Positive electrode sheet and manufacturing method therefor, secondary battery, and electrical device

Abstract

The present application provides a positive electrode sheet and a manufacturing method therefor, a secondary battery, and an electrical device. The positive electrode sheet of the present application has high compaction density.

Description

Positive electrode sheet and preparation method thereof, secondary battery and electric device

[0001] Cross-references

[0002] This application refers to Chinese patent application No. 202410269102.7 filed on March 8, 2024, entitled “Positive electrode sheet and preparation method thereof, secondary battery and electrical device”, which is incorporated into this application in its entirety by reference. Technical Field

[0003] 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

[0004] As a positive electrode active material for secondary batteries, lithium iron phosphate offers advantages such as low cost, strong safety, and excellent cycling performance. However, compared to materials such as NCM, LMO, and LiCoO2, lithium iron phosphate has a lower powder compaction density, resulting in a lower electrode sheet compaction density. Therefore, there is a need to provide a positive electrode sheet containing lithium iron phosphate that has a higher compaction density. Summary of the Invention

[0005] The present application is made in view of the above-mentioned problems, and its purpose is to provide a positive electrode sheet and a preparation method thereof, a secondary battery and an electrical device, wherein the positive electrode sheet has a high compaction density.

[0006] The inventors have discovered that the above objectives can be achieved by adopting the technical solution of the present invention.

[0007] A first aspect of the present application provides a positive electrode plate, the positive electrode plate includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate salt particles, the lithium iron phosphate salt particles include first lithium iron phosphate salt particles, second lithium iron phosphate salt particles and third lithium iron phosphate salt particles, wherein,

[0008] The major diameter of the primary particles of the lithium iron phosphate salt particles is r1, the minor diameter of the primary particles of the lithium iron phosphate salt particles is r2, the primary particle size of the first lithium iron phosphate salt particles satisfies: 50nm≤(r1+r2) / 2≤200nm; the primary particle size of the second lithium iron phosphate salt particles satisfies: 500nm≤(r1+r2) / 2≤1000nm; the primary particle size of the third lithium iron phosphate salt particles satisfies: 1000nm<(r1+r2) / 2≤5000nm;

[0009] Based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 5-27%; the number of the second lithium iron phosphate salt particles accounts for 8-37%; and the number of the third lithium iron phosphate salt particles accounts for 36-86%.

[0010] In any embodiment, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles, and the third lithium iron phosphate salt particles:

[0011] The number of the first lithium iron phosphate particles accounts for 11.76%-20.41%;

[0012] The number of the second lithium iron phosphate particles accounts for 17.65%-23.44%; and / or

[0013] The number of third lithium iron phosphate particles accounts for 57.14%-70.59%.

[0014] The positive electrode sheet of the present application has a relatively high compaction density.

[0015] In any embodiment, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is ≥1.

[0016] In any embodiment, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is 1≤PDI≤1.6.

[0017] In any embodiment, the roundness T1 of the first lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles; and / or

[0018] The roundness T2 of the second lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles.

[0019] In any embodiment, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.6<T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.4≤T3≤1.

[0020] In any embodiment, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.8≤T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.7≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.6≤T3≤1.

[0021] In any embodiment, the first lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q1q1 , wherein Q1 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, 0.9≤x1≤1, 0.95≤y1≤1, 3.5≤z1≤4, and 0≤q1≤0.1, and / or,

[0022] The second lithium iron phosphate salt particle has a molecular formula of Li m2 Fe x2 P y2 O z2 Q2 q2 , wherein Q2 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤z2≤4, 0≤q2≤0.1, and / or,

[0023] The third lithium iron phosphate salt particle has the molecular formula Li m3 Fe x3 P y3 O z3 Q3 q3 , wherein Q3 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m3≤1.15, 0.9≤x3≤1, 0.95≤y3≤1, 3.5≤z3≤4, and 0≤q3≤0.1.

[0024] In any embodiment, Q1, Q2, and Q3 each independently include at least one of Ti, V, Mg, and Nb.

[0025] In any embodiment, the content of Ti, V, Mg and / or Nb elements is 2400-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles;

[0026] The content of Ti, V, Mg and / or Nb elements is 1300-3000 ppm based on the total weight of the second lithium iron phosphate particles; and / or

[0027] Calculated based on the total weight of the third lithium iron phosphate salt particles, the content of Ti, V, Mg and / or Nb elements is 450-1200 ppm.

[0028] A second aspect of the present application provides a method for preparing a positive electrode sheet. The method for preparing a positive electrode sheet includes a method for preparing a positive electrode active material. The method for preparing a positive electrode active material includes the following steps:

[0029] Providing raw materials containing at least a lithium source, an iron source, and a phosphorus source, and performing at least one sintering to obtain a positive electrode active material;

[0030] The iron source includes a first iron source, a second iron source and a third iron source. The average particle size of the primary particles of the first iron source is 80-150 nm, the average particle size of the primary particles of the second iron source is 200-400 nm, and the average particle size of the primary particles of the third iron source is 550-2200 nm.

[0031] In any embodiment, the sintering temperature is 750-820° C., and the sintering time is 10-14 hours.

[0032] The third aspect of the present application provides a secondary battery, which includes the positive electrode sheet of the first aspect of the present application or the positive electrode sheet obtained by the preparation method of the second aspect of the present application.

[0033] A fourth aspect of the present application provides an electrical device comprising the secondary battery according to the third aspect of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] FIG1 is a schematic diagram of a secondary battery according to an embodiment of the present application.

[0035] FIG. 2 is an exploded view of the secondary battery according to the embodiment of the present application shown in FIG. 1 .

[0036] FIG3 is a schematic diagram of a battery module according to an embodiment of the present application.

[0037] FIG4 is a schematic diagram of a battery pack according to an embodiment of the present application.

[0038] FIG. 5 is an exploded view of the battery pack shown in FIG. 4 according to an embodiment of the present application.

[0039] FIG6 is a schematic diagram of an electric device using a secondary battery as a power source according to an embodiment of the present application.

[0040] Explanation of reference numerals: 1 battery pack; 2 upper case; 3 lower case; 4 battery module; 5 secondary battery; 51 housing; 52 electrode assembly; 53 top cover assembly. DETAILED DESCRIPTION

[0041] Below, the embodiments of the positive electrode sheet and its manufacturing method, secondary battery, and electric 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.

[0042] " range " disclosed in the present application is limited in the form of lower limit and upper limit, and 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 special range. The scope limited in this way can be to include end value or not include end value, and can be arbitrarily combined, that is, any lower limit can form a range with any upper limit combination. For example, if the scope of 60-120 and 80-110 is listed for specific parameters, it is understood that the scope of 60-110 and 80-120 is also expected. In addition, if the minimum range value 1 and 2 are listed, and if the maximum range value 3,4 and 5 are listed, then the following range can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In this 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 all 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 simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0044] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0045] 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 may include 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 may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0046] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0047] Unless otherwise specified, the term "or" is used in this application to be inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0048] As a positive electrode active material for secondary batteries, lithium iron phosphate offers advantages such as low cost, strong safety, and excellent cycling performance. However, compared to materials such as NCM, LMO, and LiCoO2, lithium iron phosphate has a lower powder compaction density, resulting in a lower electrode sheet compaction density. Therefore, there is a need to provide a positive electrode sheet containing lithium iron phosphate that has a higher compaction density.

[0049] Based on this, this application proposes a technical solution to solve the above technical problems.

[0050] A first aspect of the present application provides a positive electrode plate, the positive electrode plate includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate salt particles, the lithium iron phosphate salt particles include first lithium iron phosphate salt particles, second lithium iron phosphate salt particles and third lithium iron phosphate salt particles, wherein,

[0051] The long diameter of the primary particle of the lithium iron phosphate salt particle is r1, the short diameter of the primary particle of the lithium iron phosphate salt particle is r2, the primary particle size of the first lithium iron phosphate salt particle satisfies: 50 nanometers (nm) ≤ (r1+r2) / 2 ≤ 200nm; the primary particle size of the second lithium iron phosphate salt particle satisfies: 500nm ≤ (r1+r2) / 2 ≤ 1000nm; the primary particle size of the third lithium iron phosphate salt particle satisfies: 1000nm < (r1+r2) / 2 ≤ 5000nm.

[0052] In some embodiments, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 5-27%; the number of the second lithium iron phosphate salt particles accounts for 8-37%; and the number of the third lithium iron phosphate salt particles accounts for 36-86%.

[0053] In some embodiments, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles: the number of the first lithium iron phosphate salt particles accounts for 11.76%-20.41%; the number of the second lithium iron phosphate salt particles accounts for 17.65%-23.44%; and / or the number of the third lithium iron phosphate salt particles accounts for 57.14%-70.59%.

[0054] When the above conditions are met, by mixing large particles with a larger primary particle size, medium particles with a medium primary particle size, and small particles with a smaller primary particle size in a specific quantity ratio, and by controlling the size, size difference and quantity ratio of the particles, the medium particles and small particles can densely fill the pores between the large particles, and the small particles can densely fill the pores between the medium particles and between the medium particles and the large particles, so that the positive electrode sheet containing the positive electrode active material has a higher compaction density.

[0055] In this article, "primary particles" refer to particles that do not have obvious agglomeration interfaces in the particle scanning electron microscope image, but may have tiny pores and point or line defects, which are different from the smallest unit powder particles without structures such as stacking and flocculation.

[0056] In this article, "long diameter" and "short diameter" refer to the two diagonals of a quadrilateral drawn around the center of a single primary particle, with the longer diagonal being the long diameter r1 and the shorter diagonal being the short diameter r2.

[0057] The major and minor diameters can be measured using conventional methods known in the art. For example, the major diameter r1 and minor diameter r2 of a primary particle can be measured using a Sigma 300 and Avizo software: an argon ion beam is used to cut the positive electrode piece perpendicular to its large surface, exposing the cross section. The cross section is photographed using a scanning electron microscope. A circumscribed quadrilateral is drawn around the center of a single particle, and the two diagonals of the quadrilateral are obtained. The longer diagonal is the major diameter r1, and the shorter diagonal is the minor diameter r2. Test multiple scanning electron microscope images of the positive electrode sheet cross-section, count the (r1+r2) / 2 of each particle, and record the particles whose primary particle size satisfies: 50nm≤(r1+r2) / 2≤200nm as the first lithium iron phosphate salt particles; the particles whose primary particle size satisfies: 500nm≤(r1+r2) / 2≤1000nm as the second lithium iron phosphate salt particles; the particles whose primary particle size satisfies: 1000nm<(r1+r2) / 2≤5000nm as the third lithium iron phosphate salt particles, and the number of the first lithium iron phosphate salt particles divided by the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles is used as the number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles. The proportion of the first lithium iron phosphate salt particles in the total number of lithium iron phosphate salt particles; and so on, the proportion of the second lithium iron phosphate salt particles in the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles is calculated by dividing the number of the second lithium iron phosphate salt particles by the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles; the proportion of the third lithium iron phosphate salt particles in the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles is calculated by dividing the number of the third lithium iron phosphate salt particles by the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles.

[0058] In some embodiments, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, or a range consisting of any two of the above number proportions or a numerical value in the range.

[0059] In some embodiments, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the second lithium iron phosphate salt particles accounts for 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, or a range consisting of any two of the above number proportions or a numerical value in the range.

[0060] In some embodiments, based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the third lithium iron phosphate salt particles accounts for 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, 51%, 52%, 53%, 54%, 55%, 56%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, 101%, 102%, 103%, 104%, 105%, 106%, 107%, 108%, 109%, 110%, 111%, 112%, 113%, 114%, 115%, 116%, 117%, 118%, 119%, 120%, 121%, 122%, 123%, 124%, 125%, 126%, 127%, 128%, 129%, 130%, 131%, 132%, 57%, 58%, 59%, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, or a range consisting of any two of the above percentages or a value within that range.

[0061] In some embodiments, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is ≥1.

[0062] In this article, "particle size distribution index" or "PDI" is a parameter that describes the uniformity of particle size distribution. The smaller the PDI value, the more uniform the particle size distribution; conversely, the larger the PDI value, the more uneven the particle size distribution.

[0063] PDI can be measured by conventional methods known in the art. For example, PDI is measured using Sigma 300 and Avizo software: an argon ion beam is used to cut the positive electrode plate perpendicular to the large surface of the plate to expose the cross section, and the cross section is photographed using a scanning electron microscope. The long diameter statistical method described above is used to perform statistical analysis on the long diameter (i.e., r1) of the lithium iron phosphate salt particles to obtain the PDI of the lithium iron phosphate salt particles.

[0064] Where σ is the standard deviation of particle size, xi is the particle size value (i.e., the major diameter value), is the average particle size (i.e., the average major diameter), and n is the total number of particles counted. Because particles with a diameter of 0 < major diameter < 50 nm tend to clumping together, this can lead to significant statistical errors and makes it difficult to clearly identify them individually. Therefore, particles with a diameter of 0 < major diameter < 50 nm are not included in the particle size statistics.

[0065] In some embodiments, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is 1≤PDI≤1.6. In some embodiments, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is 1.1≤PDI≤1.6. In some embodiments, the particle size distribution index (PDI) of the lithium iron phosphate salt particles is 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, or a range consisting of any two of the above values ​​or a value within the range.

[0066] When the particle size distribution index PDI of the lithium iron phosphate salt particles meets the above requirements, by adjusting the appropriate particle size distribution, the powder compaction density of the lithium iron phosphate salt particles can be effectively improved, thereby increasing the compaction density of the corresponding positive electrode sheet, while also taking into account the gram capacity and material kinetic performance.

[0067] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles; and / or

[0068] The roundness T2 of the second lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles.

[0069] Herein, "roundness" refers to the ratio of the average minor diameter to the average major diameter of primary particles.

[0070] When calculating the average short diameter and the average long diameter, considering that particles with a diameter of 0 < long diameter < 50 nm are prone to adhesion, there are large statistical errors and it is difficult to clearly identify them separately. Therefore, in the particle size statistics process, particles with a diameter of 0 < long diameter < 50 nm are not included in the statistical range.

[0071] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.5≤T1≤1. In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.6≤T1≤1. In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.6<T1≤1. In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.7≤T1≤1. In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.8≤T1≤1. In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles is 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of the above values ​​or a value within the range.

[0072] In some embodiments, the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1. In some embodiments, the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.6≤T2≤1. In some embodiments, the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.7≤T2≤1. In some embodiments, the roundness T2 of the second lithium iron phosphate salt particles is 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of the above values ​​or a value within the range.

[0073] In some embodiments, the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.1≤T3≤1. In some embodiments, the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.4≤T3≤1. In some embodiments, the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.5≤T3≤1. In some embodiments, the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.6≤T3≤1. In some embodiments, the roundness T3 of the third lithium iron phosphate salt particles is 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1, or a range consisting of any two of the above values ​​or a value within the range.

[0074] When the roundness T3 of the third lithium iron phosphate salt particles is ≥0.4, the secondary battery including the positive electrode plate of the present application has a higher gram capacity.

[0075] When the roundness T2 of the second lithium iron phosphate salt particles is greater than 0.6, the secondary battery including the positive electrode plate of the present application has a higher energy density.

[0076] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.5≤T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.1≤T3≤1.

[0077] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.5≤T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.4≤T3≤1.

[0078] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.6<T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.4≤T3≤1.

[0079] In some embodiments, the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.8≤T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.7≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.6≤T3≤1.

[0080] By cleverly selecting the roundness T1 of the first lithium iron phosphate salt particles, the roundness T2 of the second lithium iron phosphate salt particles, and the roundness T3 of the third lithium iron phosphate salt particles, the particles can naturally form a dense stacking during the compaction process, and during the dense stacking process, the medium particles can smoothly slide into the gaps between the large particles, and the small particles can smoothly slide into the gaps between the large particles and between the large particles and the medium particles, thereby improving the powder compaction density of the lithium iron phosphate salt particles, thereby improving the compaction density of the corresponding positive electrode sheet.

[0081] In some embodiments, the first lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q1 q1 , wherein Q1 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, 0.9≤x1≤1, 0.95≤y1≤1, 3.5≤z1≤4, and 0≤q1≤0.1, and / or,

[0082] The second lithium iron phosphate salt particle has a molecular formula of Li m2 Fe x2 P y2 O z2Q2 q2 , wherein Q2 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤z2≤4, 0≤q2≤0.1, and / or,

[0083] The third lithium iron phosphate salt particle has the molecular formula Li m3 Fe x3 P y3 O z3 Q3 q3 , wherein Q3 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m3≤1.15, 0.9≤x3≤1, 0.95≤y3≤1, 3.5≤z3≤4, and 0≤q3≤0.1.

[0084] In some embodiments, the first lithium iron phosphate salt particles have the molecular formula Li m1 Fe x1 P y1 O z1 Q q1 , m1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x1 can be 0.9, 1.0, y1 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, z1 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q1 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0085] In some embodiments, the second lithium iron phosphate salt particles have the molecular formula Li m2 Fe x2 P y2 O z2 Q q2, m2 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x2 can be 0.9, 1.0, y2 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, z2 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q2 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0086] In some embodiments, the third lithium iron phosphate salt particles have the molecular formula Li m3 Fe x3 P y3 O z3 Q3 q3 , m3 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, x3 can be 0.9, 1.0, y3 can be 0.95, 0.98, 1.00, 1.03, 1.05, 1.08, 1.10, 1.13, 1.15, z3 can be 3.5, 3.6, 3.7, 3.8, 3.9, 4, and q3 can be 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1.

[0087] Modifying the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and / or the third lithium iron phosphate salt particles by using Q1, Q2 and / or Q3 elements helps to improve the ion transport capacity of the positive electrode active material. The above elements can create vacancies in the particle lattice or change the interatomic bond length, facilitating the movement of lithium ions in the lattice, thereby effectively improving the conductivity of the particles themselves, improving the kinetic properties of the positive electrode active material, and also improving the capacity. In this application, modification can be specifically manifested as doping and / or coating.

[0088] In some embodiments, Q1, Q2, and Q3 each independently include at least one of Ti, V, Mg, and Nb.

[0089] In some embodiments, the content of Ti, V, Mg and / or Nb elements is 2400-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 2600-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000, 3100, 3200 ppm, or a range consisting of any two of the above values ​​or a value within the range.

[0090] In some embodiments, the Ti content is 2400-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles. In some embodiments, the Ti content is 2600-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles. In some embodiments, the Ti content is 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000, 3100, 3200 ppm, or a range consisting of any two of the foregoing values ​​or a value within the range, calculated based on the total weight of the first lithium iron phosphate salt particles.

[0091] In some embodiments, the content of Ti, V, Mg and / or Nb elements is 1300-3000 ppm, calculated based on the total weight of the second lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 1400-2900 ppm, calculated based on the total weight of the second lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm, 2100 ppm, 2200 ppm, 2300 ppm, 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, or a range consisting of any two of the above values ​​or a value within the range.

[0092] In some embodiments, the Ti content is 1300-3000 ppm, calculated based on the total weight of the second lithium iron phosphate salt particles. In some embodiments, the Ti content is 1400-2900 ppm, calculated based on the total weight of the second lithium iron phosphate salt particles. In some embodiments, the Ti content is 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm, 2100 ppm, 2200 ppm, 2300 ppm, 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, 3000 ppm, or a range consisting of any two of the above values ​​or a value within the range.

[0093] In some embodiments, the content of Ti, V, Mg and / or Nb elements is 450-1200 ppm, calculated based on the total weight of the third lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 500-1200 ppm, calculated based on the total weight of the third lithium iron phosphate salt particles. In some embodiments, the content of Ti, V, Mg and / or Nb elements is 450 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, or a range consisting of any two of the above values ​​or a value within the range.

[0094] In some embodiments, the Ti content is 450-1200 ppm, calculated based on the total weight of the third lithium iron phosphate salt particles. In some embodiments, the Ti content is 500-1200 ppm, calculated based on the total weight of the third lithium iron phosphate salt particles. In some embodiments, the Ti content is 450 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, or a range consisting of any two of the foregoing values ​​or a value within the range.

[0095] In some embodiments, the content of Ti, V, Mg and / or Nb elements is 2400-3200 ppm, calculated based on the total weight of the first lithium iron phosphate salt particles;

[0096] The content of Ti, V, Mg and / or Nb elements is 1300-3000 ppm based on the total weight of the second lithium iron phosphate particles; and / or

[0097] Calculated based on the total weight of the third lithium iron phosphate salt particles, the content of Ti, V, Mg and / or Nb elements is 450-1200 ppm.

[0098] In some embodiments, the Ti content is H1, calculated based on the total weight of the first lithium iron phosphate salt particles, the Ti content is H3, calculated based on the total weight of the third lithium iron phosphate salt particles, and H1 / H3 is 2-6. In some embodiments, H1 / H3 is 2.6-5.8. In some embodiments, H1 / H3 is 2, 2.5, 2.6, 2.7, 3, 3.5, 3.8, 3.9, 4, 4.2, 4.5, 5, 5.2, 5.3, 5.5, 5.6, 5.8, 6, or a range consisting of any two of the above values ​​or a value within the range.

[0099] The use of gradient doping design is conducive to forming a natural particle distribution during the sintering process, increasing the width of the particle size distribution, and improving the powder compaction density, thereby increasing the compaction density of the electrode.

[0100] The Ti content can be determined using conventional methods known in the art. For example, the Ti content test can be performed with reference to GB / T 33822-2017.

[0101] In some embodiments, the carbon content of the first lithium iron phosphate salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, 2.1 weight%, 2.2 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0102] In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is coated on the surface of the particles. In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the first lithium iron phosphate salt particles is partially coated on the surface of the particles and partially embedded in the particles.

[0103] In some embodiments, the carbon content of the second lithium iron phosphate salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, 2.1 weight%, 2.2 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0104] In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is coated on the particle surface. In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the second lithium iron phosphate salt particles is partially coated on the particle surface and partially embedded in the particles.

[0105] In some embodiments, the carbon content of the third lithium iron phosphate salt particles is 0.8 weight%, 0.9 weight%, 1.0 weight%, 1.1 weight%, 1.2 weight%, 1.3 weight%, 1.4 weight%, 1.5 weight%, 1.6 weight%, 1.7 weight%, 1.8 weight%, 1.9 weight%, 2.0 weight%, 2.1 weight%, 2.2 weight%, or a range consisting of any two of the above carbon contents or a value within the range.

[0106] In some embodiments, the carbon contained in the third lithium iron phosphate salt particles is coated on the particle surface. In some embodiments, the carbon contained in the third lithium iron phosphate salt particles is embedded in the particles. In some embodiments, the carbon contained in the third lithium iron phosphate salt particles is partially coated on the particle surface and partially embedded in the particles.

[0107] In some embodiments, the carbon content of the lithium iron phosphate salt particles is 0.8 weight %, 0.9 weight %, 1.0 weight %, 1.1 weight %, 1.2 weight %, 1.3 weight %, 1.4 weight %, 1.5 weight %, 1.6 weight %, 1.7 weight %, 1.8 weight %, 1.9 weight %, 2.0 weight %, 2.1 weight %, 2.2 weight %, or a range consisting of any two of the above carbon contents or a value within the range.

[0108] A second aspect of the present application provides a method for preparing a positive electrode sheet. The method for preparing a positive electrode sheet includes a method for preparing a positive electrode active material. The method for preparing a positive electrode active material includes the following steps:

[0109] Providing raw materials containing at least a lithium source, an iron source, and a phosphorus source, and performing at least one sintering to obtain a positive electrode active material;

[0110] The iron source includes a first iron source, a second iron source and a third iron source. The average particle size of the primary particles of the first iron source is 80-150 nm, the average particle size of the primary particles of the second iron source is 200-400 nm, and the average particle size of the primary particles of the third iron source is 550-2200 nm.

[0111] In some embodiments, the lithium source is a lithium-containing compound. In some embodiments, the lithium source includes at least one of lithium dihydrogen phosphate, lithium oxalate, lithium carbonate, lithium oxide, lithium hydroxide, and lithium acetate. In some embodiments, the lithium source includes lithium carbonate.

[0112] In some embodiments, the iron source is an iron-containing compound. In some embodiments, the iron source includes at least one of ferric hydroxide, ferrous chloride, ferric oxide, ferric phosphate, ferric pyrophosphate, ferrous oxalate, iron powder, ferric nitrate, ferric oxide, and ferric oxyhydroxide. In some embodiments, the iron source includes ferric phosphate.

[0113] In some embodiments, the phosphorus source is a phosphoric acid compound. In some embodiments, the phosphorus source includes at least one of phosphoric acid, ferric phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. In some embodiments, the phosphorus source includes ferric phosphate.

[0114] In some embodiments, the iron source and the phosphorus source can be the same substance. In some embodiments, ferric phosphate is used as the iron source and the phosphorus source.

[0115] In some embodiments, the raw material further comprises a carbon source. In some embodiments, the carbon source comprises at least one of citric acid, glucose, sucrose, starch, fructose, and lactose. In some embodiments, the carbon source comprises glucose.

[0116] In some embodiments, the sintering temperature is 750-820° C., and the sintering time is 10-14 hours.

[0117] In some embodiments, the sintering temperature is 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, 810°C, 820°C, or a range consisting of any two of the foregoing values ​​or a value within the range.

[0118] In some embodiments, the sintering time is 10 h, 11 h, 12 h, 13 h, 14 h, or a range consisting of any two of the above values ​​or a value within the range.

[0119] The third aspect of the present application provides a secondary battery, which includes the positive electrode sheet of the first aspect of the present application or the positive electrode sheet obtained by the preparation method of the second aspect of the present application.

[0120] A fourth aspect of the present application provides an electrical device comprising the secondary battery according to the third aspect of the present application.

[0121] In addition, the secondary battery and the electric device of the present application will be described below with reference to the drawings as appropriate.

[0122] In one embodiment of the present application, a secondary battery is provided.

[0123] 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.

[0124] [Positive electrode]

[0125] 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, and the positive electrode film layer includes a positive electrode active material.

[0126] As an 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.

[0127] 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 and a metal layer formed on at least one surface of the polymer material base. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0128] In some embodiments, the positive electrode film layer may further optionally include a binder. As an example, the binder may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylate resin.

[0129] In some embodiments, the positive electrode film layer may further include a conductive agent. For example, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0130] 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; the positive electrode slurry is coated on the positive electrode current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained.

[0131] [Negative electrode]

[0132] 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.

[0133] As an 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.

[0134] 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 base layer and a metal layer formed on at least one surface of the polymer base material. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer base material (such as a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0135] In some embodiments, the negative electrode active material may be a negative electrode active material for a battery that is well known in the art. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may be selected from at least one of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one 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.

[0136] In some embodiments, the negative electrode film layer may further include a binder. The binder may be selected from at least one 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).

[0137] In some embodiments, the negative electrode film layer may further include a conductive agent, which may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0138] In some embodiments, the negative electrode film layer may optionally include other additives, such as a thickener (eg, sodium carboxymethyl cellulose (CMC-Na)).

[0139] In some embodiments, the negative electrode sheet can be prepared by the following method: 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 (such as deionized water) to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0140] [Electrolytes]

[0141] The electrolyte conducts ions between the positive and negative electrodes. This application does not specify the type of electrolyte, and the electrolyte can be selected based on the needs. For example, the electrolyte can be liquid, gel, or solid.

[0142] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0143] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalatoborate, lithium dioxalatoborate, lithium difluorodioxalatophosphate, and lithium tetrafluorooxalatophosphate.

[0144] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, fluoroethylene carbonate, 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.

[0145] 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.

[0146] [Isolation film]

[0147] 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.

[0148] In some embodiments, the material of the separator can be selected from at least one 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.

[0149] 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.

[0150] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0151] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. Alternatively, the outer packaging of the secondary battery can be a soft shell, such as a pouch-type soft shell. The soft shell can be made of plastic, such as polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0152] [Secondary battery]

[0153] The present application provides a secondary battery, which includes the positive electrode sheet of the present application.

[0154] The present application has no particular limitation on the shape of the secondary battery, which may be cylindrical, square, or any other shape. For example, FIG1 shows a secondary battery 5 with a square structure as an example.

[0155] In some embodiments, referring to Figure 2, 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 secondary battery 5 can be one or more, and those skilled in the art can select according to specific actual needs.

[0156] In some embodiments, secondary batteries can be assembled into a battery module. The number of secondary batteries 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.

[0157] Figure 3 shows an example battery module 4. Referring to Figure 3 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured together using fasteners.

[0158] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0159] 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.

[0160] Figures 4 and 5 illustrate an example battery pack 1. Referring to Figures 4 and 5 , the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the battery box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 can be arranged in any manner within the battery box.

[0161] [Electrical devices]

[0162] In addition, the present application also provides an electrical device, which includes the positive electrode sheet of the present application or the secondary battery of the present application.

[0163] In some embodiments, the electrical device of the present application may further include at least one of a battery module or a battery pack. Secondary batteries, battery modules, or battery packs can be used as power sources for electrical devices, or as energy storage units for electrical devices. Electrical devices may include, but are not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships, satellites, energy storage systems, etc.

[0164] As an electrical device, a secondary battery, a battery module or a battery pack can be selected according to its usage requirements.

[0165] Figure 6 shows an example of an electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the secondary battery, a battery pack or battery module can be used.

[0166] 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.

[0167] Example

[0168] Below, the embodiment of the present application is described. The embodiment described below is exemplary and is only used to explain the present application, and is not to be construed as limiting the present application. Where specific 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. Reagents or instruments used that do not specify the manufacturer are conventional products that can be obtained commercially.

[0169] 1. Preparation method

[0170] Example 1

[0171] 1) Preparation of positive electrode active materials

[0172] The preparation methods of the first, second, and third ferric phosphate salts are as follows: TiO2, a ferric sulfate solution, and a diammonium phosphate solution are uniformly mixed according to the Fe / P molar ratio and Ti content shown in Table 1. A H2O2 solution with a molar ratio of 0.5 to 10 times that of Fe is added. After reacting for 1 to 2 hours, the solid is separated and sintered at a high temperature of 300°C to 600°C for 3 to 5 hours. By controlling the reaction time with the H2O2 solution and the high-temperature sintering time, the first, second, and third ferric phosphate salts are obtained, respectively.

[0173] The first iron phosphate salt, the second iron phosphate salt, and the third iron phosphate salt were mixed in a weight ratio of 1:4:10 to serve as an iron source.

[0174] The Fe source, Li2CO3 and glucose were mixed evenly at a ratio of Li / P (molar ratio) = 1.01 and a finished product carbon content of 1.25%, and then transferred to a sand mill for thorough grinding until the slurry particle size Dv50 was 0.38±0.01μm, and then spray-dried. The spray-dried material was sintered at a temperature of 800°C and a high-purity nitrogen atmosphere for 12h. After natural cooling, it was pulverized by air flow until the powder Dv50 was 1.4±0.2μm to obtain the positive electrode active material.

[0175] 2) Preparation of positive electrode sheet

[0176] The positive electrode active material, binder polyvinylidene fluoride (PVDF), and conductive agent acetylene black were added to solvent N-methylpyrrolidone at a mass ratio of 96.5:2.0:1.5, and stirred evenly to obtain a positive electrode slurry;

[0177] Then the positive electrode slurry was prepared at 200 mg / cm 2 The single-sided coating density is coated on an aluminum foil with a thickness of 13 μm. After coating, it is dried, cold pressed, and cut to obtain the positive electrode sheet.

[0178] 3) Preparation of negative electrode sheet

[0179] The negative electrode active material graphite, the thickener sodium carboxymethyl cellulose, the binder styrene-butadiene rubber, and the conductive agent acetylene black were mixed in a mass ratio of 97:1:1:1, and deionized water was added to obtain a negative electrode slurry under the action of a vacuum mixer;

[0180] Then the negative electrode slurry was adjusted to 140 mg / cm 2 The single-sided coating is evenly coated on a copper foil with a thickness of 8 μm. After coating, it is dried, cold pressed and cut to obtain the negative electrode sheet.

[0181] 4) Preparation of isolation membrane

[0182] Polypropylene film is used as the isolation film.

[0183] 5) Preparation of electrolyte

[0184] In an argon atmosphere glove box (H2O<0.1ppm, O2<0.1ppm), organic solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed in a volume ratio of 1 / 1, and lithium salt LiPF6 was added and dissolved in the organic solvent. The content of LiPF6 in the solution was 1 mol / L. The mixture was stirred evenly to obtain an electrolyte.

[0185] 6) Preparation of batteries

[0186] After the electrode is placed in a high-temperature oven at 110°C and baked for 7 hours to remove water, the positive electrode, isolation film, and negative electrode are stacked in order, so that the isolation film is between the positive and negative electrode sheets to play an isolation role. Then, the bare battery cell is wound to obtain the electrode ear, and the bare battery cell is placed in an aluminum shell and baked in a vacuum oven at 100°C for 8 hours. Then, the electrolyte is injected and sealed to obtain an uncharged battery. The uncharged battery is then subjected to the processes of static standing, hot and cold pressing, formation, shaping, capacity testing, etc. in sequence to obtain the lithium-ion battery of Example 1.

[0187] Examples 2-16 and Comparative Examples 1-2 are similar to Example 1, and the main differences in the positive electrode active materials are shown in Table 1, and the main differences in the preparation methods are shown in Table 2.

[0188] 2. Battery performance test

[0189] 1. Performance test of positive electrode active materials

[0190] 1) Test method for particle size and roundness of primary particles of lithium iron phosphate salt particles

[0191] The long diameter r1 and short diameter r2 of the primary particle were measured using Sigma300 and Avizo software (default parameters): an argon ion beam was used to cut the electrode perpendicular to the large surface of the positive electrode to expose the cross section, which was photographed using a scanning electron microscope. A circumscribed quadrilateral was drawn with the center of a single particle to obtain the two diagonals of the quadrilateral. The longer diagonal was the long diameter r1, and the shorter diagonal was the short diameter r2.

[0192] The roundness T of the primary particles = average minor diameter / average major diameter.

[0193] Since particles with a size of 0<r1<50nm are prone to adhesion, there are large errors in statistics and it is difficult to clearly identify them separately. Therefore, during the particle size statistics process, particles with a size of 0<r1<50nm are not included in the statistical range.

[0194] 2) PDI test method for lithium iron phosphate particles

[0195] PDI was measured using Sigma 300 and Avizo software (default parameters): an argon ion beam was used to cut the positive electrode perpendicular to the large surface of the electrode to expose the cross section, which was photographed using a scanning electron microscope. The particle size of the lithium iron phosphate salt particles (i.e., r1) was statistically analyzed using the above-mentioned long diameter statistical method to obtain the PDI of the lithium iron phosphate salt particles.

[0196] Where σ is the standard deviation of particle size, xi is the particle size value (i.e., the major diameter value), is the average particle size (ie, the average major diameter), and n is the total number of particles counted. As mentioned above, in the particle size statistics process, particles with a particle size of 0 < r1 < 50 nm are not within the statistical range.

[0197] 3) Ti content test method

[0198] The Ti content test was carried out in accordance with GB / T 33822-2017.

[0199] 4) Testing method for the number of first lithium iron phosphate salt particles, the number of second lithium iron phosphate salt particles, and the number of third lithium iron phosphate salt particles

[0200] Measurements were performed using a Sigma 300 and Avizo software (default parameters): An argon ion beam was used to cut the positive electrode perpendicular to its broad surface, exposing the cross-section. The cross-section was then photographed using a scanning electron microscope. The long and short diameters of the lithium iron phosphate particles were statistically analyzed using the length-diameter method. The particles were then classified as first, second, or third lithium iron phosphate particles and counted separately. As mentioned above, particles with a diameter of 0 < r1 < 50 nm were excluded from the statistical analysis.

[0201] 4) Compaction density test method of positive electrode

[0202] When the electrode is coated on one side, the compaction density of the film layer on one side of the electrode = m / (V1-V2). When the electrode is coated on both sides, the compaction density of the film layer on one side of the electrode = m / [2×(V1-V2)], where m represents the weight of the film layer, V1 represents the volume of the electrode, and V2 represents the volume of the current collector. m can be obtained by subtracting the weight of the current collector from the weight of the electrode. The product of the surface area of ​​the electrode and the thickness of the electrode is the volume V1 of the electrode, and the product of the surface area of ​​the electrode and the thickness of the current collector is V2. The thickness of the current collector and the thickness of the electrode are obtained by measuring the thickness of the empty foil in the tab area with a micrometer.

[0203] 2. Battery performance test

[0204] 1) Test method for charging gram capacity and discharging gram capacity of secondary batteries

[0205] 0.1C rate performance test: At room temperature (25°C), the battery was charged at a constant current of 0.1C to 3.75V, then charged at a constant voltage of 3.75V to a current of 0.05C, and left for 5 minutes; then discharged at 0.1C to 2.0V to obtain the charge and discharge capacity.

[0206] 2) Energy density test method of secondary batteries

[0207] Allow the battery cell to rest at 25°C for 2 hours, ensuring the cell temperature is 25°C. Charge the battery cell at 0.33C at 25°C to a charge cutoff voltage of 3.65V. Continue constant voltage charging at this charge cutoff voltage until the current reaches 0.05C, at which point charging is terminated (where C represents the rated capacity of the battery cell). Allow the battery cell to rest at 25°C for 1 hour, then discharge it at 0.33C at 25°C to a discharge cutoff voltage of 3.65V. Record the total discharge energy of the battery cell as E0.

[0208] Place the battery cell on an electronic balance until the weight stabilizes, and read the battery cell weight value M0.

[0209] Battery energy density = battery cell discharge energy E0 / battery cell weight M0.

[0210] 3. Analysis of test results of various embodiments and comparative examples

[0211] Batteries of various embodiments and comparative examples were prepared according to the above method, and various performance parameters were measured. The parameters of the positive electrode active material are shown in Table 1, the preparation method parameters are shown in Table 2, and the performance test results are shown in Table 3.

[0212] Table 3. Performance test results

[0213] According to the above results, the positive electrode plates in Examples 1-16 all include positive electrode active materials, the positive electrode active materials include lithium iron phosphate particles, the lithium iron phosphate particles include first lithium iron phosphate particles, second lithium iron phosphate particles and third lithium iron phosphate particles, wherein the major diameter of the primary particles of the lithium iron phosphate particles is r1, the minor diameter of the primary particles of the lithium iron phosphate particles is r2, and the primary particle size of the first lithium iron phosphate particles satisfies: 50nm≤(r1+r2) / 2≤200nm; the second lithium iron phosphate particles The primary particle size of the first lithium iron phosphate salt particles satisfies the following conditions: 500 nm ≤ (r1 + r2) / 2 ≤ 1000 nm; the primary particle size of the third lithium iron phosphate salt particles satisfies the following conditions: 1000 nm < (r1 + r2) / 2 ≤ 5000 nm; based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles, and the third lithium iron phosphate salt particles, the first lithium iron phosphate salt particles account for 5-27%; the second lithium iron phosphate salt particles account for 8-37%; and the third lithium iron phosphate salt particles account for 36-86%. A comparison of Examples 1-16 with Comparative Example 1 shows that the positive electrode sheets of the present invention have a good compaction density.

[0214] From the comparison between Examples 1-11, 13-16 and Example 12, it can be seen that when the roundness T3 of the third lithium iron phosphate salt particles is ≥0.4, the secondary battery including the positive electrode plate of the present application has a higher gram capacity.

[0215] From the comparison between Examples 1-5, 7-11, 13, 15-16 and Examples 6, 12, and 14, it can be seen that when the roundness T2 of the second lithium iron phosphate salt particles is greater than 0.6, the secondary battery including the positive electrode plate of the present application has a higher energy density.

[0216] 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 plate, comprising a positive electrode active material, wherein the positive electrode active material comprises lithium iron phosphate salt particles, wherein the lithium iron phosphate salt particles comprise first lithium iron phosphate salt particles, second lithium iron phosphate salt particles, and third lithium iron phosphate salt particles, wherein: The long diameter of the primary particles of the lithium iron phosphate salt particles is r1, the short diameter of the primary particles of the lithium iron phosphate salt particles is r2, the primary particle size of the first lithium iron phosphate salt particles satisfies: 50nm≤(r1+r2) / 2≤200nm; the primary particle size of the second lithium iron phosphate salt particles satisfies: 500nm≤(r1+r2) / 2≤1000nm; the primary particle size of the third lithium iron phosphate salt particles satisfies: 1000nm<(r1+r2) / 2≤5000nm; Based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles, the number of the first lithium iron phosphate salt particles accounts for 5-27%; the number of the second lithium iron phosphate salt particles accounts for 8-37%; and the number of the third lithium iron phosphate salt particles accounts for 36-86%.

2. The positive electrode sheet according to claim 1, wherein: Based on the total number of the first lithium iron phosphate salt particles, the second lithium iron phosphate salt particles and the third lithium iron phosphate salt particles: The number of the first lithium iron phosphate particles accounts for 11.76%-20.41%; The amount of the second lithium iron phosphate particles accounts for 17.65%-23.44%; and / or The third lithium iron phosphate salt particles account for 57.14%-70.59% of the total number. 3 . The positive electrode plate according to claim 1 , wherein the particle size distribution index (PDI) of the lithium iron phosphate salt particles is ≥1. 4 . The positive electrode sheet according to claim 1 , wherein the particle size distribution index (PDI) of the lithium iron phosphate salt particles is 1≤PDI≤1.

6.

5. The positive electrode according to any one of claims 1 to 4, wherein the roundness T1 of the first lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles; and / or The roundness T2 of the second lithium iron phosphate salt particles is greater than or equal to the roundness T3 of the third lithium iron phosphate salt particles.

6. The positive electrode plate according to any one of claims 1 to 5, wherein the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.6<T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.5≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.4≤T3≤1.

7. The positive electrode plate according to any one of claims 1 to 6, wherein the roundness T1 of the first lithium iron phosphate salt particles satisfies: 0.8≤T1≤1; the roundness T2 of the second lithium iron phosphate salt particles satisfies: 0.7≤T2≤1; and / or the roundness T3 of the third lithium iron phosphate salt particles satisfies: 0.6≤T3≤1.

8. The positive electrode according to any one of claims 1 to 7, wherein the first lithium iron phosphate salt particles have a molecular formula of Li m1 Fe x1 P y1 O z1 Q1 q1 , wherein Q1 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m1≤1.15, 0.9≤x1≤1, 0.95≤y1≤1, 3.5≤z1≤4, and 0≤q1≤0.1, and / or, The second lithium iron phosphate salt particles have a molecular formula of Li m2 Fe x2 P y2 O z2 Q2 q2 , wherein Q2 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, Br, 0.95≤m2≤1.15, 0.9≤x2≤1, 0.95≤y2≤1, 3.5≤z2≤4, 0≤q2≤0.1, and / or, The third lithium iron phosphate salt particles have a molecular formula of Li m3 Fe x3 P y3 O z3 Q3 q3 , wherein Q3 includes at least one of Al, Na, K, Mg, Cu, Mn, Cr, Zn, Pb, Ca, Co, Ni, Sr, Nb, V, Ti, B, S, Si, N, F, Cl, and Br, 0.95≤m3≤1.15, 0.9≤x3≤1, 0.95≤y3≤1, 3.5≤z3≤4, and 0≤q3≤0.

1. 9 . The positive electrode sheet according to claim 8 , wherein Q1 , Q2 and Q3 each independently include at least one of Ti, V, Mg and Nb.

10. The positive electrode sheet according to claim 8 or 9, wherein the content of Ti, V, Mg and / or Nb elements is 2400-3200 ppm, calculated based on the total weight of the first lithium iron phosphate particles; The content of Ti, V, Mg and / or Nb elements is 1300-3000 ppm, calculated based on the total weight of the second lithium iron phosphate particles; and / or Calculated based on the total weight of the third lithium iron phosphate salt particles, the content of Ti, V, Mg and / or Nb elements is 450-1200 ppm.

11. A method for preparing a positive electrode sheet, the method comprising preparing a positive electrode active material, the method comprising the following steps: Providing raw materials containing at least a lithium source, an iron source, and a phosphorus source, and performing at least one sintering to obtain the positive electrode active material; The iron source includes a first iron source, a second iron source and a third iron source. The average particle size of primary particles of the first iron source is 80-150 nm, the average particle size of primary particles of the second iron source is 200-400 nm, and the average particle size of primary particles of the third iron source is 550-2200 nm. 12 . The preparation method according to claim 11 , wherein the sintering temperature is 750-820° C. and the sintering time is 10-14 hours. 13 . A secondary battery, comprising the positive electrode sheet according to claim 1 or the positive electrode sheet obtained by the preparation method according to claim 11 or 12. 14 . An electric device comprising the secondary battery according to claim 13 .

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