Composite cell and battery including the same

Abstract

To provide a composite cell that sufficiently exercises a synergy superiority of a positive electrode active material of a different type, and further improve an energy density of a battery, a rate characteristic, and a cycle performance.SOLUTION: A composite cell contains a positive electrode sheet, and each positive electrode sheet satisfies a ratio of a unit area capacity and a ratio of a unit area capacity of the other one of the positive electrode sheets so as to be 0.9 to 1.1. Each positive electrode sheet contains a first positive electrode sheet. A positive electrode active material contained in the first positive electrode sheet contains a first positive electrode active material and a second positive electrode active material. An energy density of the first positive electrode active material is larger than that of the second positive electrode active material. When a mass amount of the first positive electrode active material to be contained in the first positive electrode sheet is m1, and a total mass of the positive electrode active material contained to the first positive electrode sheet is m0, the following equation is satisfied. A mean one surface density of a positive electrode active coating is 50 to 650 g / m2, and a ratio surface resistance is 0.0001 to 0.1500 Ω / mm2*g.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention belongs to the field of battery technology, and specifically relates to a composite cell and a battery including the same.

Background Art

[0002] The mainstream cathode materials for lithium-ion batteries mainly include oxide-based cathode active material systems and phosphate-based cathode active material systems. Oxide-based cathode active materials have a high energy density but low structural stability, while phosphate-based cathode active materials have a low energy density but a long cycle life, and have obvious advantages in terms of cost and safety. How to fully utilize the complementary advantages of different materials has always been a difficult problem in the industry. In the current industry, a blending method is generally used in which a cathode slurry formed by mixing different types of cathode active materials is directly coated on the surface of a current collector to produce a cathode sheet. However, due to the differences in the particle size and surface energy of different types of cathode active materials, the mixed slurry aggregates. In the multi-layer coating method, the above problems can be avoided to a certain extent. However, when the difference in the ratio of different types of cathode active materials is large, the coating surface density of the relatively small material is too small to realize the coating process.

[0003] Based on this, how to integrate different types of cathode sheets in the same cell and fully utilize the synergistic advantages of different types of cathode active materials is a technical problem that those skilled in the art should quickly solve in their research.

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present invention provides a composite cell and a battery including the same, so as to fully utilize the synergistic advantages of different types of cathode active materials and further improve the energy density, rate performance, and cycle performance of the battery.

Means for Solving the Problems

[0005] According to the first aspect of the present invention, a composite cell is provided. The composite cell includes a positive electrode sheet, and the number of positive electrode sheets is greater than 1. Each positive electrode sheet satisfies that the ratio of its unit area capacity to the unit area capacity of any other one positive electrode sheet is 0.9 to 1.1. The positive electrode sheet includes a first positive electrode sheet, and the positive electrode active material included in the first positive electrode sheet includes a first positive electrode active material and a second positive electrode active material. Here, the energy density of the first positive electrode active material is greater than the energy density of the second positive electrode active material. In each first positive electrode sheet, the mass of the first positive electrode active material is smaller than the mass of the second positive electrode active material. Let the mass of the first positive electrode active material included in the first positive electrode sheet be m1, and the total mass of the positive electrode active material included in the first positive electrode sheet be m0.

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[0006] In the composite cell according to the present invention, by controlling the ratio of the positive electrode active material in the first positive electrode sheet, the surface density of the first positive electrode sheet, and the specific surface resistance, the potential distribution of the first positive electrode sheet is made uniform, which is advantageous for alleviating the lithium precipitation phenomenon caused by the excessive local current density of the electrode sheet, and is also advantageous for improving the energy density of the electrode sheet. Moreover, between different positive electrode sheets in the composite cell, the capacity balance state is almost achieved, further ensuring the uniformity of the current density of the composite cell and reducing the impedance level of the electrode sheet. Based on this, the first positive electrode sheet according to the present invention has a stable structure, can fully exert the advantages of different positive electrode active materials, and can effectively alleviate the problems of impedance and current inequality occurring between different types of positive electrode active materials in the composite cell, enabling the composite cell to have excellent energy density, high and low temperature performance, and cycle performance.

[0007] If the mass ratio of the first positive electrode active material is too small, the accuracy of the coating process is limited, processing problems are likely to occur, and in the high-rate charge and discharge process, the non-uniform distribution of lithium ion concentration and potential is obvious, and the lithium precipitation phenomenon is likely to be caused. If the mass ratio of the first positive electrode active material is too large, it is disadvantageous to the structural stability of the first positive electrode sheet and the cost increases.

[0008] If the surface density and specific surface resistance of the first positive electrode sheet are too large, the electron transmission path is long, the transmission speed is slow, the electronic conductivity of the electrode sheet decreases, and peeling is likely to occur between the active material with a large volume effect and the current collector during the lithium insertion and extraction process, making the structure of the electrode sheet unstable. If the surface density and specific surface resistance of the first positive electrode sheet are too low, the coating accuracy of the electrode sheet is limited, processing problems are likely to occur, which is disadvantageous to the uniformity of the lithium ion insertion and extraction process, and the energy density of the cell decreases.

[0009] Specifically, the average surface density of the positive electrode active coating provided on the positive electrode sheet is Di, the average gram capacity of the positive electrode active material contained in the positive electrode sheet is Ci, the average initial efficiency of the positive electrode active material is Ei, the average mass ratio of the positive electrode active material is Wi, and its unit area capacity Qi = Di* Ci * It is Wi / Ei.

[0010] Specifically, the specific surface resistance of the positive electrode active coating = the surface resistance of the positive electrode active coating / the mass of the positive electrode active coating.

[0011] Preferably, the positive electrode sheet further includes a second positive electrode sheet, and the average single-sided surface density of the positive electrode active coating provided on the second positive electrode sheet is 65 to 700 g / m 2 and the specific surface resistance is 0.0002 to 0.2000 Ω / mm 2 ·g. The combination effect of the second positive electrode sheet with a specific surface density and specific surface resistance and the above-mentioned first positive electrode sheet is excellent, and the second positive electrode sheet can exhibit a synergistic effect with the first positive electrode sheet, ensuring the uniformity of the current density between different positive electrode sheets, and avoiding phenomena such as destabilization of the electrode sheet structure, precipitation of lithium, and polarization due to poor combination effect of the electrode sheets, thereby improving the energy density, high and low temperature performance, and cycle performance of the composite cell according to the present invention as a whole.

[0012] Preferably,

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[0013] Preferably, the average single-sided surface density of the positive electrode active coating provided on the first positive electrode sheet is 100 to 500 g / m 2 and the specific surface resistance is 0.0001 to 0.1200 Ω / mm 2 ·g.

[0014] Preferably, the average single-sided surface density of the positive electrode active coating provided on the second positive electrode sheet is 200 to 600 g / m 2 and the specific surface resistance is 0.0002 to 0.1600 Ω / mm 2 ·g.

[0015] Preferably, in the composite cell, the total mass of the first positive electrode active material is M1, and the total mass of the positive electrode active material is M0,

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[0016] Preferably, the first positive electrode active material is a ternary positive electrode material, and the second positive electrode active material contains at least one of lithium iron phosphate and lithium manganese iron phosphate.

[0017] Preferably, on the same side of the current collector of the first positive electrode sheet, along the direction away from the surface of the current collector, the positive electrode active coating formed by the first active material and the positive electrode active coating formed by the second positive electrode active material are sequentially provided.

[0018] Preferably, the positive electrode active material adopted in the second positive electrode sheet contains one of lithium iron phosphate, lithium manganese iron phosphate, and lithium manganate.

[0019] According to another aspect of the present invention, a battery including the above composite cell is provided.

[0020] Preferably, the composite cell includes a negative electrode sheet, and the negative electrode active material adopted in the negative electrode sheet includes a carbon negative electrode material and a non-carbon negative electrode material. Here, the carbon negative electrode material includes at least one of artificial graphite, natural graphite, hard carbon, and soft carbon, and the non-carbon negative electrode material includes at least one of a silicon-based material, a titanium-based material, and a tin-based material.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] To enable those skilled in the art to better understand the solution of the present invention, the following clearly and completely describes the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are some of the embodiments of the present invention, not all of them.

[0023] Example 1

[0024] This example provides a battery, and its preparation method is as follows:

[0025] 1. Preparation of the second positive electrode sheet A 1-1 Lithium iron phosphate, binder PVDF, and conductive agent acetylene black are uniformly mixed at a mass ratio of 98:1:1 to prepare a positive electrode slurry A. The positive electrode slurry A is coated on both sides of a positive electrode current collector and dried to form a positive electrode active coating A. The average single-sided surface density S A is 210 g / m 2 . The positive electrode sheet prepared in this way is denoted as the second positive electrode sheet A 1-1, and the specific surface resistance of the second positive electrode sheet A 1-1 is 0.1230 Ω / mm 2 ·g.

[0026] 2. Preparation of the first positive electrode sheet D1 1-4 Lithium iron phosphate, binder PVDF, and conductive agent acetylene black were uniformly mixed at a mass ratio of 98:1:1 to prepare positive electrode slurry A. A ternary cathode material, binder PVDF, and conductive agent acetylene black were uniformly mixed at a mass ratio of 90:5:5 to prepare positive electrode slurry C. Then, the positive electrode slurry C was coated on both sides of the positive electrode current collector, dried, and the first positive electrode active coating 1-41 was formed. Next, the positive electrode slurry A was coated on the surface of the first positive electrode active coating 1-41, dried, and the second positive electrode active coating 1-42 was formed. The first positive electrode active coating 1-41 and the second positive electrode active coating 1-42 jointly constituted a composite positive electrode active coating provided on the first positive electrode sheet D1. The first positive electrode sheet prepared in this way was designated as the first positive electrode sheet D1 1-4. In the first positive electrode sheet D1 1-4, the average single-sided surface density S of the composite positive electrode active coating D1 is 165 g / m 2 , and the specific surface resistance R D1 is 0.0500 Ω / mm 2 ·g, and moreover

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[0027] 3. Preparation of the composite cell The structural schematic diagram of the composite cell is shown in Figure 1. The composite cell is sequentially stacked with separator 1-2, negative electrode sheet 1-3, separator 1-2, second positive electrode sheet A 1-1, separator 1-2, negative electrode sheet 1-3, separator 1-2, first positive electrode sheet D1 1-4, separator 1-2, and negative electrode sheet 1-3 (here, there are 6 second positive electrode sheets A 1-1 and 44 first positive electrode sheets D1 1-4). The composite cell

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[0028] 4. Preparation of the battery Put the composite cell into the battery housing, then perform processes such as tab welding and firing on the battery housing. After passing the moisture content inspection, inject an appropriate amount of electrolyte into the battery housing, package it, age it, form it, and perform evacuation packaging to prepare the battery.

[0029] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0030] Example 2

[0031] This example provides a battery, and its preparation method is as follows:

[0032] 1. Preparation of the second positive electrode sheet B 1-1 Mix lithium iron manganese phosphate, binder PVDF, and conductive agent acetylene black uniformly at a mass ratio of 98:1:1 to prepare positive electrode slurry B. Coat the positive electrode slurry B on both sides of the positive electrode current collector, dry it to form a positive electrode active coating B, and the average single-sided areal density S B of the positive electrode active coating B is 175 g / m 2 . The positive electrode sheet prepared in this way is designated as the second positive electrode sheet B 1-1, and the specific surface resistance R B of the second positive electrode sheet B 1-1 is 0.1230 Ω / mm 2 ·g.

[0033] 2. Preparation of the first positive electrode sheet D2 1-4 Mix lithium iron manganese phosphate, binder PVDF, and conductive agent acetylene black in a mass ratio of 98:1:1 to prepare positive electrode slurry B. Mix ternary cathode material, binder PVDF, and conductive agent acetylene black in a mass ratio of 90:5:5 to prepare positive electrode slurry C. Then, apply the above positive electrode slurry C to both sides of the positive electrode current collector, dry it to form the first positive electrode active coating 1-41. Next, apply positive electrode slurry B to the surface of the first positive electrode active coating 1-41, dry it to form the second positive electrode active coating 1-42. The first positive electrode active coating 1-41 and the second positive electrode active coating 1-42 jointly constitute a composite positive electrode active coating provided on the first positive electrode sheet D2 1-4. The first positive electrode sheet prepared in this way is denoted as the first positive electrode sheet D2 1-4. In the first positive electrode sheet D2 1-4, the average single-sided surface density S of the composite positive electrode active coating D2 is 160 g / m 2 and the specific surface resistance R D1 is 0.0800 Ω / mm 2 ·g, and moreover

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[0034] 3. Preparation of the composite cell The structural schematic diagram of the composite cell is shown in Figure 1. The composite cell is sequentially stacked with separator 1-2, negative electrode sheet 1-3, separator 1-2, second positive electrode sheet B 1-1, separator 1-2, negative electrode sheet 1-3, separator 1-2, first positive electrode sheet D2 1-4, separator 1-2, and negative electrode sheet 1-3 (here, there are 6 second positive electrode sheets B 1-1 and 44 first positive electrode sheets D2 1-4). The composite cell

Number

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[0035] 4. Preparation of the battery Put the composite cell into the battery housing, then perform processes such as tab welding and firing on the battery housing. After passing the moisture content inspection, inject an appropriate amount of electrolyte into the battery housing, package it, age it, form it, and perform evacuation packaging to prepare the battery.

[0036] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0037] Example 3

[0038] This example provides a battery, and its preparation method is as follows:

[0039] 1. Preparation of the second positive electrode sheet A 1-1 For the preparation method of the second positive electrode sheet A, refer to the preparation of the second positive electrode sheet A in Example 1.

[0040] 2. Preparation of the first positive electrode sheet D2 1-4 For the preparation method of the first positive electrode sheet D2, refer to the preparation of the first positive electrode sheet D2 in Example 2.

[0041] 3. Preparation of the composite cell The structural schematic diagram of the composite cell is shown in Figure 1. The composite cell is stacked in the order of separator 1-2, negative electrode sheet 1-3, separator 1-2, second positive electrode sheet A 1-1, separator 1-2, negative electrode sheet 1-3, separator 1-2, first positive electrode sheet D2 1-4, separator 1-2, and negative electrode sheet 1-3 (here, the second positive electrode sheet A 1-3 is 6 sheets, and the first positive electrode sheet D2 1-4 is 44 sheets). The composite cell is

number

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[0042] 4. Battery Preparation The composite cell is put into a battery housing, and then the battery housing is subjected to tab welding, firing and other processes. After passing the moisture content test, an appropriate amount of electrolyte is injected into the battery housing, packaged, aged, formed, and packaged to prepare the battery.

[0043] The structural schematic diagram of the composite cell prepared in this example is shown in FIG.

[0044] Example 4 This embodiment provides a battery, the preparation method of which is as follows:

[0045] 1. Preparation of second positive electrode sheet A 1-1 The preparation method of the second positive electrode sheet A was the same as that of the second positive electrode sheet A in Example 1.

[0046] 2. Preparation of the first positive electrode sheet D3 1-4 A ternary system cathode material, a binder PVDF, and a conductive agent acetylene black were uniformly mixed at a mass ratio of 90:5:5 to prepare a cathode slurry C. Lithium manganate, a binder PVDF, and a conductive agent acetylene black were uniformly mixed at a mass ratio of 94:3:3 to prepare a cathode slurry D. Then, the cathode slurry C was coated on both sides of the cathode current collector, dried, and a first cathode active coating 1-41 was formed. Next, the cathode slurry D was coated on the surface of the first cathode active coating 1-41, dried, and a second cathode active coating 1-42 was formed. The first cathode active coating 1-41 and the second cathode active coating 1-42 jointly constitute a composite cathode active coating provided on the first cathode sheet D3 1-4. The first cathode sheet prepared in this way is denoted as the first cathode sheet D3 1-4. In the first cathode sheet D3 1-4, the average single-sided surface density S of the composite cathode active coating D3 is 175 g / m 2 , and the specific surface resistance R D1 is 0.1000 Ω / mm 2 ·g, and moreover

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[0047] 3. Preparation of the composite cell The structural schematic diagram of the composite cell is shown in Figure 1. The composite cell is sequentially stacked with a separator 1-2, a negative electrode sheet 1-3, a separator 1-2, a second cathode sheet A 1-1, a separator 1-2, a negative electrode sheet 1-3, a separator 1-2, a first cathode sheet D3 1-4, a separator 1-2, and a negative electrode sheet 1-3 (here, there are 6 second cathode sheets A 1-1 and 44 first cathode sheets D3 1-4). The composite cell

Number

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[0048] 4. Battery Preparation The composite cell is put into a battery housing, and then the battery housing is subjected to tab welding, firing and other processes. After passing the moisture content test, an appropriate amount of electrolyte is injected into the battery housing, packaged, aged, formed, and packaged to prepare the battery.

[0049] The structural schematic diagram of the composite cell prepared in this example is shown in FIG.

[0050] Example 5

[0051] In this example, a battery was prepared with reference to Example 1, and the differences between this example and Example 1 are as follows: in the first positive electrode sheet D1 1-4,

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[0052] The structural schematic diagram of the composite cell prepared in this example is shown in FIG.

[0053] Example 6

[0054] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the first positive electrode sheet D1 1-4,

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[0055] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0056] Example 7

[0057] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the first positive electrode sheet D1 1-4,

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[0058] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0059] Example 8

[0060] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the first positive electrode sheet D1 1-4,

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[0061] The structural schematic diagram of the composite cell prepared in this example is shown in FIG. 1.

[0062] Example 9

[0063] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the second positive electrode sheet A1-1, the average single-sided surface density of the positive electrode active coating A is 50 g / m 2 and the specific surface resistance is 0.0001 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0064] The structural schematic diagram of the composite cell prepared in this example is shown in FIG. 1.

[0065] Example 10

[0066] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the second positive electrode sheet A1-1, the average single-sided surface density of the positive electrode active coating A is 65 g / m 2 and the specific surface resistance is 0.0002 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0067] The structural schematic diagram of the composite cell prepared in this example is shown in FIG. 1.

[0068] Example 11

[0069] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the second positive electrode sheet A1-1, the average single-sided surface density of the positive electrode active coating A is 700 g / m 2 and the specific surface resistance is 0.2 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0070] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0071] Example 12

[0072] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the second positive electrode sheet A1-1, the average single-sided surface density of the positive electrode active coating A is 750 g / m 2 and the specific surface resistance is 0.25 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0073] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0074] Example 13

[0075] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the first positive electrode sheet D1 1-4, the surface density S of the composite positive electrode active coating D1 is 650 g / m 2 and the specific surface resistance R D1 is 0.0001 Ω / mm 2 ·g 2 . Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0076] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0077] Example 14

[0078] In this example, a battery was prepared with reference to Example 1. The differences between this example and Example 1 are as follows: In the first positive electrode sheet D1 1-4, the surface density S of the composite positive electrode active coating D1 is 50 g / m 2 and the specific surface resistance R D1 is 0.0500 Ω / mm 2 ·g 2 . Except for the above differences, the materials and process operations adopted in this example are exactly the same as those in Example 1.

[0079] The structural schematic diagram of the composite cell prepared in this example is shown in Figure 1.

[0080] Comparative Example 1

[0081] This comparative example provides a battery, and its preparation method is as follows:

[0082] 1. Preparation of the second positive electrode sheet B 2-1 Lithium iron manganese phosphate, binder PVDF, and conductive agent acetylene black were uniformly mixed at a mass ratio of 98:1:1 to prepare a positive electrode slurry B. The positive electrode slurry B was coated on both sides of the positive electrode current collector and dried to form a positive electrode active coating B. The average single-sided surface density S of the positive electrode active coating B B is 175 g / m 2 and the specific surface resistance is 0.1200 Ω / mm 2 ·g. The positive electrode sheet thus prepared is labeled as the second positive electrode sheet B 2-1.

[0083] 2. Preparation of the second positive electrode sheet C 2-4 A ternary system material, binder PVDF, and conductive agent acetylene black were uniformly mixed at a mass ratio of 90:5:5 to prepare a positive electrode slurry C. The positive electrode slurry C was coated on both sides of the positive electrode current collector and dried to form a positive electrode active coating C. The average single-sided surface density S of the positive electrode active coating C C is 150 g / m 2and the surface resistivity is 0.1250 Ω / mm 2 ·g. The positive electrode sheet label prepared in this way is the second positive electrode sheet C 2-4.

[0084] 3. Preparation of composite cell The structural schematic diagram of the composite cell is shown in Fig. 2. The composite cell is sequentially loaded with separator 2-2, negative electrode sheet 2-3, second positive electrode sheet B 2-1, separator 2-2, negative electrode sheet 2-3, separator 2-2, second positive electrode sheet C 2-4, separator 2-2, negative electrode sheet 2-3 (here, there are 44 second positive electrode sheets B 2-1 and 6 second positive electrode sheets C 2-4), and the composite cell 1 satisfies

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[0085] 4. Preparation of battery Put the composite cell into the battery housing, then perform processes such as tab welding and firing on the battery housing. After passing the moisture content inspection, inject an appropriate amount of electrolyte into the battery housing, package it, age it, form it, and perform evacuation packaging to prepare the battery.

[0086] The structural schematic diagram of the composite cell prepared in this comparative example is shown in Fig. 2.

[0087] Comparative Example 2

[0088] In this comparative example, a battery was prepared with reference to Example 1. The differences between this comparative example and Example 1 are as follows: In the first positive electrode sheet D1 1-4,

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[0089] The structural schematic diagram of the composite cell prepared in this comparative example is shown in Fig. 1.

[0090] Comparative Example 3

[0091] In this comparative example, a battery was prepared with reference to Example 1. The differences between this comparative example and Example 1 are as follows: In the first positive electrode sheet D1 1-4, the average single-sided surface density S of the composite positive electrode active coating D1 is 30 g / m 2 , and the specific surface resistance R D1 is 0.00007 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this comparative example are exactly the same as those in Comparative Example 1.

[0092] The structural schematic diagram of the composite cell prepared in this comparative example is shown in Fig. 1.

[0093] Comparative Example 4

[0094] In this comparative example, a battery was prepared with reference to Example 1. The differences between this comparative example and Example 1 are as follows: In the first positive electrode sheet D1 1-4, the surface density S of the composite positive electrode active coating D1 is 700 g / m 2 , and the specific surface resistance R D1 is 0.20 Ω / mm 2 ·g. Except for the above differences, the materials and process operations adopted in this comparative example are exactly the same as those in Comparative Example 1.

[0095] The structural schematic diagram of the composite cell prepared in this comparative example is shown in Fig. 1.

[0096] Test Example

[0097] 1. Test Subject This test example uses the batteries prepared in Examples 1 to 14 and Comparative Examples 1 to 4 as the test subjects of this test example.

[0098] 2. Test content

[0099] (1) Energy density The test battery was charged to 4.25 V at a constant current and constant voltage of 0.33C, cut off at 0.02C, and then discharged to 2.8 V at 0.33C. The capacity, average voltage, and cell mass were recorded, and the energy density of the battery was calculated using the following formula: Energy density = Capacity * Average voltage / Battery mass.

[0100] (2) DC impedance The test battery was charged to 4.25 V at a constant current and constant voltage of 0.33C, cut off at 0.02C, and then discharged at 0.33C for 90 minutes, left standing for 10 minutes, and the terminal voltage V1 at the end of standing was recorded. Then, it was discharged at 2C (current I) for 10 seconds, and the terminal voltage V2 at the end of discharge was recorded. The DC impedance of the sodium-ion battery was calculated using the following formula: DC impedance = |V1 - V2| / I.

[0101] (3) Cycle performance The test battery was placed in an incubator at 45°C, charged at a constant current and constant voltage of 1C, cut off at 0.02C, and then discharged at 1C, cycled to 80% SOC, and the number of cycles was recorded.

[0102] 3. Test results

Table 1

Table 2

[0103] The test results of the correlation performance of the batteries prepared in Examples 1 to 14 and Comparative Examples 1 to 4 are shown in Table 1.

[0104] Compare the performance test results corresponding to Example 1 and Comparative Example 1. As can be seen from Table 1, under the same conditions of other materials and operations for preparing the battery, the impedance levels of the batteries prepared in Examples 1 to 14 are low, and they are excellent in energy density and cycle performance. In contrast, only the second positive electrode sheet is provided in the composite cell prepared in Comparative Example 1, and the cycle performance of the battery thus obtained is significantly lower than that of the battery prepared in Example 1. From this, compared with Comparative Example 1, the batteries according to Examples 1 to 14 are provided with the first positive electrode sheet, and the structure of the first positive electrode sheet is stable, and the advantages of different positive electrode active materials can be fully exerted. It can effectively alleviate the problems of impedance and current inequality generated between different types of positive electrode active materials in the composite cell, thereby improving the energy density and cycle performance of the battery.

[0105] Compare the performance test results corresponding to Example 1 and Comparative Example 2. As can be seen from Table 1, under the same conditions of other materials and operations for preparing the battery, the impedance levels of the batteries prepared in Examples 1 to 14 are low, and they are excellent in energy density and cycle performance. In contrast, in the first positive electrode sheet prepared in Comparative Example 2, the mass ratio of the first positive electrode active material exceeds the range of 10% to 50%, and the cycle performance of the battery thus obtained is significantly lower than that of the battery prepared in Example 1. From this, compared with Comparative Example 2, the batteries according to Examples 1 to 14 are advantageous in making the potential distribution of the first positive electrode sheet uniform and alleviating the lithium precipitation phenomenon caused by the excessive local current density of the electrode sheet by reasonably setting the ratio of the first positive electrode active material in the first positive electrode sheet, thereby improving the energy density and cycle performance of the battery.

[0106] Compare the performance test results corresponding to Example 1 and Comparative Examples 3 to 4. As can be seen from Table 1, under the same conditions of other materials and operations for preparing the battery, the impedance levels of the batteries prepared in Examples 1 to 14 are low, and they are excellent in energy density and cycle performance. In contrast, the average single-sided surface density of the composite positive electrode active coating provided on the first positive electrode sheet prepared in Comparative Example 3 is <50 g / m 2and the specific surface resistance < 0.0001 Ω / mm 2 ·g, and the average single-sided surface density of the composite positive electrode active coating provided on the first positive electrode sheet prepared in Comparative Example 4 > 650 g / m 2 and the specific surface resistance > 0.1500 Ω / mm 2 ·g. The cycle performance of the battery thus obtained is significantly lower than that of the battery prepared in Example 1. From this, for Comparative Examples 3 to 4, the batteries according to Examples 1 to 12, by reasonably setting the surface density and specific surface resistance of the composite positive electrode active coating on the first positive electrode sheet, make the potential distribution of the first positive electrode sheet uniform, and are advantageous for alleviating the lithium precipitation phenomenon caused by the excessive local current density of the electrode sheet, thereby improving the energy density and cycle performance of the battery.

[0107] Compare the performance test results of Example 1 with those of Examples 5 to 8. As can be seen from Table 1, under the same conditions of other materials and operations for preparing the battery, the mass ratios of the first positive electrode active material in all the positive electrode active materials in the composite cell are < 5% and > 40% in Examples 5 and 8 respectively. The cycle performance of the battery thus obtained is lower than that of the battery prepared in Example 1. From this, for Examples 5 and 8, the composite cells according to Examples 1 and 6 to 7 are advantageous for improving the combination effect between different positive electrode active materials by reasonably setting the content of the first positive electrode active material in the composite cell, realizing the uniformity of the overall current density of the composite cell, and exerting the synergistic superiority on the positive electrode active materials with different energy densities, thereby improving the energy density and cycle performance of the battery.

[0108] Compare the performance test results of Example 1 with those of Examples 9 to 12. As can be seen from Table 1, under the same conditions of other materials and operations for preparing the battery, the average surface density of the positive electrode active coating provided on the second positive electrode sheet in Example 9 < 65 g / m 2 and the specific surface resistance < 0.0002 Ω / mm 2 ·g, and the average surface density of the positive electrode active coating provided on the second positive electrode sheet in Example 12 > 700 g / m 2 and the specific surface resistance > 0.2000 Ω / mm2 ·g, and the cycle performance of the battery thus obtained is lower than that of the battery prepared in Example 1. From this, for Examples 9 and 12, the composite cells according to Examples 1 and 10 to 11, by reasonably providing the surface density and specific surface resistance of the second positive electrode sheet, are excellent in the combined effect of the second positive electrode sheet with a specific surface density and specific surface resistance and the first positive electrode sheet, can exhibit a synergistic effect with the first positive electrode sheet, ensure the uniformity of the current density between different positive electrode sheets, and avoid phenomena such as destabilization of the electrode sheet structure, lithium precipitation, and polarization due to poor combined effect of the electrode sheets, thereby improving the energy density and cycle performance of the battery.

[0109] The above examples are only used to explain the technical solution of the present invention and do not limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the above examples, those skilled in the art can modify or equivalently replace the technical solution of the present invention, but it should be understood that all these modifications or replacements are within the protection scope of the present invention.

Claims

1. A composite cell, The composite cell includes a positive electrode sheet, the number of the positive electrode sheets is greater than 1, and each of the positive electrode sheets satisfies a ratio of its unit area capacity to the unit area capacity of any one of the other positive electrode sheets of 0.9 to 1.1; the positive electrode sheet includes a first positive electrode sheet, and the positive electrode active material included in the first positive electrode sheet includes a first positive electrode active material and a second positive electrode active material, wherein an energy density of the first positive electrode active material is greater than an energy density of the second positive electrode active material; In each of the first positive electrode sheets, a mass of the first positive electrode active material is smaller than a mass of the second positive electrode active material, a mass of the first positive electrode active material contained in the first positive electrode sheet is defined as m1, and a total mass of the positive electrode active material contained in the first positive electrode sheet is defined as m0, [0010] and The number of layers of the positive electrode active coating provided on each of the first positive electrode sheets is greater than 1, and the average single-sided areal density of the positive electrode active coating is 50 to 650 g / m ２ and the specific surface resistivity is 0.0001 to 0.1500 Ω / mm ２ and the positive electrode active coating comprises a first positive electrode coating and a second positive electrode active coating, a positive electrode active material included in the first positive electrode active coating is composed of the first positive electrode active material, and a positive electrode active material included in the second positive electrode active coating is composed of the second positive electrode active material.

2. The positive electrode sheet further includes a second positive electrode sheet, and the average single-sided areal density of the positive electrode active coating provided on the second positive electrode sheet is 65 to 700 g / m ２ and the specific surface resistivity is 0.0002 to 0.2000 Ω / mm ２ 2. The composite cell according to claim 1, characterized in that: 【Request 3】 【Number 2】 3. The composite cell according to claim 2, wherein:

4. The average single-sided areal density of the positive electrode active coating applied to the first positive electrode sheet is 100 to 500 g / m ２ and the specific surface resistivity is 0.0001 to 0.1200 Ω / mm ２ 4. The composite cell according to claim 3, characterized in that:

5. The average single-sided areal density of the positive electrode active coating applied to the second positive electrode sheet is 200 to 600 g / m ２ and the specific surface resistivity is 0.0002 to 0.1600 Ω / mm ２ 5. The composite cell according to claim 4, characterized in that:

6. In the composite cell, a total mass of the first positive electrode active material is M1, and a total mass of the positive electrode active material is M0; [0030] 2. The composite cell of claim 1 ,

7. 2. The composite cell of claim 1 , wherein the first active cathode material is a ternary cathode material and the second active cathode material includes at least one of lithium iron phosphate, lithium manganese iron phosphate, and lithium manganate.

8. 8. The composite cell of claim 7, wherein a positive active coating formed of the first active material and a positive active coating formed of the second positive active material are sequentially provided on the same side of the current collector of the first positive sheet along a direction away from the current collector surface.

9. 3. The composite cell of claim 2, wherein the positive active material employed in the second positive sheet comprises one of lithium iron phosphate, lithium manganese iron phosphate, and lithium manganate.

10. A battery comprising the composite cell according to any one of claims 1 to 9.

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