] In any embodiment, both the first positive electrode active material and the second positive electrode active material are nickel cobalt manganese ternary materials with a single crystal particle morphology, the average particle size Dv50 of the first positive electrode active material is 3 μm to 7 μm, and the average particle size Dv50 of the second positive electrode active material is 9 μm to 21 μm.

[0023] In any embodiment, both the first positive electrode active material and the second positive electrode active material are nickel cobalt manganese ternary materials with a polycrystalline particle morphology, the average particle size Dv50 of the first positive electrode active material is 8 μm to 20 μm, and the average particle size Dv50 of the second positive electrode active material is 20 μm to 40 μm.

[0024] In any embodiment, the first discharge specific capacity of the first positive electrode active material is C1, and the first discharge specific capacity of the second positive electrode active material is C2; based on the total mass of the positive electrode active material layer, the mass fraction of the first positive electrode active material is W1, and the mass fraction of the second positive electrode active material is W2; the positive electrode sheet satisfies: C1*W1 + C2*W2 = k*C1*(W1 + W2), where 0.8 ≤ k < 1. Thus, while effectively improving the cycle life of the battery, the battery can have a relatively low DC resistance DCR.

[0025] In any embodiment, 0.9 ≤ k ≤ 0.97. Thus, a better balance between the battery cycle life and the DC resistance can be obtained, and the secondary battery can have better comprehensive performance.

[0026] In any embodiment, 12% ≤ W1 < 100%, 0% < W2 ≤ 85%. Thus, while effectively improving the cycle life of the battery, it is beneficial for the battery to have a low DC resistance.

[0027] In any embodiment, 60% ≤ W1 < 100% and 0% < W2 ≤ 40%. Thus, it is more beneficial to reduce the DC impedance of the battery and better achieve the balance between the battery cycle life and the DC impedance.

[0028] In any embodiment, the positive electrode active material layer includes the first positive electrode active material and the second positive electrode active material which are mixed with each other. Thus, a positive electrode active material layer with a single-layer structure containing both the first positive electrode active material and the second positive electrode active material is formed, and the coating process is relatively simple.

[0029] In any embodiment, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer which are stacked. The first positive electrode active material layer includes the first positive electrode active material, and the second positive electrode active material layer includes the second positive electrode active material. Thus, a positive electrode active material layer with a stacked structure containing the first positive electrode active material and the second positive electrode active material respectively is formed, which is more beneficial to the capacity utilization of the active material.

[0030] The second aspect of the present application further provides a secondary battery, which includes the positive electrode sheet of the first aspect of the present application.

[0031] The third aspect of the present application further provides an electrical device, which includes the secondary battery of the second aspect of the present application.

[0032] Details of one or more embodiments of the present application are set forth in the following drawings and description. Other features, objects, and advantages of the present application will become apparent from the specification, the drawings, and the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To better describe and illustrate the embodiments or examples provided by the present application, one or more drawings can be referred to. Additional details or examples used to describe the drawings should not be considered as limiting the scope of any of the disclosed application, the currently described embodiments or examples, and the currently understood best mode of these applications. Moreover, in all the drawings, the same reference numerals are used to represent the same components. In the drawings:

[0034] FIG. 1 is a capacity decay curve of a secondary battery in which all the positive electrode active material is lithium iron phosphate and part of it is lithium iron phosphate and part of it is passivated lithium iron phosphate;

[0035] FIG. 2 is a schematic diagram of a positive electrode sheet according to an embodiment of the present application;

[0036] FIG. 3 is a schematic diagram of a positive electrode sheet according to another embodiment of the present application;

[0037] FIG. 4 is a schematic diagram of a positive electrode sheet according to another embodiment of the present application;

[0038] FIG5 is a schematic diagram of a battery cell according to an embodiment of the present application;

[0039] FIG6 is an exploded view of the battery cell according to one embodiment of the present application shown in FIG5 ;

[0040] FIG. 7 is a schematic diagram of an electrical device using a secondary battery as a power source according to an embodiment of the present application.

[0041] Description of reference numerals:

[0042] 1. Positive electrode sheet; 11. First positive electrode active material; 12. Second positive electrode active material; 13. Positive electrode current collector; 121. Passivation layer; 5. Battery cell; 51. Housing; 52. Electrode assembly; 53. Cover plate; 6. Electrical device. DETAILED DESCRIPTION

[0043] Below, the embodiments of the positive electrode sheet, secondary battery and electrical device of the present application are described in detail with appropriate reference to the accompanying drawings. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present application and are not intended to limit the subject matter described in the claims.

[0044] The "ranges" disclosed in this application can be defined in the form of lower limits and upper limits. A given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of the particular range. Ranges defined in this way can be inclusive or exclusive of the end values, any end value can be included or excluded independently, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60 to 120 and 80 to 110 are listed for a particular parameter, it is understood that ranges of 60 to 110 and 80 to 120 are also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4, and 5 are also listed, the following ranges are all expected: 1 to 3, 1 to 4, 1 to 5, 2 to 3, 2 to 4, and 2 to 5. In this application, unless otherwise specified, the numerical range "a to b" is an abbreviation for any combination of real numbers between a and b, where a and b are both real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is merely an abbreviation for a combination of these values. Furthermore, when a parameter is expressed as an integer greater than or equal to 2, this is equivalent to listing the parameter as, for example, the integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on. For example, when a parameter is expressed as an integer selected from "2-10," this is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9, and 10.

[0045] In this application, "a plurality of" or "a plurality of" refers to a number greater than or equal to 2 unless otherwise specified. For example, "one or more" means one or more than or equal to two.

[0046] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0047] References to "embodiments" herein mean that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of such phrases in various locations in the specification does not necessarily refer to the same embodiment, nor does it necessarily refer to independent or alternative embodiments that are mutually exclusive with other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments. References to "implementations" herein have a similar understanding.

[0048] It will be appreciated by those skilled in the art that, in the methods of various embodiments or examples, the order in which the steps are written does not imply a strict order of execution and does not constitute any limitation on the implementation process, and the detailed order of execution of each step should be determined by its function and possible inherent logic. Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0049] In this application, in the open technical features or technical solutions described with words such as "contain", "include", and "include", unless otherwise specified, additional members other than the listed members are not excluded, and it can be regarded as providing both closed features or solutions consisting of the listed members and open features or solutions that also include additional members in addition to the listed members. For example, A includes a1, a2, and a3. Unless otherwise specified, it may also include other members or may not include additional members. It can be regarded as providing both the feature or solution of "A consists of a1, a2, and a3" and the feature or solution of "A includes not only a1, a2, and a3, but also other members". In this application, unless otherwise specified, A (such as B) means that B is a non-limiting example of A, and it can be understood that A is not limited to B.

[0050] In this application, the terms "optionally," "optional," and "optional" mean optional or dispensable, i.e., they refer to either option being selected from two parallel options: "with" or "without." If a technical solution contains multiple "optional" clauses, each "optional" clause is considered independent unless otherwise specified and there are no conflicts or constraints.

[0051] With the rapid development of secondary batteries, higher requirements have been placed on their cycle life. Improving the cycle life of secondary batteries has become an important research direction in this field. In this regard, the present application improves the positive electrode sheet to effectively extend the cycle life of secondary batteries.

[0052] In some embodiments, the first aspect of the present application provides a positive electrode plate 1, which includes a positive electrode active material layer, the positive electrode active material layer includes a first positive electrode active material 11 and a second positive electrode active material 12, the first positive electrode active material 11 and the second positive electrode active material 12 each independently include a lithium-containing transition metal oxide; and, the outer surface of the second positive electrode active material 12 includes a passivation layer 121; and / or the average particle size Dv50 of the second positive electrode active material 12 is greater than the average particle size Dv50 of the first positive electrode active material 11.

[0053] The above-mentioned positive electrode plate 1 of the present application makes the first positive electrode active material 11 and the second positive electrode active material 12 independently include a lithium-containing transition metal oxide, and the outer surface of the second positive electrode active material 12 includes a passivation layer 121, and / or the average particle size Dv50 of the second positive electrode active material 12 is greater than the average particle size Dv50 of the first positive electrode active material 11; the passivation layer 121 on the outer surface of the second positive electrode active material 12 can passivate the second positive electrode active material 12, so that the first discharge gram capacity of the second positive electrode active material 12 is reduced; during the battery cycle, the passivation layer 121 on the surface of the second positive electrode active material 12 will gradually break, so that the capacity of the second positive electrode active material 12 is gradually restored and released, forming a self-lithium replenishment effect, so that the cycle attenuation of the secondary battery is slowed down and the cycle life is improved.

[0054] Similarly, the average particle size Dv50 of the second positive electrode active material 12 is larger than that of the first positive electrode active material 11. Due to its larger particle size, the corresponding specific surface area is smaller, which reduces the active sites exposed to the electrolyte. This can also passivate the second positive electrode active material 12 and reduce the initial discharge capacity of the second positive electrode active material 12. During the battery cycle, the second positive electrode active material 12 will gradually crack, gradually exposing more active sites, so that the capacity of the second positive electrode active material 12 is gradually restored and released, which can also achieve a self-lithium replenishment effect, slowing the cycle decay of the secondary battery and increasing the cycle life.

[0055] It should be noted that the above-mentioned second positive electrode active material 12 having a passivation layer 121 on its outer surface and the second positive electrode active material 12 having a larger particle size do not conflict with each other and can be used simultaneously. In other words, the positive electrode active material layer can contain either the second positive electrode active material 12 including the passivation layer 121 or the second positive electrode active material 12 with a large particle size, or can contain both the second positive electrode active material 12 including the passivation layer 121 and the second positive electrode active material 12 with a large particle size. In some cases, the two passivation methods of coating passivation and large particle size passivation can also be combined. That is, the passivation layer 121 is coated on the surface of the second positive electrode active material 12 with a large particle size.

[0056] It can be understood that the first discharge gram capacity refers to the capacity per unit mass of the positive electrode active material at the end of the first discharge, and its unit is mAh / g.

[0057] Figure 1 shows the capacity decay curves of secondary batteries whose positive electrode active material layer is entirely lithium iron phosphate, and whose positive electrode active material layer is partially lithium iron phosphate and whose surface includes a passivation layer 121. Curves ① and ② in Figure 1 represent the capacity decay curves after cycling for the secondary battery whose positive electrode active material is entirely lithium iron phosphate; curves ③ and ④ represent the capacity decay curves for the secondary battery whose positive electrode active material is partially lithium iron phosphate and whose surface includes a passivation layer 121. As shown in Figure 1, the capacity decays rapidly in the initial cycle, as shown in curve ①, and then slowly decays, as shown in curve ②. In the secondary battery whose positive electrode active material is partially lithium iron phosphate and whose surface includes a passivation layer 121, the passivated material slowly activates in the initial cycle, continuously releasing lithium ions, forming activation curve ③. After activation is complete, the capacity decays normally, as shown in curve ④. The cycle life of the secondary battery is significantly improved compared to the secondary battery whose positive electrode active material is partially lithium iron phosphate and whose surface includes a passivation layer 121.

[0058] In some embodiments, the first positive electrode active material 11 and the second positive electrode active material 12 each independently include one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich lithium manganese oxide or lithium cobalt oxide.

[0059] In some embodiments, the first positive electrode active material 11 and the second positive electrode active material 12 are simultaneously selected from one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich lithium manganese oxide, or lithium cobalt oxide. Thus, the first positive electrode active material 11 and the second positive electrode active material 12 are made of the same material, which can reduce the adverse effects of mixing different types of positive electrode active materials on other performance of the secondary battery besides cycle life, such as safety, reliability, storage performance, and energy density.

[0060] In some specific examples, the first positive electrode active material 11 and the second positive electrode active material 12 are both lithium iron phosphate.

[0061] In some specific examples, the second positive electrode active material 12 has the same molecular formula as the first positive electrode active material 11. Using the same molecular formula for the second positive electrode active material 12 and the first positive electrode active material 11 can further mitigate the impact on battery performance other than cycle life. Furthermore, the first positive electrode active material 11 can be lithium iron phosphate; the second positive electrode active material 12 can be lithium iron phosphate with a passivation layer 121 on its surface, or lithium iron phosphate with an average particle size Dv50 greater than that of the first positive electrode active material 11, or a mixture of the two.

[0062] It should be noted that when the second positive electrode active material 12 is a positive electrode active material having a passivation layer 121 on its surface, the molecular formula of the second positive electrode active material 12 refers to the molecular formula of the positive electrode active material coated inside the passivation layer 121 .

[0063] In some embodiments, the positive electrode sheet 1 may further include a positive electrode current collector 13 , and the positive electrode active material layer is disposed on at least one surface of the positive electrode current collector 13 .

[0064] Referring to Figure 2, in some embodiments, the second positive electrode active material 12 is a positive electrode active material having a surface-coated passivation layer 121. Specifically, the passivation treatment is performed by coating the surface of the positive electrode active material with the passivation layer 121, thereby forming the second positive electrode active material 12. By coating the surface with the passivation layer 121, the initial discharge capacity per gram of the second positive electrode active material 12 can be reduced relative to the initial discharge capacity per gram of the positive electrode active material before the passivation treatment. Furthermore, during the secondary battery cycle, the passivation layer 121 is gradually activated, and its capacity is slowly released, resulting in a self-lithium replenishment effect, thereby extending the cycle life of the secondary battery.

[0065] In some embodiments, the passivation layer 121 may be one or more of an oxide passivation layer or a nitride passivation layer. Specifically, the passivation layer 121 may be one or more of an aluminum oxide layer, a zirconium oxide layer, a titanium oxide layer, or a titanium nitride layer. The passivation layer 121 described above can effectively passivate the surface of the positive electrode active material.

[0066] The above-mentioned passivation layer 121 can be formed by wet coating or atomic layer deposition (ALD). Specifically, in some embodiments, in the process of forming the passivation layer 121 by wet coating, the positive electrode active material is dispersed in a buffer solution of a certain pH, and ultrasonicated for a period of time to evenly disperse the material; then the passivation material solution is added, heated, filtered, washed, and dried, and then calcined at 400°C for 2 hours in a nitrogen atmosphere in an atmosphere furnace to obtain a second positive electrode active material 12 with a surface coated with a passivation layer 121. Among them, the buffer solution can be a formic acid-ammonium formate buffer solution with a pH of 3.8 to 4.5; the passivation material solution can be a salt solution such as aluminum sulfate. Aluminum sulfate can provide an aluminum donor, precipitate aluminum hydroxide in the buffer solution, and form an aluminum oxide coating layer after sintering.

[0067] It should be noted that when the passivation layer 121 is prepared by atomic layer deposition, since the density of the passivation layer 121 is relatively large, the passivation layer 121 is difficult to break during the battery cycle. Therefore, the thickness of the passivation layer 121 needs to be set relatively thin, generally around 2 nm.

[0068] In some embodiments, the thickness of the passivation layer 121 is 1 nm to 50 nm. The thickness of the passivation layer 121 coated on the surface of the second positive electrode active material 12 affects the passivation effect of the positive electrode active material, as well as the cracking and capacity release of the passivation layer 121 during battery cycling. By controlling the thickness of the passivation layer 121 on the surface of the second positive electrode active material 12 within the above range, while achieving a good passivation effect, the capacity of the active material within the passivation layer 121 can be effectively released during subsequent cycles.

[0069] It can be understood that the thickness of the passivation layer 121 on the surface of the second positive electrode active material 12 can be 1nm, 2nm, 5nm, 8nm, 10nm, 12nm, 15nm, 18nm, 20nm, 22nm, 25nm, 28nm, 30nm, 32nm, 35nm, 38nm, 40nm, 42nm, 45nm, 48nm, 50nm and any value within the range formed by any two of the above values.

[0070] In some embodiments, the thickness of the passivation layer 121 is 3 nm to 10 nm. Controlling the thickness of the passivation layer 121 on the surface of the second positive electrode active material 12 within the above range can achieve a good passivation effect while allowing the capacity of the active material inside the passivation layer 121 to be better released during the cycle process.

[0071] Referring to FIG. 3 , in some embodiments, the second positive electrode active material 12 has the same molecular formula as the first positive electrode active material 11, and the particle size of the second positive electrode active material 12 is larger than that of the first positive electrode active material 11. That is, the first positive electrode active material 11 and the second positive electrode active material 12 are the same positive electrode active material. By using the same positive electrode active material with a larger particle size than the first positive electrode active material 11 as the second positive electrode active material 12, the second positive electrode active material 12 has a smaller specific surface area due to its larger particle size, which reduces its active sites exposed to the electrolyte. During the cycling process, the second positive electrode active material 12 expands and contracts, and the second positive electrode active material 12 with larger particles will crack, exposing more active sites and releasing capacity, thereby achieving the effect of improving the cycle life of the secondary battery.

[0072] It should be noted that the second positive electrode active material 12 having a larger particle size than the first positive electrode active material 11 can be obtained by increasing the particle size of the precursor used in preparing the positive electrode active material, or by performing secondary sintering on the first positive electrode active material 11.

[0073] In some embodiments, the ratio of the average particle size Dv50 of the second positive electrode active material 12 to the average particle size Dv50 of the first positive electrode active material 11 is 2 to 18. Controlling the ratio of the average particle size Dv50 of the second positive electrode active material 12 to the first positive electrode active material 11 within the above range can effectively passivate the second positive electrode active material 12 and effectively release its capacity during cycling. It is understood that the ratio of the average particle size Dv50 of the second positive electrode active material 12 to the first positive electrode active material 11 can be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, and any value within the range formed by any two of the above values.

[0074] In some specific examples, the first positive electrode active material 11 and the second positive electrode active material 12 are both lithium iron phosphate, the average particle size Dv50 of the first positive electrode active material 11 is 0.5 μm to 3 μm, and the average particle size Dv50 of the second positive electrode active material 12 is 1.5 μm to 9 μm.

[0075] In some specific examples, the first positive electrode active material 11 and the second positive electrode active material 12 are both nickel-cobalt-manganese ternary materials with the morphology of single crystal particles, the average particle size Dv50 of the first positive electrode active material 11 is 3μm~7μm, and the average particle size Dv50 of the second positive electrode active material 12 is 9μm~21μm.

[0076] In some specific examples, the first positive electrode active material 11 and the second positive electrode active material 12 are both nickel-cobalt-manganese ternary materials with the morphology of polycrystalline particles, the average particle size Dv50 of the first positive electrode active material 11 is 8μm~20μm, and the average particle size Dv50 of the second positive electrode active material 12 is 20μm~40μm.

[0077] In some embodiments, the first discharge gram capacity of the first positive electrode active material 11 is C1, and the unit is mAh / g; the first discharge gram capacity of the second positive electrode active material 12 is C2, and the unit is mAh / g; based on the total mass of the positive electrode active material layer, the mass fraction of the first positive electrode active material 11 is W1, and the mass fraction of the second positive electrode active material 12 is W2; and the positive electrode sheet 1 satisfies: C1*W1+C2*W2=k*C1*(W1+W2), where 0.8≤k<1.

[0078] In the above relationship, k represents the percentage of the electrode sheet capacity using both the first positive electrode active material 11 and the second positive electrode active material 12 to the electrode sheet capacity using the first positive electrode active material 11 alone; C1*W1+C2*W2 represents the weighted capacity of the first positive electrode active material 11 and the second positive electrode active material 12; C1*(W1+W2) represents the electrode sheet capacity using the first positive electrode active material 11 alone.

[0079] When the total amount of positive electrode active material in the positive electrode active material layer remains unchanged, the capacity of the electrode sheet will decrease after adding part of the second positive electrode active material 12; the positive electrode sheet 1 is controlled according to the relationship C1*W1+C2*W2=k*C1*(W1+W2), 0.8≤k<1. The addition ratio of the second positive electrode active material 12 can effectively improve the cycle life of the battery while making the battery have a relatively low DC impedance.

[0080] It can be understood that in the above relationship, the value of k can be 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 and any value within the range formed by any two of the above values.

[0081] In some embodiments, 0.9≤k≤0.97. In this way, the secondary battery can have a lower DC impedance while further improving the cycle life of the secondary battery, achieving a better balance between battery cycle life and DC impedance, and providing the secondary battery with better overall performance.

[0082] In some of these embodiments, 12% ≤ W1 < 100%, 0% < W2 ≤ 85%. Thus, while effectively improving the battery cycle life, the battery has a relatively low DC impedance. It can be understood that the mass fraction W1 of the first positive electrode active material 11 can be 12%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, 98%, 99%, and any value within the range formed by any two of the above values. The mass fraction W2 of the second positive electrode active material 12 can be 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, and any value within the range formed by any two of the above values.

[0083] In some of these embodiments, 60% ≤ W1 < 100%, 0% < W2 ≤ 40%. Controlling W1 and W2 within the above ranges can effectively improve the battery cycle life while further reducing the DC impedance of the battery and making the capacity of the positive electrode active material easier to release and recover during subsequent cycles.

[0084] It should be noted that in the positive electrode active material layer, the first positive electrode active material 11 and the second positive electrode active material 12 can be mixed with each other to form a single-layer structure in which the first positive electrode active material 11 and the second positive electrode active material 12 are evenly distributed; or the first positive electrode active material 11 and the second positive electrode active material 12 can be separated as a two-layer laminated structure.

[0085] Please refer to FIGS. 2 and 3. In some of these embodiments, the first positive electrode active material 11 and the second positive electrode active material 12 in the positive electrode active material layer are mixed with each other. That is, the first positive electrode active material 11 and the second positive electrode active material 12 are mixed with each other to form a positive electrode active material layer with a single-layer structure in which the first positive electrode active material 11 and the second positive electrode active material 12 are evenly distributed.

[0086] Specifically, the above-mentioned positive electrode active material layer with a single-layer structure can be formed by mixing the first positive electrode active material 11 and the second positive electrode active material 12 in a positive electrode slurry and coating it on the positive electrode current collector 13. The coating process of its positive electrode slurry is relatively simple.

[0087] Referring to FIG. 4 , in some embodiments, the positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer stacked together. The first positive electrode active material layer includes a first positive electrode active material 11, and the second positive electrode active material layer includes a second positive electrode active material 12. Thus, the first positive electrode active material 11 and the second positive electrode active material 12 are located in different layers.

[0088] Specifically, the aforementioned stacked positive electrode active material layer can be formed using a layered coating process. That is, a positive electrode slurry containing one positive electrode active material is first coated onto the positive electrode current collector 13 to form a first positive electrode active material layer; then, a positive electrode slurry containing another positive electrode active material is coated onto the first positive electrode active material layer to form a second positive electrode active material layer. While the layered coating process is relatively complex, the conductive network formed by the first positive electrode active material 11 is not affected by the passivation of the second positive electrode active material 12, resulting in a higher capacity than a single-layer positive electrode active material layer.

[0089] It should be noted that in the aforementioned stacked structure of positive electrode active material layers, there is no restriction on the number of single active material layers, but the number of layers should not be too large to avoid significantly increasing the difficulty of the coating process. There is also no restriction on the order in which the first and second positive electrode active material layers are applied; the first positive electrode active material layer can be applied first, followed by the second positive electrode active material layer; the second positive electrode active material layer can be applied first, followed by the first positive electrode active material layer; or the first and second positive electrode active material layers can be applied alternately.

[0090] In some embodiments, the second aspect of the present application provides a secondary battery comprising the positive electrode sheet 1 of the first aspect of the present application. By employing the positive electrode sheet 1 of the first aspect of the present application, the secondary battery of the present application has a long cycle life. It is understood that the secondary battery may further comprise a negative electrode sheet, a separator, an electrolyte, and the like.

[0091] In some embodiments, a third aspect of the present application provides an electrical device, which includes the secondary battery of the second aspect of the present application.

[0092] The secondary battery and the electric device of the present application will be described below with reference to the accompanying drawings as appropriate.

[0093] Unless otherwise specified, the components, material types, or contents of the batteries mentioned are applicable to both lithium-ion secondary batteries and sodium-ion secondary batteries.

[0094] In one embodiment of the present application, a secondary battery is provided.

[0095] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During the battery's charge and discharge processes, active ions are inserted and removed between the positive and negative electrodes. The electrolyte conducts ions between the positive and negative electrodes. The separator, located between the positive and negative electrodes, primarily prevents short circuits between the positive and negative electrodes while allowing ions to pass through.

[0096] Positive electrode

[0097] The positive electrode sheet includes a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector.

[0098] As a non-limiting example, the positive electrode current collector has two surfaces opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive electrode current collector.

[0099] In some embodiments, the positive electrode current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be obtained by forming a metal material on a polymer material substrate. In the positive electrode current collector, non-limiting examples of the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In the positive electrode current collector, non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0100] In some embodiments, the positive electrode active material may include a positive electrode active material for a battery known in the art.

[0101] As a non-limiting example, the positive electrode active material of the lithium-ion secondary battery may include one or more of the following materials: lithium-containing phosphates with an olivine structure, lithium transition metal oxides and their respective modified compounds. However, the present application is not limited to these materials, and other traditional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Among them, examples of lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide and their modified compounds. Non-limiting examples of lithium-containing phosphates with an olivine structure may include, but are not limited to, one or more of lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, and a composite material of lithium manganese iron phosphate and carbon. Non-limiting examples of lithium cobalt oxide may include LiCoO2; non-limiting examples of lithium nickel oxide may include LiNiO2; non-limiting examples of lithium manganese oxide may include LiMnO2, LiMn2O4, etc.; non-limiting examples of lithium nickel cobalt manganese oxide may include LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also referred to as NCM 333 ), LiNi 0.5 Co 0.2 Mn 0.3 O2 (also referred to as NCM 523 ), LiNi 0.5 Co 0.25 Mn 0.25 O2 (also referred to as NCM 211 ), LiNi 0.6 Co 0.2 Mn 0.2 O2 (also referred to as NCM 622 ), LiNi 0.8 Co 0.1 Mn 0.1 O2 (also referred to as NCM 811 ) etc.; Non-limiting examples of lithium nickel cobalt aluminum oxide may include LiNi 0.85 Co 0.1 Al 0.05 O2.

[0102] It is understandable that the battery will be accompanied by lithium (Li) deintercalation and consumption during the charge and discharge process, and the content of Li in the positive electrode plate is different when the battery is discharged to different states. In the list of positive electrode materials in this application, unless otherwise specified, the Li content is the initial state of the material. The positive electrode material is applied to the positive electrode plate in the battery system, and after the charge and discharge cycle, the Li content in the positive electrode material contained in the plate will usually change. Among them, the Li content can be measured by molar content, but is not limited to this. Regarding "the Li content is the initial state of the material", the initial state of the material refers to the state before the material is added to the positive electrode slurry. It is understandable that new materials obtained by appropriate modification on the basis of the listed positive electrode materials are also within the scope of positive electrode materials. The aforementioned appropriate modification refers to acceptable modification methods for positive electrode materials, and non-limiting examples include coating modification.

[0103] In the examples of positive electrode materials in this application, the oxygen (O) content is only a theoretical value. Lattice oxygen release will cause the molar content of oxygen to change, and the actual O content will fluctuate. The O content can be measured by molar content, but is not limited to this.

[0104] As non-limiting examples, the positive electrode active material of a sodium ion secondary battery may include one or more of the following materials: sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. However, the present application is not limited to these materials, and other conventionally known materials that can be used as positive electrode active materials for sodium ion batteries may also be used.

[0105] As an optional technical solution of the present application, in the sodium transition metal oxide, the transition metal can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Sodium transition metal oxide is, for example, Na x MO2, wherein M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, 0 <x≤1。

[0106] As an optional technical solution of the present application, the polyanionic compound can be a compound having sodium ions, transition metal ions and tetrahedral (YO4) n- A class of compounds with anionic units. The transition metal can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; Y can be one or more of P, S and Si; n represents (YO4) n- valence.

[0107] Polyanionic compounds can also be sodium ions, transition metal ions, tetrahedral (YO4) n-A class of compounds containing anion units and halogen anions. The transition metal can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be one or more of P, S, and Si, and n represents (YO4) n- valence state; the halogen can be one or more of F, Cl and Br.

[0108] Polyanionic compounds can also be sodium ions, tetrahedral (YO4) n- Anion unit, polyhedron unit (ZO y ) m+ and an optional halogen anion. Y can be one or more of P, S and Si, and n represents (YO4) n- valence state; Z represents a transition metal, which can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce; m represents (ZO y ) m+ valence state; the halogen can be one or more of F, Cl and Br.

[0109] Polyanionic compounds include NaFePO4, Na3V2(PO4)3 (sodium vanadium phosphate, abbreviated as NVP), Na4Fe3(PO4)2(P2O7), NaM'PO4F (M' is one or more of V, Fe, Mn and Ni) and Na3(VO y )2(PO4)2F 3-2y One or more of (0≤y≤1).

[0110] Prussian blue compounds can be sodium ions, transition metal ions and cyanide ions (CN - ) compounds. The transition metal can be one or more of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr and Ce. Prussian blue compounds are, for example, Na a Me b Me' c (CN)6, wherein Me and Me' are each independently one or more of Ni, Cu, Fe, Mn, Co and Zn, 0 <a≤2,0<b<1,0<c<1。

[0111] The weight ratio of the positive electrode active material in the positive electrode film layer is 80 wt % to 100 wt %, based on the total weight of the positive electrode film layer.

[0112] In some embodiments, the positive electrode film layer may further optionally include a binder. As a non-limiting example, the binder may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorine-containing acrylate resin. The weight ratio of the binder in the positive electrode film layer is 0% to 20% by weight, based on the total weight of the positive electrode film layer.

[0113] In some embodiments, the positive electrode film layer may further optionally include a conductive agent. As non-limiting examples, the conductive agent may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The weight ratio of the conductive agent in the positive electrode film layer is 0% to 20% by weight, based on the total weight of the positive electrode film layer.

[0114] In some embodiments, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet, such as the positive electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry, wherein the positive electrode slurry has a solid content of 40wt% to 80wt%, and the viscosity at room temperature is adjusted to 5000mPa·s to 25000mPa·s, the positive electrode slurry is coated on the surface of the positive electrode collector, and after drying, the positive electrode sheet is formed by cold rolling; the positive electrode powder coating unit area density is 150mg / m 2 ~350 mg / m 2 The compaction density of the positive electrode is 3.0g / cm 3 ~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 .

[0115] The calculation formula of the compacted density is:

[0116] Compaction density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0117] The mass M of the positive electrode active material per unit area of ​​the positive electrode membrane can be obtained by weighing using a standard balance.

[0118] The thickness T of the positive electrode film can be measured using a micrometer, for example, a Mitutoyo 293-100 with an accuracy of 0.1 μm. It should be noted that the thickness of the positive electrode film described in this application refers to the thickness of the positive electrode film in the positive electrode sheet after cold pressing and used in battery assembly.

[0119] Negative electrode

[0120] The negative electrode sheet includes a negative electrode current collector and a negative electrode film layer provided on at least one surface of the negative electrode current collector, wherein the negative electrode film layer includes a negative electrode active material.

[0121] As a non-limiting example, the negative electrode current collector has two surfaces opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0122] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be obtained by forming a metal material on a polymer material substrate. Among them, in the negative electrode current collector, non-limiting examples of the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. In the negative electrode current collector, non-limiting examples of the polymer material substrate may include one or more of substrates such as polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.

[0123] In some embodiments, the negative electrode active material may be a negative electrode active material for batteries known in the art.

[0124] As a non-limiting example, the negative electrode active material of the lithium-ion secondary battery may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials and lithium titanate. Silicon-based materials may include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxides and tin alloys. However, the present application is not limited to these materials, and other traditional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0125] As a non-limiting example, the negative active material of the sodium ion secondary battery is generally a hard carbon material, a two-dimensional metal carbide or a nitride. Preferably, the negative active material of the sodium ion secondary battery is generally a hard carbon material.

[0126] In some embodiments, the negative electrode film layer may further optionally include a binder. The binder may include one or more of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0127] In some embodiments, the negative electrode film layer may further include a conductive agent, which may include one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0128] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0129] In some embodiments, the negative electrode sheet can be prepared in the following manner: the components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components, are dispersed in a solvent (a non-limiting example of the solvent is deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on at least one side of the negative electrode collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained. The surface of the negative electrode collector coated with the negative electrode slurry can be a single surface of the negative electrode collector or on both surfaces of the negative electrode collector. The solid content of the negative electrode slurry can be 40wt% to 60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s to 10000mPa·s. When coating the negative electrode slurry, the coating unit surface density on a dry weight basis (excluding the solvent) can be 75g / m 2 ~220g / m 2 The compaction density of the negative electrode can be 1.0g / cm 3 ~1.8g / cm 3 .

[0130] electrolytes

[0131] The electrolyte conducts ions between the positive and negative electrodes. This application does not specifically limit the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0132] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0133] In some embodiments, the electrolyte salt of the lithium ion secondary battery may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0134] In some embodiments, the solvent may include one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0135] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0136] In some embodiments, the additives in the electrolyte may include, but are not limited to, one or more of fluoroethylene carbonate (FEC), difluoroethylene carbonate (DFEC), trifluoromethylethylene carbonate (TFPC), and the like.

[0137] Isolation film

[0138] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical and mechanical stability can be selected.

[0139] In some embodiments, the material of the separator can include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0140] In some embodiments, the isolation film has a thickness of 6 μm to 40 μm, and may optionally be 12 μm to 20 μm.

[0141] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly through a winding process or a lamination process.

[0142] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0143] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery can also be a soft shell, such as a pouch-type soft shell. The material of the soft shell can be plastic. Further, non-limiting examples of plastic can include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0144] A secondary battery includes at least one battery cell. A secondary battery may include one or more battery cells.

[0145] In this application, unless otherwise specified, a "battery cell" refers to a basic unit that can achieve the mutual conversion of chemical energy and electrical energy. Further, generally speaking, it includes at least a positive electrode plate, a negative electrode plate, and an electrolyte. During the battery's charge and discharge process, active ions are embedded in and out of the positive and negative electrode plates. The electrolyte plays the role of conducting active ions between the positive and negative electrode plates.

[0146] The present application has no particular limitation on the shape of the battery cell, which can be cylindrical, square, or any other shape. For example, FIG5 shows a battery cell 5 with a square structure as an example.

[0147] In some embodiments, referring to Figure 6, the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 can be covered on the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation membrane can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the battery cell 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0148] In some embodiments, the battery cells 5 can be assembled into a battery module. The number of battery cells 5 contained in the battery module can be one or more. The specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0149] In the battery module, the plurality of battery cells 5 can be arranged in sequence along the length of the battery module. Of course, they can also be arranged in any other manner. Furthermore, the plurality of battery cells 5 can be fixed by fasteners.

[0150] Optionally, the battery module may further include a housing having an accommodation space, and the plurality of battery cells 5 are accommodated in the accommodation space.

[0151] In some embodiments, the battery modules described above may also be assembled into a battery pack. The battery pack may contain one or more battery modules, and the specific number may be selected by those skilled in the art based on the application and capacity of the battery pack.

[0152] A battery pack may include a battery box and multiple battery modules disposed within the box. The battery box comprises an upper case and a lower case. The upper case can be placed over the lower case to form an enclosed space for accommodating the battery modules. The multiple battery modules can be arranged in any manner within the battery box.

[0153] In addition, the present application also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack provided in the present application. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but is not limited thereto.

[0154] As the electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0155] FIG7 shows an example of an electric device 6. The electric device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the electric device's requirements for high power and high energy density of secondary batteries, a battery pack or battery module may be used.

[0156] Another example device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is generally required to be lightweight and thin, and may use a secondary battery as a power source.

[0157] The following are some examples.

[0158] In order to make the technical problems, technical solutions and beneficial effects solved by this application clearer, the application will be further described in detail below with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of this application, 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 application and its applications. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0159] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0160] Example 1:

[0161] 1) Preparation of positive electrode sheet

[0162] Lithium iron phosphate (LiFePO4), lithium iron phosphate (LiFePO4) coated with an aluminum oxide passivation layer, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) are stirred and mixed in an NMP solvent in a weight ratio of 71.8:25.2:0.8:2.2 to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on the positive electrode current collector aluminum foil; after drying, cold pressing, and slitting, the positive electrode sheet is obtained.

[0163] The first discharge capacity in grams C1 of the lithium iron phosphate used is 156 mAh / g, and the first discharge capacity in grams C2 of the lithium iron phosphate used with an aluminum oxide passivation layer coated on the surface is 120 mAh / g; k = 0.94; since the second positive electrode active material and the first positive electrode active material before passivation treatment both use lithium iron phosphate (LiFePO4), the discharge potential difference relative to the lithium metal electrode at a discharge current of 0.33C and a temperature of 25°C is 0; the average particle size Dv50 of the lithium iron phosphate is 1.5 μm; the thickness of the passivation layer is 5 nm; since the thickness of the passivation layer is very small, the average particle size Dv50 of the lithium iron phosphate material after the surface is coated with the aluminum oxide passivation layer can be considered to be basically unchanged, that is, 1.5 μm.

[0164] 2) Preparation of negative electrode sheet

[0165] The active material artificial graphite, the conductive agent carbon black, the binder styrene-butadiene rubber (SBR), and the thickener sodium carboxymethyl cellulose (CMC) are dissolved in the solvent deionized water in a weight ratio of 96.9:0.4:1.5:1.2, and mixed evenly to prepare the negative electrode slurry; the negative electrode slurry is evenly coated on the negative electrode current collector copper foil, and the negative electrode sheet is obtained after drying, cold pressing, and slitting.

[0166] 3) Electrolyte preparation

[0167] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), organic solvents ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 3:7, and LiPF6 accounting for 12.5% ​​of the mass fraction of the electrolyte was added and dissolved in the organic solvent, and stirred evenly to obtain an electrolyte.

[0168] 4) Isolation film

[0169] Polyethylene film is used as the isolation film.

[0170] 5) Battery assembly

[0171] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator positioned between the positive and negative electrodes to provide insulation. The cells are then wound to form a bare cell. The tabs are then welded to the bare cell and placed in an aluminum shell. The shell is then baked at 80°C to remove moisture. The electrolyte is then injected and sealed to produce an uncharged battery. The uncharged battery then undergoes a series of processes, including resting, hot and cold pressing, formation, shaping, and capacity testing, to produce a lithium-ion secondary battery.

[0172] Example 2:

[0173] This embodiment is basically the same as embodiment 1, with the only difference being that the weight ratio of lithium iron phosphate, lithium iron phosphate coated with an aluminum oxide passivation layer, conductive agent carbon black, and binder polyvinylidene fluoride (PVDF) in the positive electrode slurry is 34:63:0.8:2.2; k=0.85.

[0174] Example 3:

[0175] This embodiment is basically the same as embodiment 1, except that the weight ratio of lithium iron phosphate, lithium iron phosphate coated with an aluminum oxide passivation layer, conductive agent carbon black, and binder polyvinylidene fluoride in the positive electrode slurry is 12.9:84.1:0.8:2.2; k=0.80.

[0176] Example 4:

[0177] This embodiment is basically the same as embodiment 1, with the only difference being that the weight ratio of lithium iron phosphate, lithium iron phosphate coated with an aluminum oxide passivation layer, conductive agent carbon black, and binder polyvinylidene fluoride in the positive electrode slurry is 84.4:12.6:0.8:2.2; k=0.97.

[0178] Example 5:

[0179] This embodiment is basically the same as embodiment 1 except that: lithium nickel cobalt manganese oxide (LiNi 0.5 Co 0.2 Mn 0.3 O2) instead of the lithium iron phosphate in Example 1; lithium nickel cobalt manganese oxide (LiNi) coated with an aluminum oxide passivation layer 0.5 Co 0.2 Mn 0.3 O2) replaces the lithium iron phosphate with an aluminum oxide passivation layer on the surface in Example 1; the first discharge gram capacity C1 of the lithium nickel cobalt manganese oxide used is 174 mAh / g, and the first discharge gram capacity C2 of the lithium nickel cobalt manganese oxide with an aluminum oxide passivation layer used is 145 mAh / g; k = 0.96.

[0180] Example 6:

[0181] This embodiment is basically the same as embodiment 1 except that: lithium nickel cobalt manganese oxide (LiNi 0.5 Co 0.2 Mn 0.3 O2) instead of the lithium iron phosphate in Example 1; lithium manganese iron phosphate (LiFe 0.6 Mn 0.4 PO4) replaces the lithium iron phosphate with an aluminum oxide passivation layer on the surface in Example 1; the first discharge gram capacity C1 of the lithium nickel cobalt manganese oxide used is greater than the first discharge gram capacity C2 of the lithium manganese iron phosphate with an aluminum oxide passivation layer used; k = 0.96.

[0182] Example 7:

[0183] This embodiment is basically the same as Example 1, with the only difference being that lithium iron phosphate with a titanium nitride passivation layer coated on its surface is used instead of the lithium iron phosphate with an aluminum oxide passivation layer coated on its surface in Example 1; the first discharge capacity C2 of the lithium iron phosphate with a titanium nitride passivation layer coated on its surface is 122 mAh / g, and k=0.94.

[0184] Example 8:

[0185] This embodiment is basically the same as Example 1, with the only difference being that lithium iron phosphate with a zirconium oxide passivation layer coated on its surface is used instead of the lithium iron phosphate with an aluminum oxide passivation layer coated on its surface in Example 1; the first discharge capacity C2 of the lithium iron phosphate with a zirconium oxide passivation layer coated on its surface is 121 mAh / g, and k=0.94.

[0186] Example 9:

[0187] This embodiment is basically the same as Example 1, with the only difference being that in the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface, the thickness of the aluminum oxide passivation layer on the surface of the lithium iron phosphate is 1 nm; the first discharge capacity C2 of the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface is 145 mAh / g, and k=0.98.

[0188] Example 10:

[0189] This embodiment is basically the same as Example 1, with the only difference being that: in the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface, the thickness of the aluminum oxide passivation layer on the surface of the lithium iron phosphate is 3 nm; the first discharge capacity C2 of the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface is 134 mAh / g, and k=0.96.

[0190] Example 11:

[0191] This embodiment is basically the same as Example 1, with the only difference being that in the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface, the thickness of the aluminum oxide passivation layer on the surface of the lithium iron phosphate is 10 nm; the first discharge capacity C2 of the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface is 96 mAh / g, and k=0.9.

[0192] Example 12:

[0193] This embodiment is basically the same as Example 1, with the only difference being that: in the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface, the thickness of the aluminum oxide passivation layer on the surface of the lithium iron phosphate is 25 nm; the first discharge capacity C2 of the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface is 72 mAh / g, and k=0.86.

[0194] Example 13:

[0195] This embodiment is basically the same as Example 1, with the only difference being that: in the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface, the thickness of the aluminum oxide passivation layer on the surface of the lithium iron phosphate is 50 nm; the first discharge capacity C2 of the lithium iron phosphate with an aluminum oxide passivation layer coated on the surface is 38 mAh / g, and k=0.8.

[0196] Example 14:

[0197] This embodiment is basically the same as Example 1, with the only difference being that large-particle lithium iron phosphate (LiFePO4) is used in the positive electrode slurry instead of the lithium iron phosphate (LiFePO4) coated with an aluminum oxide passivation layer in Example 1; the average particle size Dv50 of the large-particle lithium iron phosphate is 6.2 μm; the weight ratio of small-particle lithium iron phosphate, large-particle lithium iron phosphate, conductive agent carbon black, and binder polyvinylidene fluoride in the positive electrode slurry is 73.7:23.3:0.8:2.2; the first discharge gram capacity C2 of the large-particle lithium iron phosphate is 117 mAh / g; k=0.94.

[0198] Example 15:

[0199] This embodiment is basically the same as Example 14, with the only difference being that: the average particle size Dv50 of the large-particle lithium iron phosphate is 1.5 μm, and the corresponding average particle size Dv50 of the small-particle lithium iron phosphate is 0.5 μm; the first discharge capacity C1 of the small-particle lithium iron phosphate is 160 mAh / g, and the first discharge capacity C2 of the large-particle lithium iron phosphate is 156 mAh / g; k = 0.99.

[0200] Example 16:

[0201] This embodiment is basically the same as Example 14, with the only difference being that: the average particle size Dv50 of the large-particle lithium iron phosphate is 9 μm, and the corresponding average particle size Dv50 of the small-particle lithium iron phosphate is 3 μm; the first discharge capacity C1 of the small-particle lithium iron phosphate is 154 mAh / g, and the first discharge capacity C2 of the large-particle lithium iron phosphate is 130 mAh / g; k = 0.96.

[0202] Example 17:

[0203] This embodiment is basically the same as the embodiment 1, except that the positive electrode slurry uses a nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 O2) instead of the lithium iron phosphate in Example 1; a nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 O2) replaces the lithium iron phosphate with a surface coated with an aluminum oxide passivation layer in Example 1; the average particle size Dv50 of the small-particle nickel-cobalt-manganese ternary material is 3 μm, and the average particle size Dv50 of the large-particle nickel-cobalt-manganese ternary material is 9 μm; the first discharge gram capacity C1 of the small-particle nickel-cobalt-manganese ternary material is 182 mAh / g, and the first discharge gram capacity C2 of the large-particle nickel-cobalt-manganese ternary material is 165 mAh / g; k = 0.98.

[0204] Example 18:

[0205] This embodiment is basically the same as embodiment 17 except that: the small particle nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 The average particle size Dv50 of large-particle nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 The average particle size Dv50 of the small-particle nickel-cobalt-manganese ternary material is 21 μm; the first discharge gram capacity C1 of the small-particle nickel-cobalt-manganese ternary material is 174 mAh / g, and the first discharge gram capacity C2 of the large-particle nickel-cobalt-manganese ternary material is 146 mAh / g; k = 0.96.

[0206] Example 19:

[0207] This embodiment is basically the same as the embodiment 1, except that the positive electrode slurry uses a nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3O2) instead of the lithium iron phosphate in Example 1; a nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 O2) replaces the lithium iron phosphate with a surface coated with an aluminum oxide passivation layer in Example 1; the average particle size Dv50 of the small-particle nickel-cobalt-manganese ternary material is 8 μm, and the average particle size Dv50 of the large-particle nickel-cobalt-manganese ternary material is 20 μm; the first discharge gram capacity C1 of the small-particle nickel-cobalt-manganese ternary material is 170 mAh / g, and the first discharge gram capacity C2 of the large-particle nickel-cobalt-manganese ternary material is 148 mAh / g; k = 0.96.

[0208] Example 20:

[0209] This embodiment is basically the same as embodiment 19 except that: the small particle nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 The average particle size Dv50 of large-particle nickel-cobalt-manganese ternary material (LiNi O2) is 20 μm. 0.5 Co 0.2 Mn 0.3 The average particle size Dv50 of the nickel-cobalt-manganese ternary material is 40 μm; the first discharge capacity C1 of the small-particle nickel-cobalt-manganese ternary material is 148 mAh / g, and the first discharge capacity C2 of the large-particle nickel-cobalt-manganese ternary material is 120 mAh / g; k = 0.95.

[0210] Example 21:

[0211] 1) Preparation of positive electrode sheet

[0212] Lithium iron phosphate (LiFePO4), conductive agent carbon black, and binder polyvinylidene fluoride are stirred and mixed in an NMP solvent in a weight ratio of 97:0.8:2.2 to obtain positive electrode slurry I; lithium iron phosphate (LiFePO4) with an aluminum oxide passivation layer on the surface, conductive agent carbon black, and binder polyvinylidene fluoride are stirred and mixed in an NMP solvent in a weight ratio of 97:0.8:2.2 to obtain positive electrode slurry II; the above-mentioned positive electrode slurry I is evenly coated on the positive electrode current collector aluminum foil and dried; then the above-mentioned positive electrode slurry II is evenly coated on the positive electrode slurry I coating; drying, cold pressing, and cutting are carried out to obtain a positive electrode sheet with a double-layer stacked structure of positive electrode active material layer.

[0213] The quality of the lithium iron phosphate and the lithium iron phosphate coated with an aluminum oxide passivation layer used is the same as that in Example 1.

[0214] 2) negative electrode sheet preparation, 3) electrolyte preparation, 4) separator preparation and 5) battery assembly steps are the same as in Example 1.

[0215] Comparative Example 1:

[0216] This comparative example is basically the same as Example 1, except that the positive electrode slurry does not contain the lithium iron phosphate material with an aluminum oxide passivation layer on the surface, and the lithium iron phosphate material with an aluminum oxide passivation layer on the surface in Example 1 is completely replaced by lithium iron phosphate.

[0217] Comparative Example 2:

[0218] This comparative example is basically the same as Example 18 except that the positive electrode slurry does not contain large particles of nickel-cobalt-manganese ternary material (LiNi 0.5 Co 0.2 Mn 0.3 O2); the positive electrode active materials are all made of small particles of nickel-cobalt-manganese ternary materials (LiNi 0.5 Co 0.2 Mn 0.3 O2).

[0219] Test method:

[0220] 1. Volume average particle size Dv50 test

[0221] Equipment model: Malvern 2000 (MasterSizer 2000) laser particle size analyzer; Reference standard process: GB / T19077-2016 / ISO 13320:2009.

[0222] Specific test process: Take an appropriate amount of the sample to be tested (the sample concentration is sufficient to ensure 8% to 12% shading), add 20mL of NMP, and simultaneously operate under external ultraviolet light for 5 minutes (53KHz / 120W) to ensure that the sample is completely dispersed. Then, measure the sample according to GB / T19077-2016 / ISO13320:2009 standard.

[0223] 2. Coating passivation layer thickness test

[0224] The electrode was cut into 6cm x 6cm samples using scissors and polished using an IB-19500CP ion cross-section polisher to obtain polished samples with cut surfaces. The samples were then tested using a ZEISS Sigma 300 instrument in accordance with standard JY / T010-1996. Ten locations were randomly selected within the test sample for testing, and EDS was used to identify the coating elements.

[0225] 3. Positive electrode active material content test

[0226] Take a 5cm*5cm electrode and scrape the powder on the substrate without leaking the substrate. Perform a thermogravimetric test on the scraped powder in an ambient gas atmosphere with a heating rate of 5℃ / min and a temperature range of 25~600℃. The remaining mass percentage is the mass of the active material.

[0227] 4. First discharge capacity test of positive electrode active material

[0228] The electrode is punched into small discs with a diameter of 14 mm, and assembled into a button battery together with the separator, lithium sheet, and electrolyte. Charge and discharge for one week at room temperature at a charge and discharge rate of 0.1C / 0.1C within the appropriate voltage range (LFP2.5~3.65V, NCM2.8~4.25V). The first discharge capacity in grams = discharge capacity / mass of active material.

[0229] 5. Battery capacity retention test

[0230] At 25°C, charge the battery at a constant current of 1 / 3C to 3.65V, then charge at a constant voltage of 3.65V to a current of 0.05C, let it rest for 5 minutes, and then discharge it at 1 / 3C to 2.5V. The resulting capacity is recorded as the initial capacity C0. Repeat these steps for the same battery and simultaneously record the battery's discharge capacity Cn after the nth cycle; the battery capacity retention rate after each cycle Pn = Cn / C0*100%, which is the battery capacity retention rate.

[0231] 6. Battery DC impedance test

[0232] At 25°C, charge the battery at a constant current of 1 / 3C to 3.65V, then charge it at a constant voltage of 3.65V to a current of 0.05C. After 5 minutes of rest, discharge it at a constant current of 1 / 3C for 90 minutes. After 120 minutes of rest, record the voltage V1. Then discharge it at 4C for 30 seconds, and record the voltage V2. The internal resistance DCR of the battery is calculated as (V2-V1) / 4C.

[0233] The parameters of the batteries of the above embodiments and comparative examples are shown in Table 1, and the performance test data are shown in Table 2. Wherein, D1 represents the average particle size Dv50 of the first positive electrode active material; D2 represents the average particle size Dv50 of the second positive electrode active material.

[0234] Table 1

[0235] Table 2

[0236] It can be seen from Table 1 and Table 2 that the secondary batteries of the above embodiments of the present application have a relatively high cycle capacity retention rate, which indicates that the secondary batteries have a relatively high cycle life.

[0237] In Comparative Example 1, the positive electrode active material used entirely unpassivated lithium iron phosphate, and no mixing of large and small particles was performed. The cycle capacity retention rate of the secondary battery was significantly lower than that of the secondary batteries in the various embodiments that also used lithium iron phosphate as the positive electrode active material. Similarly, in Comparative Example 2, the positive electrode active material used entirely unpassivated nickel-cobalt-manganese ternary material, and no mixing of large and small particles was performed. The cycle capacity retention rate of the secondary battery was significantly lower than that of the secondary batteries in the various embodiments that also used the nickel-cobalt-manganese ternary material as the positive electrode active material.

[0238] The above description of the various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced with each other and will not be repeated herein for the sake of brevity.

[0239] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet comprising a positive electrode active material layer, wherein the positive electrode active material layer comprises a first positive electrode active material and a second positive electrode active material, wherein the first positive electrode active material and the second positive electrode active material each independently comprise a lithium-containing transition metal oxide; The outer surface of the second positive active material comprises a passivation layer; and / or The average particle size Dv50 of the second positive electrode active material is greater than the average particle size Dv50 of the first positive electrode active material.

2. The positive electrode sheet according to claim 1, wherein: The first positive electrode active material and the second positive electrode active material each independently include one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich lithium manganese oxide or lithium cobalt oxide.

3. The positive electrode sheet according to claim 1 or 2, wherein: The first positive electrode active material and the second positive electrode active material are simultaneously selected from one or more of lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium-rich lithium manganese oxide or lithium cobalt oxide.

4. The positive electrode sheet according to any one of claims 1 to 3, wherein: The passivation layer includes one or more of an oxide passivation layer and a nitride passivation layer.

5. The positive electrode sheet according to any one of claims 1 to 4, wherein: The passivation layer includes one or more of an aluminum oxide layer, a zirconium oxide layer, a titanium oxide layer, or a titanium nitride layer.

6. The positive electrode sheet according to any one of claims 1 to 5, wherein: The thickness of the passivation layer is 1 nm to 50 nm.

7. The positive electrode sheet according to any one of claims 1 to 6, wherein: The thickness of the passivation layer is 3 nm to 10 nm.

8. The positive electrode sheet according to any one of claims 1 to 7, wherein: The ratio of the average particle size Dv50 of the second positive electrode active material to the average particle size Dv50 of the first positive electrode active material is 2-18.

9. The positive electrode sheet according to any one of claims 1 to 8, wherein: The first positive electrode active material and the second positive electrode active material are both lithium iron phosphate, the average particle size Dv50 of the first positive electrode active material is 0.5 μm to 3 μm, and the average particle size Dv50 of the second positive electrode active material is 1.5 μm to 9 μm.

10. The positive electrode sheet according to any one of claims 1 to 8, wherein: The first positive electrode active material and the second positive electrode active material are both nickel-cobalt-manganese ternary materials with the morphology of single crystal particles. The average particle size Dv50 of the first positive electrode active material is 3μm~7μm, and the average particle size Dv50 of the second positive electrode active material is 9μm~21μm.

11. The positive electrode sheet according to any one of claims 1 to 8, wherein: The first positive electrode active material and the second positive electrode active material are both nickel-cobalt-manganese ternary materials with the morphology of polycrystalline particles. The average particle size Dv50 of the first positive electrode active material is 8μm~20μm, and the average particle size Dv50 of the second positive electrode active material is 20μm~40μm.

12. The positive electrode sheet according to any one of claims 1 to 11, wherein: The first discharge gram capacity of the first positive electrode active material is C1, and the first discharge gram capacity of the second positive electrode active material is C2; based on the total mass of the positive electrode active material layer, the mass fraction of the first positive electrode active material is W1, and the mass fraction of the second positive electrode active material is W2; the positive electrode sheet satisfies: C1*W1+C2*W2=k*C1*(W1+W2), where 0.8≤k<1.

13. The positive electrode sheet according to claim 12, wherein: 0.9≤k≤0.97。 14. The positive electrode sheet according to claim 12, wherein: 12%≤W1<100%, 0% <W2≤85%。 15. The positive electrode sheet according to claim 14, wherein: 60%≤W1<100%, 0% <W2≤40%。 16. The positive electrode sheet according to any one of claims 1 to 15, wherein: The positive electrode active material layer includes the first positive electrode active material and the second positive electrode active material mixed with each other.

17. The positive electrode sheet according to any one of claims 1 to 15, wherein: The positive electrode active material layer includes a first positive electrode active material layer and a second positive electrode active material layer that are stacked. The first positive electrode active material layer includes the first positive electrode active material, and the second positive electrode active material layer includes the second positive electrode active material.

18. A secondary battery comprising the positive electrode sheet according to any one of claims 1 to 17.

19. An electrical device comprising the secondary battery according to claim 18.

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