Positive pole piece and preparation method thereof, secondary battery and electric device

Abstract

The invention relates to a positive pole piece and a preparation method thereof, a secondary battery and an electric device, and belongs to the technical field of secondary batteries. The positive electrode plate comprises a first positive electrode active material layer, a positive electrode current collector and a second positive electrode active material layer which are sequentially stacked, the first positive electrode active material layer comprises a first positive electrode active material of which the discharge platform voltage is greater than or equal to 3.6 V, and the second positive electrode active material layer comprises a second positive electrode active material with a layered structure. The second positive electrode active material comprises a ternary material; the positive pole piece effectively prolongs the cycle life of the secondary battery.

Description

Technical Field

[0001] The present application relates to the technical field of secondary batteries, and in particular to a positive electrode plate and a preparation method thereof, 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] Secondary batteries are widely used in electric vehicles, portable electronic products, aerospace and other fields due to their outstanding features such as light weight, no pollution and no memory effect. The positive electrode material is one of the key materials of secondary batteries and has an important impact on the performance of secondary batteries. The commercial positive electrode materials mainly include ternary materials, lithium iron phosphate and lithium cobalt oxide. Among them, the cycle life of secondary batteries containing ternary materials still needs to be improved. Summary of the invention

[0004] Based on this, the present application provides a positive electrode plate and a preparation method thereof, a secondary battery and an electrical device, aiming to improve the cycle life of a secondary battery containing ternary materials.

[0005] In a first aspect of the present application, a positive electrode plate is provided, comprising a first positive electrode active material layer, a positive electrode current collector and a second positive electrode active material layer stacked in sequence, wherein the first positive electrode active material layer comprises a first positive electrode active material having a discharge platform voltage greater than or equal to 3.6V, and the second positive electrode active material layer comprises a second positive electrode active material having a layered structure, wherein the second positive electrode active material comprises a ternary material.

[0006] The above-mentioned positive electrode plate adopts the design of the first positive electrode active material layer and the second positive electrode active material layer, so that the first positive electrode active material layer and the second positive electrode active material layer can independently face the negative electrode plate. When the discharge voltage of the positive electrode plate is lower than the discharge platform voltage of the ternary material, the discharge current borne by the second positive electrode active material layer is relatively small, and the discharge current borne by the first active material layer is relatively large. The first active material layer mainly discharges, thereby improving the problem of long-term low-voltage discharge of the ternary material in the traditional positive electrode plate, which leads to the destruction of the structure of the ternary material. The discharge platform voltage of the first positive electrode active material is greater than or equal to 3.6V, which can reduce the risk of lithium chromatography of the first active material, thereby improving the cycle life of the secondary battery.

[0007] In some embodiments, in the discharge curve of the positive electrode sheet relative to the negative electrode sheet containing graphite in the discharge voltage range of 2.5V-4.4V and the discharge rate of 0.33C, the discharge capacity of the positive electrode sheet less than or equal to 3.5V accounts for the proportion of its total discharge capacity is Q1, and the discharge capacity of the positive electrode sheet greater than 3.5V accounts for the proportion of its total discharge capacity is Q2, and Q1 and Q2 meet the following conditions: 49%≤Q2 / Q1≤340%;

[0008] Optionally, 52%≤Q2 / Q1≤215%; further optionally, 52%≤Q2 / Q1≤147%;

[0009] Optionally, the discharge curve is a discharge curve from the first discharge to the 50th discharge of the positive electrode plate relative to the negative electrode plate containing graphite.

[0010] In some embodiments, 23%≤Q1≤67%, optionally 30%≤Q1≤67%, further optionally 40%≤Q1≤67%.

[0011] In some embodiments, based on the weight of the first positive electrode active material layer, the weight fraction of the first positive electrode active material is 18%-90%, and optionally 21%-63%.

[0012] In some embodiments, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based materials, and lithium cobalt oxide.

[0013] In some embodiments, the first positive electrode active material layer further includes lithium iron phosphate.

[0014] In some embodiments, based on the total weight of the first positive electrode active material and the lithium iron phosphate in the first positive electrode active material layer, the weight fraction of the lithium iron phosphate is ≤80%, and may be 30%-70%.

[0015] In some embodiments, based on the weight of the second positive electrode active material layer, the weight fraction of the second positive electrode active material is 63%-81%.

[0016] In some embodiments, the ternary material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof.

[0017] In some embodiments, the modifying element in the modifying compound includes one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb.

[0018] In some embodiments, at least one of the lithium nickel cobalt manganese oxide, the lithium nickel cobalt aluminum oxide, and modified compounds thereof is a single crystalline primary particle.

[0019] In some embodiments, the chemical molar ratio of cobalt ions in at least one of the lithium nickel cobalt manganese oxide, the lithium nickel cobalt aluminum oxide, and modified compounds thereof is ≥ 0.07.

[0020] In some embodiments, the ternary material includes a chemical formula of Li a Ni b Co c M 1 d M 2 e O f R g Materials, where 0.75≤a≤1.2, 0 <b<1,0<c<1,0<d<1,0≤e≤0.2,1≤f≤2.5,0≤g≤1,b+c+d+e=1,f+g≤3,M 1 Including one or more of Mn and Al, M 2 Includes one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb, and R includes one or more of N, F, S and Cl;

[0021] Optionally, 0.07≤c<1.

[0022] In some embodiments, the chemical formula is Li a Ni b Co c M 1 d M 2 e O f R g The material is single crystal primary particles;

[0023] In some embodiments, the ternary material further includes a first coating layer, wherein the first coating layer is disposed on the surface of the ternary material having the chemical formula Li a Ni b Co c M 1 d M 2 e O f R g at least a portion of the surface of the material; further optionally, the first coating layer contains one or more of Al2O3 and ZrO2.

[0024] In some embodiments, the second positive electrode active material layer further comprises a third positive electrode active material, and the third positive electrode active material comprises one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based material and lithium cobalt oxide.

[0025] In some embodiments, based on the total weight of the second positive electrode active material and the third positive electrode active material, the weight fraction of the third positive electrode active material is 10%-30%.

[0026] In some embodiments, the mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is (1.18-1.34):1, and can be optionally (1.19-1.31):1.

[0027] The second aspect of the present application provides a method for preparing the positive electrode sheet according to the first aspect of the present application, comprising the following steps:

[0028] Disposing a first positive electrode slurry containing the first positive electrode active material on one side of the positive electrode current collector to form a first positive electrode active material layer;

[0029] A second positive electrode slurry containing the second positive electrode active material is disposed on a side of the positive electrode current collector away from the first positive electrode active material layer to form a second positive electrode active material layer.

[0030] The above preparation method is simple to operate and is conducive to the large-scale production of positive electrode sheets.

[0031] In some embodiments, the first cathode slurry further comprises lithium iron phosphate;

[0032] In some embodiments, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based materials, and lithium cobalt oxide.

[0033] In some embodiments, the second positive electrode slurry further includes a third positive electrode active material, and the third positive electrode active material includes one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based materials and lithium cobalt oxide.

[0034] The third aspect of the present application provides a secondary battery, comprising at least one of the positive electrode sheet described in the first aspect of the present application and the positive electrode sheet prepared by the preparation method described in the second aspect of the present application.

[0035] The secondary battery of the present application includes the positive electrode sheet provided by the present application, and thus has at least the same advantages as the positive electrode sheet.

[0036] In some embodiments, the operating voltage of the secondary battery is 2.5V-4.4V.

[0037] In some embodiments, the secondary battery further includes a negative electrode sheet, and the negative electrode active material layer of the negative electrode sheet includes graphite.

[0038] A fourth aspect of the present application provides an electrical device comprising the secondary battery described in the third aspect of the present application.

[0039] The electric device of the present application includes the secondary battery provided by the present application, and thus has at least the same advantages as the secondary battery.

[0040] The 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 description, drawings, and claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] In order to better describe and illustrate the embodiments or examples provided by the present application, reference may be made to one or more drawings. The additional details or examples used to describe the drawings should not be considered as limiting the scope of the disclosed application, the embodiments or examples currently described, and any of the best modes of these applications currently understood. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0042] Figure 1 This is the sixth discharge curve of the battery prepared in Example 11 of the present application at 25° C. and 2.5V to 4.4V.

[0043] Figure 2 It is a schematic diagram of a battery cell according to an embodiment of the present application.

[0044] Figure 3 for Figure 2 An exploded view of a battery cell according to an embodiment of the present application is shown.

[0045] Figure 4 It is a schematic diagram of a battery module according to one embodiment of the present application.

[0046] Figure 5 A schematic diagram of a battery pack according to an embodiment of the present application.

[0047] Figure 6 for Figure 5 An exploded view of a battery pack according to an embodiment of the present application is shown.

[0048] Figure 7 FIG. 1 is a schematic diagram of an electrical device using a secondary battery according to an embodiment of the present application as a power source.

[0049] Description of reference numerals:

[0050] 1 battery pack; 2 upper box; 3 lower box; 4 battery module; 5 battery cell; 51 shell; 52 electrode assembly; 53 cover plate; 6 electrical device. DETAILED DESCRIPTION

[0051] Below, some embodiments of the positive electrode sheet and its preparation method, secondary battery and electric device of the present application are described in detail with appropriate reference to the 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 descriptions 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.

[0052] "Scope" disclosed in the present application can be limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or excluding end values, and any end value can be included or not included independently, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 listed, and if the maximum range values ​​3,4 and 5 are also listed, the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, and "0-5" is just an abbreviation for these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to listing the parameter as, for example, integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc. For example, when a parameter is expressed as an integer selected from "2-10", it is equivalent to listing the integers 2, 3, 4, 5, 6, 7, 8, 9 and 10.

[0053] In the present application, "plurality", "multiple" and the like, unless otherwise specified, refer to a number greater than or equal to 2. For example, "one or more" means one or greater than or equal to two.

[0054] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0055] Reference to "embodiments" herein means that a particular feature, structure, or characteristic described in conjunction with the embodiments may be included in at least one embodiment or implementation of the present application. The appearance of the phrase in various locations in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment that is mutually exclusive with other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein may be combined with other embodiments. The "implementation methods" mentioned herein have a similar understanding.

[0056] Those skilled in the art will appreciate that, in the methods of each embodiment or example, the order in which each step is written does not mean 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 internal logic. If there is no special explanation, all steps of the present application can 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 also include step (c), indicating that step (c) can 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.

[0057] In the present application, in the open technical features or technical solutions described by the words "contain", "include", "comprise", etc., 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 including 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 not only includes a1, a2 and a3, but also includes other members". In the present 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.

[0058] In this application, "optionally", "optional", and "optional" mean optional, that is, any one of the two parallel schemes of "yes" or "no". If there are multiple "options" in a technical solution, unless otherwise specified and there is no contradiction or mutual restriction, each "option" is independent.

[0059] Ternary materials have been widely used in the field of energy storage due to their high energy density and high voltage platform. However, the material itself has the problem of unstable cycle and rapid decay in the later stage of the cycle. Therefore, the cycle life of secondary batteries containing ternary materials needs to be improved. In traditional technology, positive electrode active materials with good cycle performance are mixed with ternary materials to make positive electrode sheets. Although synergistic advantages can be achieved to a certain extent, it is easy to cause the ternary materials in the positive electrode sheets to discharge at low voltage for a long time, resulting in the destruction of the structure of the ternary materials and deteriorating the cycle life of the secondary battery.

[0060] In view of this, the present application provides a positive electrode plate, comprising a first positive electrode active material layer, a positive electrode current collector and a second positive electrode active material layer stacked in sequence, the first positive electrode active material layer comprising a first positive electrode active material having a discharge platform voltage greater than or equal to 3.6V, the second positive electrode active material layer comprising a second positive electrode active material having a layered structure, and the second positive electrode active material comprising a ternary material; the above-mentioned positive electrode plate adopts the design of the first positive electrode active material layer and the second positive electrode active material layer, so that the first positive electrode active material layer and the second positive electrode active material layer can independently face the negative electrode plate, when the discharge voltage of the positive electrode plate is lower than the discharge platform voltage of the ternary material, the discharge current borne by the second positive electrode active material layer is relatively small, the discharge current borne by the first active material layer is relatively large, and the first active material layer mainly discharges, thereby improving the problem that the ternary material in the traditional positive electrode plate is discharged at a low voltage for a long time, resulting in the destruction of the structure of the ternary material, the discharge platform voltage of the first positive electrode active material is greater than or equal to 3.6V, which can reduce the risk of lithium stratification of the first active material, thereby improving the cycle life of the secondary battery.

[0061] Positive electrode

[0062] An embodiment of the present application provides a positive electrode plate, comprising a first positive electrode active material layer, a positive electrode current collector and a second positive electrode active material layer stacked in sequence, the first positive electrode active material layer comprising a first positive electrode active material with a discharge platform voltage greater than or equal to 3.6V, the second positive electrode active material layer comprising a second positive electrode active material having a layered structure, and the second positive electrode active material comprises a ternary material.

[0063] The above-mentioned positive electrode plate adopts the design of the first positive electrode active material layer and the second positive electrode active material layer, so that the first positive electrode active material layer and the second positive electrode active material layer can independently face the negative electrode plate. When the discharge voltage of the positive electrode plate is lower than the discharge platform voltage of the ternary material, the discharge current borne by the second positive electrode active material layer is relatively small, and the discharge current borne by the first active material layer is relatively large. The first active material layer mainly discharges, thereby improving the problem of long-term low-voltage discharge of the ternary material in the traditional positive electrode plate, resulting in the destruction of the structure of the ternary material. The discharge platform voltage of the first positive electrode active material is greater than or equal to 3.6V, which can reduce the risk of lithium chromatography of the first active material, thereby improving the cycle life of the secondary battery.

[0064] In the above embodiment, the discharge platform voltage of the first positive electrode active material is greater than or equal to 3.6V, which can reduce the risk of lithium plating in the first active material layer. This is because when the discharge platform voltage of the first positive electrode active material is low, the positive electrode plate is charged. There may be a phenomenon that the first positive electrode active material layer is already full and the second positive electrode active material layer has just started to charge. As the second positive electrode active material layer is charged, the first positive electrode active material layer is at risk of lithium plating.

[0065] When the first positive electrode active material is a specific positive electrode active material, the corresponding discharge platform voltage is certain. Optionally, the first positive electrode active material in the positive electrode plate can be tested by the following method: wipe off the second positive electrode active material layer of the positive electrode plate with N-methylpyrrolidone (NMP) solution, and then test the remaining positive electrode plate using ICP. Take about 0.4g of dry positive electrode sheet or about 1g of wet positive electrode sheet (both accurate to 0.0001g) in a 25ml beaker, add 2mL~5mL nitric acid, leave overnight, then place on a hot plate, heat at about 100°C (use a voltage regulating transformer to adjust the input voltage to control the temperature), add 0.5mL of perchloric acid after the positive electrode sheet is digested, and heat to digest at about 140°C until the white smoke is gone. The residue should be white, otherwise nitric acid and perchloric acid should be added again to repeat the digestion, and finally use 7% (volume % of acid) hydrochloric acid to dissolve and extract. After the volume is adjusted to an appropriate volume depending on the content of the element to be measured, start testing on an ICP-OES test instrument to confirm the content of elements such as Ni, Co, Mn, Al, Li, Fe, and P. According to the results, qualitative and quantitative analysis can be performed to obtain the type of the first positive active material. Similarly, the second positive electrode active material in the positive electrode plate can be tested; during the test, the first positive electrode active material layer of the positive electrode plate needs to be wiped off with NMP solution, and then the remaining positive electrode plate is subjected to ICP test using the above method to obtain the type of the second positive electrode active material.

[0066] In some embodiments, in the discharge curve of the positive electrode sheet relative to the negative electrode sheet containing graphite in the discharge voltage range of 2.5V-4.4V and the discharge rate of 0.33C, the proportion of the discharge capacity of the positive electrode sheet less than or equal to 3.5V to its total discharge capacity is Q1, and the proportion of the discharge capacity of the positive electrode sheet greater than 3.5V to its total discharge capacity is Q2, and Q1 and Q2 meet the following conditions: 49%≤Q2 / Q1≤340%. Controlling the ratio of Q2 to Q1 within the above range is conducive to maintaining the structure of the ternary material and further reducing the risk of lithium chromatography of the first positive active material, thereby further improving the cycle life of the secondary battery. It is understood that Q2 / Q1 includes but is not limited to: 49%, 52%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, 110%, 120%, 130%, 140%, 150%, 160%, 170%, 180%, 190%, 200%, 210%, 215%, 230%, 250%, 270%, 300%, 320%, 340%. Further, 52%≤Q2 / Q1≤215%. Further, 52%≤Q2 / Q1≤147%. Optionally, the above discharge curve is a discharge curve from the first discharge to the 50th discharge of the positive electrode sheet relative to the negative electrode sheet containing graphite. Exemplarily, the discharge curve may be a discharge curve of the first discharge of the positive electrode sheet relative to the negative electrode sheet containing graphite, or a discharge curve of the second discharge, or a discharge curve of the tenth discharge, or a discharge curve of the 30th discharge, or a discharge curve of the 50th discharge, etc.

[0067] In some embodiments, 23%≤Q1≤67%. Q1 is within the above range, which is conducive to maintaining the structure of the ternary material and further reducing the risk of lithium chromatography of the first positive electrode active material, thereby further improving the cycle life of the secondary battery. Q1 includes but is not limited to: 23%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 67%. Further, 30%≤Q1≤67%. Further, 40%≤Q1≤67%.

[0068] In some embodiments, based on the weight of the first positive electrode active material layer, the weight fraction of the first positive electrode active material is 18%-90%. Controlling the weight fraction of the first positive electrode active material to the above range is conducive to further improving the cycle life of the secondary battery. The weight fraction of the above-mentioned first positive electrode active material includes but is not limited to: 18%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90%. Optionally, based on the weight of the first positive electrode active material layer, the weight fraction of the first positive electrode active material is 21%-63%.

[0069] It is understandable that the secondary battery will be accompanied by the deintercalation and consumption of lithium (Li) during the charge and discharge process, and the content of Li in the positive electrode plate is different when the secondary battery is discharged to different states. In the enumeration of positive electrode active materials in this application, unless otherwise specified, the content of Li is the initial state of the material. The positive electrode active material is applied to the positive electrode plate in the secondary battery system, and after the charge and discharge cycle, the content of Li in the positive electrode active material contained in the plate usually changes. Among them, the content of Li can be measured by molar content, but is not limited to this. Regarding "the content of Li 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 the new material obtained by appropriate modification on the basis of the listed positive electrode active materials is also within the scope of positive electrode active materials. The aforementioned appropriate modification refers to the acceptable modification method for the positive electrode active material, and non-limiting examples include coating modification.

[0070] In the list of positive electrode active materials in this application, the content of oxygen (O) is only a theoretical state value. The release of oxygen from the lattice will cause the molar content of oxygen to change, and the actual content of O will fluctuate. Among them, the content of O can be measured by molar content, but is not limited to this.

[0071] In some embodiments, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, a lithium-rich manganese-based material, and lithium cobalt oxide.

[0072] In the present application, lithium manganese iron phosphate includes a chemical formula of Li 1+x Mn y Fe j A 1-y-j P 1-z R 1 z O4 material, wherein -0.100≤x≤0.100, 0.001≤y≤0.800, 0.001≤j≤0.800, 0.001≤z≤0.100, A includes one or more of Zn, Al, Na, K, Mg, Mo, W, Ti, V, Zr, Ni, Co, Ga, Sn, Sb, Nb and Ge, R 1 Including one or more of B, Si, N, S, F, Cl and Br. Optionally, A includes one or more of Ti, V, Ni, Co and Mg; Optionally, R 1 Including one or more of B, Si, N and S. Further optionally, the lithium manganese iron phosphate further includes a second coating layer, wherein the second coating layer is provided on a surface having a chemical formula of Li 1+x Mn y Fe j A 1-y-j P 1-z R 1 zAt least a portion of the surface of the O4 material. Further optionally, the second coating layer includes one or more of pyrophosphate, phosphate and carbon material. The second coating layer can be a single layer structure, or a multi-layer structure.

[0073] In the present application, lithium-rich manganese-based materials include materials with a chemical formula of nLi2MnO3·(1-n)LiMO2, wherein 0.2≤n≤0.55, and M includes one or more of Ni, Co, and Mn. Lithium manganate includes materials with a chemical formula of LiMn2O4 and modified compounds thereof. Lithium cobaltate includes materials with a chemical formula of LiCoO2 and modified compounds thereof.

[0074] In some embodiments, the first positive electrode active material layer also includes lithium iron phosphate. Compared with ternary materials, lithium iron phosphate has a longer cycle life, so traditional technology usually mixes ternary materials and lithium iron phosphate to form a positive electrode sheet, so that the secondary battery containing the positive electrode sheet takes into account both energy density and cycle life. However, due to the low discharge platform of lithium iron phosphate (about 3.2V), it is easy to cause the ternary material in the positive electrode sheet to discharge at a low voltage for a long time, resulting in the structure of the ternary material being destroyed, deteriorating the cycle life of the secondary battery. In the above-mentioned embodiment of the present application, the design of the first positive electrode active material layer and the second positive electrode active material layer is adopted, and the discharge platform voltage of the first positive electrode active material in the first positive electrode active material layer is greater than or equal to 3.6V, which can effectively improve the problem of long-term low-voltage discharge of the ternary material in the traditional positive electrode sheet, resulting in the structure of the ternary material being destroyed, and can also effectively reduce the risk of lithium chromatography of the first positive electrode active material, thereby further improving the cycle life of the secondary battery.

[0075] In some embodiments, lithium iron phosphate includes a chemical formula of Li p Fe r M 0 (1-r) PO4 materials, where 1≤p≤1.1, 0.95≤r≤1, M 0 The lithium iron phosphate further comprises one or more of Cu, Mn, Mg, V, Cr, Zn, Pb, Ca, Co, Ni, Sr and Ti. Optionally, the lithium iron phosphate further comprises a third coating layer, wherein the third coating layer is provided on a surface of the lithium iron phosphate. p Fe r M 0 (1-r) At least a portion of the surface of the PO4 material. Further optionally, the third coating layer includes a carbon material.

[0076] In some embodiments, based on the total weight of the first positive electrode active material and lithium iron phosphate in the first positive electrode active material layer, the weight fraction of lithium iron phosphate is ≤80%. The weight fraction of lithium iron phosphate is within the above range, which can further reduce the risk of lithium chromatography of the first positive electrode active material and further improve the cycle life of the secondary battery. It is understood that the weight fraction of lithium iron phosphate includes but is not limited to: 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%. Optionally, based on the total weight of the first positive electrode active material and lithium iron phosphate in the first positive electrode active material layer, the weight fraction of lithium iron phosphate is 30%-70%.

[0077] In some embodiments, based on the total weight of the first positive electrode active material and lithium iron phosphate in the first positive electrode active material layer, the weight fraction of the first positive electrode active material is ≥ 20%. The weight fraction of the first positive electrode active material includes, but is not limited to, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 95%. Optionally, it is 30%-70%.

[0078] In some embodiments, based on the weight of the second positive electrode active material layer, the weight fraction of the second positive electrode active material is 63%-81%. Thus, the cycle performance of the secondary battery can be further improved. The weight fraction of the second positive electrode active material includes but is not limited to: 63%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 81%.

[0079] In some embodiments, the ternary material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and modified compounds thereof. Non-limiting examples of lithium nickel cobalt manganese oxide may include 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 0.8 Co 0.15 Al 0.05O2. Optionally, the modifying element in the modifying compound includes one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb.

[0080] In some embodiments, at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof is a single crystal primary particle, thereby improving the structural stability of the ternary material, thereby further improving the cycle life and rate performance of the secondary battery.

[0081] In some embodiments, the chemical molar ratio of cobalt ions in at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof is ≥ 0.07. Thus, the structural stability of the ternary material can be improved, thereby further improving the cycle life and rate performance of the secondary battery.

[0082] Optionally, the present application can use the following method to test the chemical molar ratio of cobalt ions: take about 0.4g (accurate to 0.0001g) of the second positive electrode active material in a 25mL beaker, add 2mL-5mL nitric acid, leave overnight, then place on a hot plate, heat at about 100°C (use a voltage regulator to adjust the input voltage and temperature control) until the second positive electrode active material is digested, add 0.5mL of perchloric acid, and heat at about 140°C to digest until the white smoke is gone. The residue should be white, otherwise nitric acid and perchloric acid should be added again. Repeat digestion, and finally use 7% (volume % of acid) hydrochloric acid to dissolve and extract. Depending on the content of the element to be tested, the volume is fixed to an appropriate volume and then tested on the ICP-OES test instrument. The test elements are Ni, Co, Mn, and Al. The instrument test results are the mass percentages of the four elements. The one with a larger proportion of Al and Mn is selected for subsequent calculations. The molar amounts of Ni, Co, Mn, and Al are confirmed according to the molar masses of the elements and the mass of the positive electrode material. Finally, the chemical molar ratio of the cobalt ion can be calculated.

[0083] In some embodiments, the ternary material includes a chemical formula of Li a Ni b Co c M 1 d M 2 e O f R g Materials, where 0.75≤a≤1.2, 0 <b<1,0<c<1,0<d<1,0≤e≤0.2,1≤f≤2.5,0≤g≤1,b+c+d+e=1,f+g≤3,M 1 Including one or more of Mn and Al, M 2Including one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb, R includes one or more of N, F, S and Cl. Optionally, 0.07≤c<1. Thus, the structural stability of the ternary material can be improved, thereby further improving the cycle life and rate performance of the secondary battery.

[0084] In some embodiments, the chemical formula is Li a Ni b Co c M 1 d M 2 e O f R g The material is a single crystal primary particle. As a result, the structural stability of the ternary material can be further improved, thereby further improving the cycle life and rate performance of the secondary battery.

[0085] In some embodiments, the ternary material further comprises a first coating layer, wherein the first coating layer is provided on a surface having a chemical formula of Li a Ni b Co c M 1 d M 2 e O f R g Optionally, the first coating layer comprises one or more of Al2O3 and ZrO2.

[0086] In some embodiments, the second positive electrode active material layer further includes a third positive electrode active material, and the third positive electrode active material includes one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based material and lithium cobalt oxide.

[0087] In some embodiments, based on the total weight of the second positive electrode active material and the third positive electrode active material, the weight fraction of the third positive electrode active material is 10%-30%. Controlling the weight fraction of the third positive electrode active material within the above range is conducive to further improving the cycle life of the secondary battery. It is understood that the weight fraction of the third positive electrode active material includes but is not limited to: 10%, 12%, 15%, 18%, 20%, 22%, 25%, 28%, 30%.

[0088] In some embodiments, based on the total weight of the second positive electrode active material and the third positive electrode active material, the weight fraction of the second positive electrode active material is 70%-90%. Controlling the weight fraction of the second positive electrode active material within the above range is conducive to maintaining the structure of the ternary material, thereby further improving the cycle life of the secondary battery. It is understood that the weight fraction of the above-mentioned second positive electrode active material includes but is not limited to: 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%.

[0089] In some embodiments, the mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is (1.18-1.34):1, and can be optionally (1.19-1.31):1.

[0090] 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.

[0091] In some embodiments, the first positive electrode active material layer and the second positive electrode active material layer may also optionally and independently 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.

[0092] In some embodiments, the first positive electrode active material layer and the second positive electrode active material layer may also optionally and independently include a conductive agent. As a non-limiting example, 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.

[0093] Another embodiment of the present application provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0094] Disposing a first positive electrode slurry containing a first positive electrode active material on one side of the positive electrode current collector to form a first positive electrode active material layer;

[0095] A second positive electrode slurry containing a second positive electrode active material is disposed on a side of the positive electrode current collector away from the first positive electrode active material layer to form a second positive electrode active material layer.

[0096] The above preparation method is simple to operate and is conducive to the large-scale production of positive electrode sheets.

[0097] In some embodiments, the first cathode slurry further includes lithium iron phosphate.

[0098] In some embodiments, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, a lithium-rich manganese-based material, and lithium cobalt oxide.

[0099] In some embodiments, the second positive electrode slurry further includes a third positive electrode active material, and the third positive electrode active material includes one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based material and lithium cobalt oxide.

[0100] 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 first positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent to form a first positive electrode slurry; the first positive electrode slurry is coated on one side surface of the positive electrode current collector, and the first positive electrode active material layer is formed after drying; the second positive electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent to form a second positive electrode slurry, and the second positive electrode active material layer is formed after drying, and the positive electrode sheet is obtained after cold pressing. The type of solvent can be selected from but not limited to any one of the aforementioned embodiments, such as N-methylpyrrolidone (NMP). The solid content of the first positive electrode slurry and the second positive electrode slurry can be independently 40wt%-80wt%. The viscosity of the first positive electrode slurry and the second positive electrode slurry at room temperature can be independently adjusted to 5000mPa·s-25000mPa·s. When the first positive electrode slurry and the second positive electrode slurry are coated, the coating unit surface density based on dry weight (excluding solvent) can be 15 mg / cm 2 -35mg / cm 2 The compaction density of the positive electrode can be 3.0g / cm 3 -3.6g / cm 3 , optional 3.3g / cm 3 -3.5g / cm 3 .

[0101] Another embodiment of the present application provides a secondary battery, comprising at least one of the above-mentioned positive electrode sheet and the positive electrode sheet prepared by the above-mentioned preparation method.

[0102] In some embodiments, the operating voltage of the secondary battery is 2.5V-4.4V.

[0103] In some embodiments, the secondary battery further includes a negative electrode sheet, and the negative electrode active material layer of the negative electrode sheet includes graphite.

[0104] Another embodiment of the present application further provides an electrical device, comprising the above-mentioned secondary battery.

[0105] In addition, the secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

[0106] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is set between the positive electrode sheet and the negative electrode sheet, mainly to prevent the positive and negative electrodes from short-circuiting, while allowing ions to pass through.

[0107] Positive electrode

[0108] The above-mentioned positive electrode plate of the present application is adopted.

[0109] Negative electrode

[0110] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer includes a negative electrode active material.

[0111] As a non-limiting example, the negative electrode current collector has two surfaces opposite to each other in its own thickness direction, and the negative electrode active material layer is disposed on any one or both of the two opposite surfaces of the negative electrode current collector.

[0112] 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. 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.

[0113] In some embodiments, the negative electrode active material may adopt the negative electrode active material for the battery known in the art. As a non-limiting example, the negative electrode active material may include one or more of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, lithium titanate, etc. Silicon-based materials may include one or more of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may include one or more of elemental tin, tin oxide compounds, 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.

[0114] In some embodiments, the negative electrode active material layer may further 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).

[0115] In some embodiments, the negative electrode active material 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.

[0116] In some embodiments, the negative electrode active material layer may further optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0117] 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 surface 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 on 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%-60wt%. The viscosity of the negative electrode slurry at room temperature can be adjusted to 2000mPa·s-10000mPa·s. When the negative electrode slurry is coated, 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 sheet can be 1.0g / cm 3 -1.8g / cm 3 .

[0118] Electrolytes

[0119] The electrolyte has the function of conducting ions between the positive electrode and the negative electrode. The present application has no particular restrictions on the type of electrolyte, which can be selected according to needs. For example, the electrolyte can be liquid, gel or all-solid.

[0120] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0121] In some embodiments, the electrolyte salt 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 difluorophosphate (LiPO2F2), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0122] In some embodiments, the solvent may include 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), ethylene carbonate One or more of fluoroethylene carbonate (FEC), methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclopentane, dimethyl sulfone, methyl ethyl sulfone and diethyl sulfone.

[0123] In some embodiments, the electrolyte may further include additives, such as 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, additives that improve battery high or low temperature performance, etc.

[0124] 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.

[0125] Isolation film

[0126] 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 stability and mechanical stability can be selected.

[0127] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different, without particular limitation.

[0128] In some embodiments, the isolation film has a thickness of 6 μm-40 μm, and optionally 12 μm-20 μm.

[0129] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator may be formed into an electrode assembly by a winding process or a lamination process.

[0130] In some embodiments, the secondary battery may include an outer package, which may be used to encapsulate the electrode assembly and the electrolyte.

[0131] In some embodiments, the outer packaging of the secondary battery may be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery may also be a soft package, such as a bag-type soft package. The material of the soft package may be plastic, and further, non-limiting examples of plastic may include one or more of polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0132] The secondary battery includes at least one battery cell. The secondary battery may include one or more battery cells.

[0133] In this application, unless otherwise specified, "battery cell" refers to a basic unit that can realize the mutual conversion of chemical energy and electrical energy, and further, generally speaking, at least includes a positive electrode sheet, a negative electrode sheet and an electrolyte. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting active ions between the positive electrode sheet and the negative electrode sheet.

[0134] The present application has no particular limitation on the shape of the battery cell, which may be cylindrical, square or any other shape. Figure 2 The battery cell 5 is a square structure as an example.

[0135] In some embodiments, reference Figure 3, the outer packaging may include a shell 51 and a cover plate 53. Among them, 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 form 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 actual needs.

[0136] The secondary battery may be a battery module 4 or a battery pack 1 .

[0137] The battery module includes at least one battery cell. The number of battery cells contained in the battery module can be one or more, and those skilled in the art can select a suitable number according to the application and capacity of the battery module.

[0138] Figure 4 4 is an example of a battery module. Figure 4 In the battery module 4, the plurality of battery cells 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of battery cells 5 may be fixed by fasteners.

[0139] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of battery cells 5 are received in the receiving space.

[0140] In some embodiments, the battery modules described above may also be assembled into a battery pack. The number of battery modules contained in the battery pack may be one or more. Those skilled in the art may select a suitable number according to the application and capacity of the battery pack.

[0141] Figure 5 and Figure 6 1 is a battery pack 1 as an example. Figure 5 and Figure 6 The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0142] In addition, the present application also provides an electrical device, which includes a secondary battery provided by the present application. The secondary battery can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device may include mobile devices, electric vehicles, electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto. Among them, the mobile device may be, for example, a mobile phone, a laptop computer, etc.; the electric vehicle may be, for example, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc., but are not limited thereto.

[0143] As an electrical device, a secondary battery can be selected according to its usage requirements.

[0144] Figure 7 The power consumption device 6 is taken as an example. The power consumption device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module can be used.

[0145] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0146] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are only used to explain the present application, and should not be construed as limiting the present application. If the techniques or conditions are not specified in the embodiments, the techniques or conditions described in the literature in this area or the product specifications are used. The reagents or instruments used that do not specify the manufacturer are all conventional products that can be obtained commercially.

[0147] Example 1

[0148] (1) Preparation of positive electrode sheet

[0149] The positive electrode active material (LiFePO4 and the first positive electrode active material lithium manganese iron phosphate (LiMn 0.25 Fe 0.75 PO4), the mass ratio of the two is 7:3), the binder polyvinylidene fluoride (PVDF), and the conductive carbon are added to a certain amount of NMP, and the mass ratio of the positive electrode active material: binder: conductive agent is 90:5:5, and stirred in a drying room to form a uniform slurry, and the viscosity is controlled to be 3000-10000mPa·s. The slurry is coated on one side of the aluminum foil and dried to form a first positive electrode active material layer; then the positive electrode active material (the second positive electrode active material NCM (LiNi 0.55 Co 0.12 Mn 0.33 O2) and the third positive electrode active material lithium manganese iron phosphate (LiMn 0.25 Fe0.75 PO4), the mass ratio of the two is 7:3), binder polyvinylidene fluoride (PVDF), and conductive carbon are added to a certain amount of N-methylpyrrolidone (NMP), the mass ratio of positive electrode active material: binder: conductive agent is 90:5:5, stirred in a drying room to form a uniform slurry, the viscosity is controlled to be 3000-10000mPa·s, the above slurry is coated on the other side of the aluminum foil, and dried to form a second positive electrode active material layer to obtain a positive electrode sheet, the mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is 1.22:1.

[0150] (2) Preparation of negative electrode sheet

[0151] Graphite, sodium carboxymethyl cellulose (CMC-Na), styrene-butadiene rubber (SBR), and conductive carbon are added to a certain amount of deionized water, with the mass ratio of graphite: sodium carboxymethyl cellulose: styrene-butadiene rubber: conductive agent being 90:2:3:5, and stirred to form a uniform slurry with a viscosity controlled at 3000-10000 mPa·s. The slurry is applied on both sides of a copper foil and dried to form a negative electrode sheet.

[0152] (3) Isolation film

[0153] Polyethylene (PE) porous polymer film is used.

[0154] (4) Electrolyte

[0155] The concentration of LiPF6 in the electrolyte is 1 mol / L, and the solvent is ethylene carbonate (EC), diethyl carbonate (DEC) and dimethyl carbonate (DMC) in a volume ratio of 1:1:1.

[0156] (5) Preparation of soft-pack laminated batteries

[0157] The prepared positive electrode sheet, negative electrode sheet and isolation membrane are made into corresponding battery cells according to the Z-shaped stacking structure, and the battery cells are vacuum dried at 90°C for 12 hours, followed by ultrasonic welding of the positive and negative electrode tabs. The positive electrode uses aluminum tabs and the negative electrode uses nickel tabs. The positive and negative electrode tabs are located on the same side of the battery cell. The battery cell after the tab welding is loaded into an aluminum-plastic film of a suitable size for top-side sealing, and the electrolyte is injected, allowed to stand, formed, aged, exhausted, sealed twice, and capacity tested to obtain the prepared soft-pack stacked battery.

[0158] Example 2-41

[0159] It is basically the same as Example 1, except that: the preparation method of the positive electrode plate is different; the parameters of the positive electrode plates of Examples 2-41 are shown in Table 1.

[0160] Comparative Example 1

[0161] It is basically the same as Example 1, except that the preparation method of the positive electrode sheet is different; the preparation method of the positive electrode sheet of Comparative Example 1 is as follows:

[0162] The positive electrode active material (LiNi 0.55 Co 0.12 Mn 0.33 O2 and LiFePO4 (LFP), the mass ratio of the two is 30:70), binder polyvinylidene fluoride (PVDF), and conductive carbon are added to a certain amount of NMP, the mass ratio of positive electrode active material: binder: conductive agent is 90:5:5, stirred in a drying room to form a uniform slurry, the viscosity is controlled to be 3000-10000mPa·s, the above slurry is coated on both sides of aluminum foil, and dried to form a first positive electrode active material layer and a second positive electrode active material layer to obtain a positive electrode sheet.

[0163] Comparative Examples 2-4

[0164] It is basically the same as Example 1, except that: the preparation method of the positive electrode plate is different; the parameters of the positive electrode plates of Comparative Examples 2-4 are shown in Table 1.

[0165] Table 1 shows the product parameters of Examples 1-41 and Comparative Examples 1-4.

[0166] Table 1

[0167]

[0168]

[0169] In Table 1, A0 is the weight fraction of lithium iron phosphate based on the total weight of the first positive electrode active material and lithium iron phosphate in the first positive electrode active material layer; A1 is the weight fraction of the first positive electrode active material based on the total weight of the first positive electrode active material and lithium iron phosphate in the first positive electrode active material layer; A2 is the weight fraction of the second positive electrode active material based on the total weight of the second positive electrode active material and the third positive electrode active material; A3 is the weight fraction of the third positive electrode active material based on the total weight of the second positive electrode active material and the third positive electrode active material.

[0170] Table 2

[0171]

[0172]

[0173] Performance Testing

[0174] (1) Cycle performance test and positive electrode interface test

[0175] The prepared battery was charged to 4.4V at 0.5C at 2.5V-4.4V in a constant temperature environment of 25℃, then charged at a constant voltage at 4.4V to a current ≤0.05C, allowed to stand for 5min, and then discharged to 2.5V at 1C. The capacity was recorded as Cn (n=1, 2, 3...), and the above operation was repeated. The capacity retention rate was calculated according to the ratio of Cn / C1. When Cn / C1×100%=80%, the corresponding number of cycles was extracted. After the cycle was completed, the battery was charged to 4.4V at 0.5C, then charged at a constant voltage at 4.4V to a current ≤0.05C, allowed to stand for 5min, and disassembled to confirm the lithium deposition situation at the negative electrode interface corresponding to the first positive electrode active material layer.

[0176] (2) Discharge capacity ratio test ≤3.5V

[0177] The prepared battery was charged to 4.4V at 0.5C at 2.5V~4.4V in a constant temperature environment of 25℃, then charged at a constant voltage at 4.4V to a current ≤0.05C, left to stand for 5min, and then discharged to 2.5V at 0.33C, with a total discharge capacity of C1, and the capacity of ≤3.5V was taken as C2. In the total discharge capacity of the battery, the discharge capacity of ≤3.5V accounted for Q1=C2 / C1, and the discharge capacity of >3.5V accounted for Q2=1-Q1. The above total discharge capacity can be the total discharge capacity of a discharge cycle in the first 50 cycles of discharge of the battery. Figure 1 The sixth discharge curve of the battery prepared in Example 11 at 25° C. and 2.5 V to 4.4 V is shown.

[0178] Table 3

[0179]

[0180]

[0181] It can be seen from Tables 1 to 3 that the first positive electrode active material layer of the positive electrode plate in Examples 1-41 contains a first positive electrode active material, the discharge platform voltage of the first positive electrode active material is greater than or equal to 3.6V, and the second positive electrode active material layer contains a ternary material with a layered structure; the first positive electrode active material layer and the second positive electrode active material layer of the positive electrode plate in Comparative Example 1 are both formed by mixing lithium iron phosphate and a ternary material, and the first active material layer of the positive electrode plate in Comparative Examples 2-4 does not contain the first positive electrode active material.

[0182] Compared with Comparative Example 1, the cycle life of the battery of Example 1-41 is significantly improved; the interface of the first positive electrode active material layer of the positive electrode plate in Example 1-41 close to the negative electrode side has no lithium deposition or slight lithium deposition, which is significantly improved compared with the interface lithium deposition of the first positive electrode active material layer of Comparative Example 2-4, and the cycle life of the battery of Example 1-41 is significantly higher than that of Comparative Example 2-4; This indicates that the differentiated design of the first positive electrode active material layer and the second positive electrode active material layer of the positive electrode plate in Example 1-41 effectively improves the cycle life of the battery.

[0183] The above description of various embodiments tends to emphasize the differences between the various embodiments. The same or similar parts can be referenced to each other, and for the sake of brevity, they will not be repeated in this article.

[0184] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are only examples, and the embodiments having the same structure as the technical idea and exerting the same effect within the scope of the technical solution of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the main purpose of the present application, various modifications that can be thought of by those skilled in the art to the embodiments and other methods of combining some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode sheet, characterized in that: The invention comprises a first positive electrode active material layer, a positive electrode current collector and a second positive electrode active material layer which are stacked in sequence, wherein the first positive electrode active material layer comprises a first positive electrode active material whose discharge platform voltage is greater than or equal to 3.6V, and the second positive electrode active material layer comprises a second positive electrode active material having a layered structure, wherein the second positive electrode active material comprises a ternary material.

2. The positive electrode sheet according to claim 1, characterized in that: In the discharge curve of the positive electrode sheet relative to the negative electrode sheet containing graphite in the discharge voltage range of 2.5V-4.4V and the discharge rate of 0.33C, the proportion of the discharge capacity of the positive electrode sheet less than or equal to 3.5V to the total discharge capacity is Q1, and the proportion of the discharge capacity of the positive electrode sheet greater than 3.5V to the total discharge capacity is Q2, and Q1 and Q2 meet the following conditions: 49%≤Q2 / Q1≤340%; Optionally, 52%≤Q2 / Q1≤215%; further optionally, 52%≤Q2 / Q1≤147%; Optionally, the discharge curve is a discharge curve from the first discharge to the 50th discharge of the positive electrode plate relative to the negative electrode plate containing graphite.

3. The positive electrode sheet according to claim 2, characterized in that: 23%≤Q1≤67%, optionally 30%≤Q1≤67%, further optionally 40%≤Q1≤67%.

4. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: Based on the weight of the first positive electrode active material layer, the weight fraction of the first positive electrode active material is 18%-90%, and optionally 21%-63%; Optionally, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based materials and lithium cobalt oxide.

5. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The first positive electrode active material layer further comprises lithium iron phosphate; Optionally, based on the total weight of the first positive electrode active material and the lithium iron phosphate in the first positive electrode active material layer, the weight fraction of the lithium iron phosphate is ≤80%, and further optionally 30%-70%.

6. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: Based on the weight of the second positive electrode active material layer, the weight fraction of the second positive electrode active material is 63%-81%.

7. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The ternary material includes one or more of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide and modified compounds thereof; Optionally, the modifying element in the modifying compound includes one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb; Optionally, at least one of the lithium nickel cobalt manganese oxide, the lithium nickel cobalt aluminum oxide and modified compounds thereof is a single crystal primary particle; Optionally, the chemical molar ratio of cobalt ions in at least one of the lithium nickel cobalt manganese oxide, the lithium nickel cobalt aluminum oxide and modified compounds thereof is ≥0.

07.

8. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The ternary material includes a chemical formula of Li a Ni b Co c M 1 d M 2 e O f R g Materials, where 0.75≤a≤1.2, 0 <b<1,0<c<1,0<d<1,0≤e≤0.2,1≤f≤2.5,0≤g≤1,b+c+d+e=1,f+g≤3,M 1 Including one or more of Mn and Al, M 2 Includes one or more of Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, Sr, Sb, Y, W and Nb, and R includes one or more of N, F, S and Cl; Optionally, 0.07≤c<1; Optionally, the chemical formula is Li a Ni b Co c M 1 d M 2 e O f R g The material is single crystal primary particles; Optionally, the ternary material further includes a first coating layer, wherein the first coating layer is provided on the surface of the chemical formula Li a Ni b Co c M 1 d M 2 e O f R g at least a portion of the surface of the material; further optionally, the first coating layer contains one or more of Al2O3 and ZrO2.

9. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The second positive electrode active material layer further comprises a third positive electrode active material, wherein the third positive electrode active material comprises one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based material and lithium cobalt oxide; Optionally, based on the total weight of the second positive electrode active material and the third positive electrode active material, the weight fraction of the third positive electrode active material is 10%-30%.

10. The positive electrode sheet according to any one of claims 1 to 3, characterized in that: The mass ratio of the first positive electrode active material layer to the second positive electrode active material layer is (1.18-1.34):1, and can be optionally (1.19-1.31):

1.

11. The method for preparing a positive electrode sheet according to any one of claims 1 to 10, characterized in that: The steps include: Disposing a first positive electrode slurry containing the first positive electrode active material on one side of the positive electrode current collector to form a first positive electrode active material layer; A second positive electrode slurry containing the second positive electrode active material is disposed on a side of the positive electrode current collector away from the first positive electrode active material layer to form a second positive electrode active material layer.

12. The preparation method according to claim 11, characterized in that: The first positive electrode slurry further comprises lithium iron phosphate; Optionally, the first positive electrode active material includes one or more of lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based materials and lithium cobalt oxide.

13. The preparation method according to claim 11 or 12, characterized in that: The second positive electrode slurry further comprises a third positive electrode active material, wherein the third positive electrode active material comprises one or more of lithium iron phosphate, lithium iron manganese phosphate, lithium manganese oxide, lithium-rich manganese-based material and lithium cobalt oxide.

14. A secondary battery, characterized in that: At least one of the positive electrode sheet according to any one of claims 1 to 10 and the positive electrode sheet prepared by the preparation method according to any one of claims 11 to 13; Optionally, the operating voltage of the secondary battery is 2.5V-4.4V; Optionally, the secondary battery further comprises a negative electrode plate, and the negative electrode active material layer of the negative electrode plate comprises graphite.

15. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 14.

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