Composite positive electrode sheet and lithium-ion battery using same

Abstract

The present application provides a composite positive electrode sheet and a lithium-ion battery using same. The composite positive electrode sheet comprises a current collector. At least one surface of the current collector is respectively provided with a first active layer and a second active layer which are compounded with each other, wherein the first active layer is close to the current collector, and the second active layer is away from the current collector. The diffusion resistance of the first active layer is formula (1), and the diffusion resistance of the second active layer is formula (2). In the composite positive electrode sheet, the following relational expression is satisfied: D1<D2, and formula (3).

Description

A composite positive electrode sheet and a lithium-ion battery using the same

[0001] This application claims the priority of the Chinese patent application with the application number 202411588887.0 filed with the Chinese Patent Office on November 7, 2024. The entire content of the above application is incorporated herein by reference. Technical Field

[0002] This application belongs to the technical field of lithium-ion batteries, and particularly relates to a composite positive electrode sheet and a lithium-ion battery using the same. Background Art

[0003] Lithium-ion batteries have great advantages in improving energy efficiency and promoting the development of new energy. At the same time, they can also reduce manufacturing costs, bringing people a more efficient, environmentally friendly and economical way of energy utilization. And lithium-ion batteries are one of the most widely used types of rechargeable batteries at present.

[0004] However, the performance requirements of lithium-ion batteries that have attracted much attention, such as high power, fast charging, and long life, are largely restricted by the lithium-ion transmission efficiency. This is because the transmission of lithium ions in the battery involves multiple steps, including diffusion inside the electrode material, transfer at the electrode / electrolyte interface, and migration through the electrolyte. And the rates of these transmission processes will directly affect the performance of the battery. Summary of the Invention

[0005] In order to expand the application scope of lithium-ion batteries, it is necessary to improve the lithium-ion transmission effect in lithium-ion batteries. At present, researchers have explored various strategies to enhance the lithium-ion transmission efficiency of lithium-ion batteries, such as improving the conductivity of the positive and negative electrode materials and using electrolytes with high conductivity. However, the research and development cycle of new materials and their applications is long, the investment is large, and the risk is high.

[0006] This application provides a composite positive electrode sheet. The composite positive electrode sheet includes a current collector, and a first active layer and a second active layer that are mutually composite are respectively provided on at least one surface of the current collector. The first active layer is close to the current collector, and the second active layer is far from the current collector; the diffusion resistance of the first active layer is ; the diffusion resistance of the second active layer is ; in the composite positive electrode sheet, the following relational expression is satisfied: D1 < D2, and ; where L1 and L2 are the thicknesses of the first active layer and the second active layer respectively; D1 and D2 are the diffusion coefficients of the first active layer and the second active layer respectively; ε1 and ε2 are the porosities of the first active layer and the second active layer respectively.

[0007] This application also provides a lithium-ion battery, and the lithium-ion battery includes the above composite positive electrode sheet. Beneficial effects

[0008] The composite positive electrode provided in this application improves the lithium-ion transport efficiency of lithium-ion batteries by optimizing the structural design of the positive electrode. Specifically, this application controls the diffusion resistance of the first and second active layers by adjusting their thickness, diffusion coefficient, and porosity to achieve different lithium-ion diffusion rates. This application cleverly controls the spatial pore distribution from the current collector to the multilayer active material and then to the electrolyte, and utilizes the aforementioned differences in lithium-ion diffusion rates to achieve beneficial synergy between different active layers, thereby improving the charge transfer impedance of the composite positive electrode. By adjusting the diffusion resistance ratio between the first and second active layers, the power, cycle life, and safety performance of the lithium-ion battery can be enhanced. Thus, this application obtains a composite positive electrode with low lithium-ion diffusion resistance by optimizing the gradient porosity of the positive electrode during use, thereby achieving the goal of improving the lithium-ion transport efficiency of lithium-ion batteries.

[0009] The lithium-ion battery provided in this application embodiment, by applying the above-mentioned composite positive electrode sheet, can have good cycle performance and power performance. Attached Figure Description

[0010] Figure 1 is a cross-sectional view of the composite positive electrode sheet provided in Embodiment 1 of this application.

[0011] Explanation of reference numerals in the attached figures:

[0012] 1. Current collector; 2. First active layer; 3. Second active layer. Embodiments of the present invention

[0013] In the description of this application, unless otherwise expressly specified and limited, the terms "connected," "linked," and "fixed" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0014] In this application, unless otherwise clearly specified and defined, the first feature being "on" or "under" the second feature may include the case where the first and second features are in direct contact, or may also include the case where the first and second features are not in direct contact but in contact through additional features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes the first feature being directly above and obliquely above the second feature, and the horizontal height of the first feature being higher than that of the second feature. The first feature being "under", "beneath" and "underneath" the second feature includes the first feature being directly below and obliquely below the second feature, and the horizontal height of the first feature being lower than that of the second feature.

[0015] In the description of this embodiment, the orientation or positional relationships such as "upper", "lower", "left", "right", "front", "rear", etc. are based on the orientation or positional relationships shown in the drawings, and are for the convenience of description and simplifying the operations, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation to this application. In addition, the terms "first" and "second" are used for differentiating in the description and have no special meanings.

[0016] In a first aspect, an embodiment of this application provides a composite positive electrode sheet, the composite positive electrode sheet includes a current collector, and a first active layer and a second active layer which are composited with each other are respectively provided on at least one surface of the current collector, the first active layer is close to the current collector, and the second active layer is far from the current collector; the diffusion resistance of the first active layer is ; the diffusion resistance of the second active layer is ; in the composite positive electrode sheet, the following relational expression is satisfied: D1 < D2, and, ; where L1 and L2 are the thicknesses of the first active layer and the second active layer respectively; D1 and D2 are the diffusion coefficients of the first active layer and the second active layer respectively; ε1 and ε2 are the porosities of the first active layer and the second active layer respectively.

[0017] This application improves the lithium-ion transport efficiency of lithium-ion batteries by optimizing the structural design of the positive electrode. Specifically, this application controls the diffusion resistance of the first and second active layers by adjusting their thickness, diffusion coefficient, and porosity to achieve different lithium-ion diffusion rates. This application cleverly controls the spatial pore distribution from the current collector to the multilayer active material and then to the electrolyte, and utilizes the aforementioned differences in lithium-ion diffusion rates to achieve beneficial synergy between different active layers, thereby improving the charge transfer impedance of the composite positive electrode. By adjusting the diffusion resistance ratio between the first and second active layers, the power, cycle life, and safety performance of the lithium-ion battery can be enhanced. Therefore, this application achieves the goal of improving the lithium-ion transport efficiency of lithium-ion batteries by optimizing the gradient porosity of the positive electrode during use, resulting in a composite positive electrode with low lithium-ion diffusion resistance.

[0018] In some embodiments, 1.0 × 10 -15 ≤D1≤1.0×10 -10 .

[0019] In some embodiments, 1.0 × 10 -12 ≤D2≤1.0×10 -10 .

[0020] In some embodiments, the thickness of the first active layer L1 ≤ 800 μm; and / or, the thickness of the second active layer L2 ≤ 600 μm.

[0021] In some embodiments, 60μm≤L1≤200μm.

[0022] In some embodiments, 10μm≤L2≤80μm.

[0023] In some embodiments, the porosity ε1 of the first active layer and the porosity ε2 of the second active layer satisfy the following relationship: 2% ≤ ε1 - ε2 ≤ 30%. The inventors unexpectedly discovered that when the porosity of the second active layer is lower than that of the first active layer, it easily affects the wetting of the electrode by the electrolyte, thereby increasing the internal resistance of the lithium-ion battery; when the porosity of the second active layer is much higher than that of the first active layer, it easily increases the transport path of lithium ions in the electrode, thereby affecting the migration resistance of lithium ions. Therefore, this application improves the DCR cycle growth and power performance of the battery cell by controlling the porosity of the second active layer to be higher than that of the first active layer and limiting it within the above-mentioned range.

[0024] In some embodiments, 5% ≤ ε1 - ε2 ≤ 10%. By defining the porosity difference between the first active layer and the second active layer, power performance and cycle stability can be enhanced.

[0025] In some embodiments, the oxide-based active material includes at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, and lithium manganese oxide.

[0026] In some embodiments, the first active layer further includes a first conductive agent, and the second active layer further includes a second conductive agent, wherein the mass content of the first conductive agent is less than the mass content of the second conductive agent, and the mass content of the first conductive agent is not higher than 1.5%.

[0027] In some embodiments, the mass content of the second conductive agent in the second active layer is 0.5-2%.

[0028] In some embodiments, the mass content of the first positive electrode active material in the first active layer is 90-99%; and / or, the mass content of the second positive electrode active material in the second active layer is 90-98.5%.

[0029] In some embodiments, the conductive agent includes at least one of conductive carbon black SP and carbon nanotubes CNT.

[0030] In some embodiments, the first active layer includes a first positive electrode active material, which includes lithium manganese iron phosphate (LiMn). 1-x Fe x PO4, wherein 0.2 ≤ x ≤ 1; and / or, the second active layer includes a second positive electrode active material, which includes an oxide-based active material.

[0031] In some embodiments, the median particle size D of lithium manganese iron phosphate 50(M1) Median particle size D of oxide-based active materials 50(M2) Satisfy the following relationship: D 50(M2) -D 50(M1) >3μm; among which, the particle size D of lithium manganese iron phosphate 50(M1) Satisfying 1μm≤D 50(M1) ≤5μm; and / or, particle size D of oxide-based active materials 50(M2) Satisfying 2μm≤D 50(M2) ≤10μm.

[0032] A second aspect of this application provides a lithium-ion battery comprising the aforementioned composite positive electrode. By applying the aforementioned composite positive electrode, this application enables the production of lithium-ion batteries with excellent cycle performance and power performance.

[0033] The embodiments of this application are described below with reference to specific examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of this application. Experimental methods in the following embodiments that do not specify specific conditions are generally performed according to the conditions recommended by the manufacturer.

[0034] Example 1

[0035] 1. Raw materials required for composite positive electrode sheets

[0036] Weigh the required raw materials according to the materials shown in Table 1.

[0037] Table 1. Raw materials for preparing composite positive electrode sheets

[0038]

[0039] 2. Methods for preparing composite positive electrode sheets and lithium-ion batteries

[0040] According to the formulations provided in Table 1, the first positive electrode active material, conductive agent, and binder are prepared into a first slurry, and the second positive electrode active material, conductive agent, and binder are prepared into a second slurry. Subsequently, the first slurry is coated onto the surface of the current collector 1, and after drying, a first active layer 2 is obtained. The second slurry is then coated onto the surface of the first active layer 2 and dried to obtain a second active layer 3. The above operations are repeated to coat both surfaces of the current collector 1 with the first active layer 2 and the second active layer 3. A cross-sectional view of the composite positive electrode sheet provided in this embodiment is shown in Figure 1. The thickness of the first active layer 2 is 68 μm, and the thickness of the second active layer 3 is 19 μm. The ratio of the diffusion resistance of the first active layer 2 to the diffusion resistance of the second active layer 3 obtained in this embodiment is 6.89 × 10⁻⁶. -4 .

[0041] The above-mentioned composite positive electrode, graphite negative electrode, separator, and electrolyte are used to make a lithium-ion battery.

[0042] Example 2

[0043] This embodiment prepares a composite positive electrode and a lithium-ion battery using the formula and method provided in Embodiment 1. The difference from Embodiment 1 is that, in this embodiment, the ratio of the diffusion resistance of the first active layer 2 to the diffusion resistance of the second active layer 3 is 2.5 × 10⁻⁶. -8 Apart from the differences mentioned above, the operation steps for preparing the composite positive electrode and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0044] Example 3

[0045] This embodiment prepares a composite positive electrode and a lithium-ion battery using the formula and method provided in Embodiment 1. The difference from Embodiment 1 is that, in this embodiment, the ratio of the diffusion resistance of the first active layer 2 to the diffusion resistance of the second active layer 3 is 2.5 × 10⁻⁶. -1 Apart from the differences mentioned above, the operation steps for preparing the composite positive electrode and lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0046] Example 4

[0047] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, when preparing the composite positive electrode sheet, the difference between the porosity of the first active layer 2 and the porosity of the second active layer 3 is 26%. Apart from the above difference, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0048] Example 5

[0049] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, when preparing the composite positive electrode sheet, the difference between the porosity of the first active layer 2 and the porosity of the second active layer 3 is 10%. Apart from the above difference, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0050] Example 6

[0051] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, the difference between the porosity of the first active layer 2 and the porosity of the second active layer 3 is 3.1% when preparing the composite positive electrode sheet. Apart from the above difference, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0052] Example 7

[0053] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, when preparing the composite positive electrode sheet, the thickness of the first active layer 2 is L1=98μm and the thickness of the second active layer 3 is L2=24μm. Apart from the above differences, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0054] Example 8

[0055] This embodiment refers to the formula and method provided in Example 1 to prepare a composite positive electrode and a lithium-ion battery. The difference from Example 1 is that in this embodiment, when preparing the composite positive electrode, the thickness of the first active layer 2 is L1=132μm and the thickness of the second active layer 3 is L2=52μm. Apart from the above differences, the operation steps for preparing the composite positive electrode and the lithium-ion battery in this embodiment are strictly consistent with those in Example 1.

[0056] Example 9

[0057] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, when preparing the composite positive electrode sheet, the thickness of the first active layer 2 is L1=218μm and the thickness of the second active layer 3 is L2=74μm. Apart from the above differences, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0058] Example 10

[0059] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, the second positive electrode active material used in the second active layer 3 is lithium cobalt oxide when preparing the composite positive electrode sheet. Apart from the above differences, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0060] Example 11

[0061] This embodiment refers to the formula and method provided in Embodiment 1 to prepare a composite positive electrode sheet and a lithium-ion battery. The difference from Embodiment 1 is that in this embodiment, the second positive electrode active material used in the second active layer 3 is lithium manganese oxide when preparing the composite positive electrode sheet. Apart from the above differences, the operation steps for preparing the composite positive electrode sheet and the lithium-ion battery in this embodiment are strictly consistent with those in Embodiment 1.

[0062] Comparative Example 1

[0063] This comparative example prepares a composite positive electrode and a lithium-ion battery using the formulation and method provided in Example 1. The difference from Example 1 is that in this comparative example, the first positive electrode active material and the second positive electrode active material are mixed and then coated onto the surface of the current collector 1 during the preparation of the composite positive electrode. Apart from the above differences, the operational steps for preparing the composite positive electrode and the lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0064] Specifically, the steps for preparing the composite positive electrode sheet are as follows: According to the formula provided in Table 1, the first positive electrode active material, the second positive electrode active material, the conductive agent, and the binder are prepared into a composite slurry. Subsequently, the composite slurry is coated onto the surface of the current collector 1 through a coating process, and after drying, a composite active layer is obtained. The above operation is repeated to coat both surfaces of the current collector 1 with the composite active layer.

[0065] Comparative Example 2

[0066] This comparative example prepares a composite positive electrode and a lithium-ion battery using the formula and method provided in Example 1. The difference from Example 1 is that in this comparative example, when preparing the composite positive electrode, a second slurry is first coated onto the surface of the current collector 1, followed by a first slurry. Apart from the above differences, the operational steps for preparing the composite positive electrode and the lithium-ion battery in this comparative example are strictly consistent with those in Example 1.

[0067] Specifically, the steps for preparing the composite positive electrode sheet are as follows: According to the formula provided in Table 1, the first positive electrode active material is prepared with a conductive agent and a binder to form a first slurry, and the second positive electrode active material is prepared with a conductive agent and a binder to form a second slurry. Subsequently, the second slurry is coated onto the surface of the current collector 1 through a coating process, and after drying, a second active layer 3 is obtained; the first slurry is then coated onto the surface of the second active layer 3 and dried to obtain a first active layer 2. The above operations are repeated so that the two surfaces of the current collector 1 are covered with the first active layer 2 and the second active layer 3.

[0068] Test case

[0069] 1. Test Object

[0070] Composite positive electrode sheets and lithium-ion batteries prepared in Examples 1-11 and Comparative Examples 1-2.

[0071] 2. Testing Methods

[0072] (1) Cycling performance: ① Charge the prepared battery with a constant current of 1 / 3C and a constant voltage of 0.05C, with a voltage range of 2.5-4.25V, and charge and discharge 3 times. The capacity C0 is calibrated in the last charge and discharge cycle; ② Let it rest for 10 minutes; ③ Charge the battery with a constant current of 0.5C0 to 4.25V, charge it with a constant voltage of 0.05C0, let it rest for 10 minutes, and then discharge it with 1C0 to 2.5V and let it rest for 10 minutes; ④ Cycle step 3 until the SOH reaches 80%.

[0073] (2) DCR: ① Charge the prepared battery with a constant current of 1 / 3C and a constant voltage of 0.05C, with a voltage range of 2.5-4.25V. Perform 3 charge-discharge cycles and calibrate the capacity C0 in the last cycle. ② Let it rest for 10 minutes. ③ Charge the battery with a constant current of 1 / 3C0 to 4.25V and a constant voltage until the cutoff current is 0.05C0. Let it rest for 10 minutes. Discharge the battery with a constant current of 1 / 3C0 to 50% SOC and let it rest for 60 minutes. ④ Discharge the battery with a constant current of 1C0 for 30 seconds and let it rest for 10 minutes. ⑤ Discharge the battery with a constant current of 1 / 3C0 until it reaches 2.5V.

[0074] (3) Power performance: ① Charge the prepared battery with a constant current of 1 / 3C and a constant voltage of 0.05C, with a voltage range of 2.5-4.25V, and perform 3 charge-discharge cycles. The capacity C0 is calibrated in the last cycle. ② Let it rest for 30 minutes. ③ Charge the battery with a constant current of 0.2C0 to 4.25V and a constant voltage to the cutoff current of 0.02C0. Let it rest for 30 minutes. ④ Discharge the battery with 1 / 3C0 to 90% SOC. Let it rest for 180 minutes. ⑤ Discharge the battery with 1800 W for 10 seconds. Let it rest for 5 minutes. ⑥ Discharge the battery with 1 / 3C0 to 30% SOC. Let it rest for 60 minutes. ⑦ Discharge the battery with 1500 W for 10 seconds. Let it rest for 5 minutes.

[0075] (4) GITT test: ① Charge the prepared battery with a constant current of 1 / 3C and a constant voltage of 0.05C, with a voltage range of 2.5-4.25V. Cycle the charge and discharge 3 times and calibrate the capacity C0 in the last cycle; ② Let it rest for 30 minutes; ③ Charge the battery with a constant current of 0.1C0 to 4.25V and a constant voltage until the cutoff current is 0.05C0. Let it rest for 60 minutes; ④ Discharge the battery with 0.1C0 for 10 minutes and let it rest for 60 minutes; ⑤ Repeat step ④ until the voltage drops to 2.5V.

[0076] 3. Test Results and Analysis

[0077] In existing technologies, lithium manganese iron phosphate (LMFP) has become a key focus in the development of lithium-ion batteries, balancing high energy density, safety, and low cost. However, its development is limited by its low ionic conductivity and double plateau effect. While researching ways to improve LMFP cathode active materials, the inventors discovered that the cycle resistance increases rapidly after LMFP is mixed with oxide-based active materials, specifically manifested as an abnormal increase in impedance at mid-to-low frequencies. Through further investigation, the inventors found that this abnormal increase is due to significant differences in particle size and particle size distribution between LMFP and oxide-based active materials during the preparation of the mixed electrode. This results in a complex pore distribution in the mixed electrode, and the porosity and tortuosity limit lithium-ion transport, leading to poor cycle performance and power performance. As seen in the electrode structure prepared in Comparative Example 1, its DCR growth rate is as high as 110.98%.

[0078] The experimental data from Examples 1-11 confirm that the present invention optimizes the structural design of the positive electrode and controls the diffusion resistance ratio between the first active layer and the second active layer to meet the requirements. This can significantly reduce the abnormal growth of DCR, with a maximum reduction of up to 67.73%. Furthermore, as shown in Comparative Example 2, the structural arrangement of the first active layer and the second active layer has a sequential relationship. When the two are interchanged, it will not only affect the diffusion resistance, but also the DCR growth rate, cycle life, and power performance of the cell.

[0079] As can be seen from Examples 1-11, the present invention can also control the porosity difference between the first active material and the second positive electrode active material, and the D of the first active material and the second positive electrode active material. 50 This allows for the utilization of the volume shrinkage and expansion differences of different active materials during lithium-ion insertion / extraction cycles to optimize the gradient porosity of the positive electrode, thereby significantly improving lithium-ion transport and enhancing the cycle performance and power performance of lithium-ion batteries.

[0080] Data from Examples 10 and 11 confirm that when the first positive electrode active material is lithium manganese iron phosphate, the second positive electrode active material, which can be an oxide active material such as lithium nickel cobalt manganese oxide, lithium cobalt oxide, or lithium manganese oxide, not only has a lower DCR growth rate but also maintains good cycle performance and power performance.

[0081] Table 2. Relevant parameters of each embodiment and comparative example in this test case.

[0082]

[0083] Table 3. Test Results of this Test Case

[0084]

Claims

1. A composite positive electrode sheet, the composite positive electrode sheet comprising a current collector, wherein a first active layer and a second active layer are respectively disposed on at least one surface of the current collector, the first active layer being close to the current collector and the second active layer being away from the current collector; The diffusion resistance of the first active layer is ； The diffusion resistance of the second active layer is ； In the composite positive electrode sheet, the following relational expression is satisfied: D1 < D2, and, ； in, L1 and L2 are the thicknesses of the first active layer and the second active layer, respectively; D1 and D2 are the diffusion coefficients of the first active layer and the second active layer, respectively; ε1 and ε2 are the porosities of the first active layer and the second active layer, respectively.

2. The composite positive electrode sheet as described in claim 1, wherein, 1.0×10 -15 ≤D1≤1.0×10 -10 。 3. The composite positive electrode sheet as described in claim 1 or 2, wherein, 1.0×10 -12 ≤D2≤1.0×10 -10 。 4. The composite positive electrode sheet according to any one of claims 1-3, wherein, The thickness of the first active layer L1 ≤ 800 μm; and / or, the thickness of the second active layer L2 ≤ 600 μm.

5. The composite positive electrode sheet as described in claim 4, wherein, 60μm≤L1≤200μm.

6. The composite positive electrode sheet as described in claim 4 or 5, wherein, 10μm≤L2≤80μm.

7. The composite positive electrode sheet according to any one of claims 1-6, wherein, The porosity ε1 of the first active layer and the porosity ε2 of the second active layer satisfy the following relationship: 2%≤ε1-ε2≤30%.

8. The composite positive electrode sheet as described in claim 7, wherein, 5%≤ε1-ε2≤10%.

9. The composite positive electrode sheet according to any one of claims 1-8, wherein: The first active layer further includes a first conductive agent, and the second active layer further includes a second conductive agent. The mass content of the first conductive agent is less than the mass content of the second conductive agent, and the mass content of the first conductive agent is not higher than 1.5%.

10. The composite positive electrode sheet as described in claim 9, wherein: In the second active layer, the mass content of the second conductive agent is 0.5~2%.

11. The composite positive electrode sheet as described in claim 9 or 10, wherein: In the first active layer, the mass content of the first positive electrode active material is 90-99%; and / or, in the second active layer, the mass content of the second positive electrode active material is 90-98.5%.

12. The composite positive electrode sheet according to any one of claims 1 to 11, wherein: The first active layer includes a first positive electrode active material, which includes lithium manganese iron phosphate (LiMn). 1-x Fe x PO4, wherein 0.2 ≤ x ≤ 1; and / or, the second active layer includes a second positive electrode active material, wherein the second positive electrode active material includes an oxide-based active material.

13. The composite positive electrode sheet as described in claim 12, wherein: The median particle size D of the lithium manganese iron phosphate 50(M1) Compared with the median particle size D of the oxide-based active material 50(M2) The following relationship must be satisfied: D 50(M2) -D 50(M1) >3μm; Among them, the median particle size D of the lithium manganese iron phosphate 50(M1) Satisfying 1μm≤D 50(M1) ≤5μm; And / or, the median particle size D of the oxide-based active material 50(M2) Satisfying 2μm≤D 50(M2) ≤10μm.

14. The composite positive electrode sheet as described in claim 12 or 13, wherein, The oxide-based active material includes at least one of lithium nickel cobalt manganese oxide, lithium cobalt oxide, and lithium manganese oxide.

15. A lithium-ion battery, the lithium-ion battery comprising a composite positive electrode sheet as described in any one of claims 1 to 14.

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