A multi-layer composite positive electrode sheet, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention provides a multi-layer composite positive electrode plate, a preparation method thereof and a lithium-ion battery. The multi-layer composite positive electrode plate includes a positive electrode current collector, an active material layer and a positive electrode tab; the positive electrode current collector includes an empty foil area and a coating area, and the empty foil area is arranged on both sides of the coating area; the positive electrode tab is fixedly arranged in the empty foil area; the active material layer is arranged in the coating area; the active material layer includes a ternary material layer, a mixed material layer and a lithium iron phosphate layer; the thickness of the ternary material layer becomes thicker from one end close to the positive electrode tab to the end far from the positive electrode tab; the thickness of the lithium iron phosphate layer becomes thinner from one end close to the positive electrode tab to the end far from the positive electrode tab; the mixed material layer has a uniform thickness. The multi-layer composite positive electrode plate provided by the present invention utilizes a three-layer structure and a gradient arrangement of thickness, so that the conductivity and ion conductivity characteristics of each area of the electrode plate are uniform, and the cycle performance, storage performance and safety performance of the battery are improved.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion batteries, and relates to a multi-layer positive electrode sheet, in particular to a multi-layer composite positive electrode sheet and its preparation method and lithium-ion battery. Background Art

[0002] In recent years, the technology of lithium-ion batteries has been developing day by day. People no longer simply pursue the highest possible energy density, but improve the energy density of the battery under the condition of ensuring high safety. The specific capacity of ternary materials is relatively high and the conductivity is good. For example, the capacity of nickel-cobalt-manganese ternary materials can reach more than 190 mAh / g, and the working voltage is 3.6 - 3.7 V. They are widely used in fields with high energy density requirements. However, they face defects such as poor cycle performance and unsatisfactory safety performance. Although the lithium iron phosphate material has a relatively low capacity and voltage platform and poor conductivity, its cycle performance and safety performance are very excellent, and it is often used in the energy storage field and fields with high safety requirements.

[0003] CN 214477542U discloses an elastic layered ternary positive electrode sheet and a lithium-ion battery based on it. The elastic layered ternary positive electrode sheet includes two sets of current collectors and two sets of active material layers. Among them, the two sets of current collectors are symmetrically adhered, and the active material layer is arranged outside the current collector; among them, the current collector is a porous aluminum foil containing a PET film substrate, the PET film substrates of the two sets of current collectors are adhered, and the active material layer includes an inner layer of lithium iron phosphate material and an outer layer of ternary positive electrode material. The inner layer of lithium iron phosphate material is coated between the porous aluminum foil and the outer layer of ternary positive electrode material. This patent uses the lithium iron phosphate material as the inner layer and the ternary positive electrode material as the outer layer, which alleviates the problems of tape breakage and edge breakage during the processing process. However, it does not consider the improvement of the safety performance of the lithium-ion battery and the defect that placing the ternary material on the outer layer is not conducive to the improvement of the cycle performance.

[0004] CN 108933242 A discloses a method for preparing a lithium-ion battery hybrid cathode, and the preparation method includes the following steps: 1) providing a first active material, a second active material, and a third active material; 2) adding a solvent into a first vacuum stirring kettle, and sequentially adding a dispersant, a binder, and a conductive agent into the first vacuum stirring kettle, stirring for 3-5 h to obtain a dispersed solution, adding the first active material particles, and stirring for 4-6 h to obtain a first slurry; 3) adding a solvent into a second vacuum stirring kettle, and sequentially adding a dispersant, a binder, and a conductive agent into the second vacuum stirring kettle, stirring for 3-5 h to obtain a dispersed solution, adding the first active material, and stirring for 4-6 h to obtain a second slurry; 4) adding a solvent into a third vacuum stirring kettle, and sequentially adding a dispersant, a binder, and a conductive agent into the third vacuum stirring kettle, stirring for 3-5 h to obtain a dispersed solution, adding the second active material, and stirring for 4-6 h to obtain a third slurry; 5) adding a solvent into a fourth vacuum stirring kettle, and sequentially adding a binder, a dispersant, and a conductive agent into the fourth vacuum stirring kettle, stirring for 3-5 h to obtain a dispersed solution, adding the third active material, and stirring for 4-6 h to obtain a fourth slurry; 6) keeping the stirring state of the second stirring kettle, slowly adding the slurry in the third stirring kettle into the second stirring kettle, and stirring for 3-5 h to obtain a fifth slurry; 7) providing a current collector, coating the first slurry on the current collector, drying, and hot pressing to obtain a first layer, then coating the fifth slurry on the first layer, drying, and hot pressing to obtain a second layer; then coating the fourth slurry on the second layer, drying, and hot pressing to obtain a third layer. This patent sets the ternary cathode material in the inner layer, alleviates the erosion of the electrolyte on the ternary cathode material, optimizes the cycle performance, and at the same time designs a buffer layer to alleviate the performance impact caused by the volume change of different materials, but it does not consider the conductivity difference between the lithium iron phosphate material and the ternary material.

[0005] Therefore, it is necessary to provide a pole piece that fully considers the conductivity difference between the ternary cathode material and the lithium iron phosphate, and utilizes the uniqueness of the structure to make the conductive and ion-conductive characteristics of each region of the pole piece uniform, so as to improve the cycle performance, storage performance, and safety performance of the battery. Summary of the Invention

[0006] The purpose of the present invention is to provide a multi-layer composite cathode pole piece, a preparation method thereof, and a lithium-ion battery. The multi-layer composite cathode pole piece makes the conductive and ion-conductive characteristics of each region of the pole piece uniform by using a three-layer structure and a gradient arrangement of thickness, so as to improve the cycle performance, storage performance, and safety performance of the battery.

[0007] To achieve the purpose of this invention, the present invention adopts the following technical solutions:

[0008] In a first aspect, the present invention provides a multi-layer composite positive electrode plate, the multi-layer composite positive electrode plate comprising a positive electrode current collector, an active material layer, and a positive electrode tab;

[0009] The positive electrode current collector includes an empty foil area and a coated area, and the empty foil area is disposed on both sides of the coated area;

[0010] The positive electrode tab is fixedly disposed in the empty foil area; the active material layer is disposed in the coated area;

[0011] The active material layer includes a ternary material layer, a mixed material layer, and a lithium iron phosphate layer;

[0012] The mixed material layer is disposed in the middle of the ternary material layer and the lithium iron phosphate layer;

[0013] The thickness of the ternary material layer becomes thicker from the end close to the positive electrode tab to the end far from the positive electrode tab;

[0014] The thickness of the lithium iron phosphate layer becomes thinner from the end close to the positive electrode tab to the end far from the positive electrode tab;

[0015] The mixed material layer has a uniform thickness.

[0016] In the present invention, the thickness of the ternary material layer uniformly increases along the direction away from the positive electrode tab, that is, its thickness is the lowest when it is close to the positive electrode tab, gradually increases towards the position far from the electrode tab, and reaches the highest at the central position between the two electrode tabs. The thickness of the lithium iron phosphate layer in the present invention is the highest from the position close to the electrode tab, gradually decreases towards the position far from the electrode tab, and reaches the lowest at the central position between the two electrode tabs.

[0017] The multi-layer composite positive electrode plate provided by the present invention utilizes a three-layer structure and a gradient arrangement of thickness, so that the conductivity and ion conductivity characteristics of each area of the electrode plate are uniform, improving the cycle performance, storage performance, and safety performance of the battery.

[0018] Preferably, the coating sequence of the active material layer includes a ternary material layer, a mixed material layer, and a lithium iron phosphate layer, or a lithium iron phosphate layer, a mixed material layer, and a ternary material layer, which are sequentially stacked.

[0019] The present invention does not limit the arrangement order of the ternary material layer, the mixed material layer, and the lithium iron phosphate layer in the active material layer.

[0020] Preferably, the positive electrode current collector includes aluminum foil.

[0021] Preferably, the mass fraction of the ternary material layer in the active material layer is 30-50 wt%, for example, it can be 30 wt%, 35 wt%, 40 wt%, 45 wt%, or 50 wt%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0022] Preferably, the mass fraction of the mixed material layer in the active material layer is 20 - 30 wt%, for example, it can be 20 wt%, 22 wt%, 24 wt%, 26 wt%, 28 wt% or 30 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0023] Preferably, the mass fraction of the lithium iron phosphate layer in the active material layer is 30 - 50 wt%, for example, it can be 30 wt%, 35 wt%, 40 wt%, 45 wt% or 50 wt%, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0024] Preferably, the total thickness of the active material layer is 60 - 110 μm, for example, it can be 60 μm, 70 μm, 80 μm, 90 μm, 100 μm or 110 μm, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0025] Preferably, the material of the ternary material layer includes any one or a combination of at least two of 5 - series nickel - cobalt - manganese ternary materials, 6 - series nickel - cobalt - manganese ternary materials, 8 - series nickel - cobalt - manganese ternary materials, and cobalt - free nickel - manganese materials;

[0026] Preferably, the mixed material layer includes a mixture of ternary material and lithium iron phosphate material.

[0027] Preferably, the mass ratio of the ternary material to the lithium iron phosphate in the mixed material layer is (1 - 3):(1 - 3), for example, it can be 1:1, 1:2, 1:3, 2:1, 2:3 or 3:1, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0028] The mixed material layer in the present invention contains ternary material and lithium iron phosphate material. The ternary material and lithium iron phosphate material in the mixed material layer are uniformly mixed, so that the conductivity and ion - conductivity of the mixed material layer are uniform; and it is arranged in the middle of the ternary material layer and the lithium iron phosphate layer. As a transition layer, it can effectively relieve the polarization, volume and structure changes caused by the differences in conductivity and ion - conductivity between the ternary material layer and the lithium iron phosphate layer, and optimize the cycle performance.

[0029] In the second aspect, the present invention provides a preparation method of a multi - layer composite positive electrode sheet as provided in the first aspect. The preparation method includes:

[0030] Welding the positive electrode tab to the empty foil area; then setting the ternary material layer, the mixed material layer and the lithium iron phosphate layer in the coating area, and obtaining the multi - layer composite positive electrode sheet after rolling.

[0031] The present invention does not limit the arrangement order of the ternary material layer, the mixed material layer, and the lithium iron phosphate layer in the multi-layer composite positive electrode sheet. Therefore, during the preparation process, the coating order of the ternary material layer, the mixed material layer, and the lithium iron phosphate layer can be adjusted according to the required multi-layer composite positive electrode sheet.

[0032] Exemplarily, the coating order can be to coat the ternary material layer, the mixed material layer, and the lithium iron phosphate layer in sequence in the upward direction of the coating area.

[0033] Preferably, the method for preparing the ternary material layer includes: dispersing the ternary material, SP, and PVDF in NMP to obtain a ternary material slurry; then performing coating and drying in sequence to obtain the ternary material layer.

[0034] Preferably, the solid content of the ternary material slurry is 70-80%, for example, it can be 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, or 80%, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0035] Preferably, the mass ratio of the ternary material, SP, and PVDF is (95-97):(1-2):(1-2), for example, it can be 95:1:1, 95:2:2, 97:1:1, 97:2:2, 96:1:1, 96:2:2, or 95:1.5:1.5, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0036] Preferably, the D50 of the ternary material is 12-16μm, for example, it can be 12μm, 12.5μm, 13μm, 13.5μm, 14μm, 14.5μm, 15μm, 15.5μm, or 16μm, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0037] Preferably, the thickness at the end close to the positive electrode tab during the coating is 10-30μm, for example, it can be 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm, or 30μm, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0038] Preferably, the thickness at the end far from the positive electrode tab during the coating is 30-50μm, for example, it can be 30μm, 32μm, 34μm, 36μm, 38μm, 40μm, 42μm, 4μm, 46μm, 48μm, or 50μm, but is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0039] The coating thickness of the ternary material layer described in the present invention is not uniform. Instead, it is the lowest near the tab and gradually increases away from the tab, reaching the highest thickness at the central position between the two tabs; the thickness at one end near the positive tab is always less than that at the other end away from the positive tab. Exemplarily, when the thickness at one end of the ternary material layer near the positive tab is 10 μm, the thickness at the other end away from the positive tab is 30 μm.

[0040] Preferably, the drying temperature is 100 - 130 °C, for example, it can be 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C or 130 °C, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0041] Preferably, the drying time is 30 - 60 min, for example, it can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0042] Preferably, the preparation method of the mixed material layer includes: mixing the ternary material and lithium iron phosphate, and obtaining the mixed material after ball milling; then dispersing the mixed material, SP, LAGP and PVDF in NMP to obtain the mixed material slurry, and then successively performing coating and drying to obtain the mixed material layer;

[0043] Preferably, the mass ratio of the mixed material, SP, LAGP and PVDF is (90 - 94):(2 - 3):(1 - 2):(2 - 3);

[0044] Preferably, the coating thickness is 20 - 30 μm, for example, it can be 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm or 30 μm, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0045] Preferably, the drying temperature is 100 - 130 °C, for example, it can be 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C or 130 °C, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0046] Preferably, the drying time is 30 - 60 min, for example, it can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min, but not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0047] Preferably, the mass ratio of the ternary material to lithium iron phosphate is (1-3):(1-3). For example, it can be 1:1, 1:2, 1:3, 2:1, 2:3, 3:1 or 3:2, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0048] Preferably, the ball milling time is 4-6h. For example, it can be 4h, 4.2h, 4.4h, 4.6h, 4.8h, 5h, 5.2h, 5.4h, 5.6h, 5.8h or 6h, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0049] Preferably, the method for preparing the lithium iron phosphate layer includes: dispersing lithium iron phosphate, SP, LAGP and PVDF in NMP to obtain a lithium iron phosphate slurry; then successively performing coating and drying to obtain the lithium iron phosphate layer.

[0050] Preferably, the mass ratio of the lithium iron phosphate, SP, LAGP and PVDF is (90-95):(2-5):(1-2):(2-5). For example, it can be 90:2:1:2, 90:5:2:5, 95:2:1:2 or 95:5:2:5, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0051] Preferably, the solid content of the lithium iron phosphate slurry is 60-70%. For example, it can be 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69% or 70%, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0052] Preferably, the D50 of the lithium iron phosphate is 0.5-3μm. For example, it can be 0.5μm, 1μm, 1.2μm, 1.4μm, 1.6μm, 1.8μm, 2μm, 2.2μm, 2.4μm, 2.6μm, 2.8μm or 3μm, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0053] Preferably, the lithium iron phosphate is carbon-coated lithium iron phosphate.

[0054] Preferably, the carbon coating amount in the carbon-coated lithium iron phosphate is 1-2wt%. For example, it can be 1wt%, 1.2wt%, 1.4wt%, 1.6wt%, 1.8wt% or 2wt%, but is not limited to the listed values. Other unlisted values within the numerical range are equally applicable.

[0055] Preferably, the thickness of the coating near the positive electrode tab is 30-50 μm. For example, it can be 30 μm, 32 μm, 34 μm, 36 μm, 38 μm, 40 μm, 42 μm, 44 μm, 46 μm, 48 μm or 50 μm. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0056] Preferably, the thickness of the coating far from the positive electrode tab is 10-30 μm. For example, it can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm or 30 μm. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0057] During the coating process of the lithium iron phosphate layer of the present invention, the thickness of the part far from the positive electrode tab is always less than that of the part near the positive electrode tab.

[0058] Exemplarily, during the coating process of the lithium iron phosphate layer, the thickness of the part far from the positive electrode tab is 10 μm, and the thickness of the part near the positive electrode tab is 30 μm.

[0059] Preferably, the drying temperature is 100-130 °C. For example, it can be 100 °C, 105 °C, 110 °C, 115 °C, 120 °C, 125 °C or 130 °C. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0060] Preferably, the drying time is 30-60 min. For example, it can be 30 min, 35 min, 40 min, 45 min, 50 min, 55 min or 60 min. However, it is not limited to the listed values, and other unlisted values within the numerical range are equally applicable.

[0061] As a preferred technical solution of the present invention, taking the sequential coating of the ternary material layer, the mixed material layer and the lithium iron phosphate layer as an example, the preparation method described in the second aspect of the present invention includes the following steps:

[0062] (1) Weld the positive electrode tab to the empty foil area;

[0063] (2) Then, sequentially coat the ternary material layer, the mixed material layer and the lithium iron phosphate layer in the coating area, and obtain the multi-layer composite positive electrode plate after rolling.

[0064] (2.1)Disperse ternary material with a D50 of 12 - 16 μm, SP, and PVDF in NMP at a mass ratio of (95 - 97):(1 - 2):(1 - 2) to obtain a ternary material slurry with a solid content of 70 - 80% and a viscosity of 4500 - 5500 mPas; coat it on the coated area, and then dry it at 100 - 130 °C for 30 - 60 min to obtain a ternary material layer with a thickness of 10 - 30 μm at one end close to the positive electrode tab and a thickness of 30 - 50 μm at the other end far from the positive electrode tab;

[0065] (2.2)Disperse the mixed material, SP, LAGP, and PVDF in NMP at a mass ratio of (90 - 94):(2 - 3):(1 - 2):(2 - 3) to obtain a mixed material slurry, then coat it on the ternary material layer obtained in step (2.1), and then dry it at 100 - 130 °C for 30 - 60 min to obtain a mixed material layer with a thickness of 20 - 30 μm;

[0066] The mixed material includes a mixture obtained by ball - milling ternary material and lithium iron phosphate at a mass ratio of (1 - 3):(1 - 3) for 4 - 6 h;

[0067] (2.3)Disperse carbon - coated lithium iron phosphate with a D50 of 0.5 - 3 μm, SP, LAGP, and PVDF in NMP at a mass ratio of (90 - 95):(2 - 5):(1 - 2):(2 - 5) to obtain a lithium iron phosphate slurry with a solid content of 60 - 70%; coat it on the mixed material layer obtained in step (2.2), and then dry it at 100 - 130 °C for 30 - 60 min to obtain a lithium iron phosphate layer with a thickness of 30 - 50 μm at one end close to the positive electrode tab and a thickness of 10 - 30 μm at the other end far from the positive electrode tab;

[0068] The carbon coating amount of the carbon - coated lithium iron phosphate is 1 - 2 wt%.

[0069] In the third aspect, the present invention provides a lithium - ion battery, which includes a multi - layer composite positive electrode sheet obtained by using the preparation method provided in the second aspect.

[0070] The numerical ranges described in the present invention not only include the exemplified point values above, but also include any point values between the above - mentioned numerical ranges not exemplified. Due to space limitations and for the sake of simplicity, the present invention does not exhaustively list the specific point values included in the ranges.

[0071] Compared with the prior art, the present invention has the following beneficial effects:

[0072] (1) During the charge and discharge process of the multi-layer composite positive electrode sheet provided by the present invention, lithium ions near the tab are transmitted more rapidly. Considering that the ternary material has a two-dimensional layered structure and lithium ions diffuse faster in the ternary material, while the lithium ion transport channels in lithium iron phosphate are one-dimensional channels and lithium ion diffusion is slower, the coating thickness of the ternary material layer near the tab is reduced and the thickness of lithium iron phosphate is increased to ensure the lithium ion transport of the lithium iron phosphate material and the balance of lithium ion transport speed on the entire positive electrode sheet, avoiding local overheating;

[0073] (2) The mixed material layer between the lithium iron phosphate layer and the ternary material layer in the multi-layer composite positive electrode sheet provided by the present invention alleviates the difference in electrical conductivity and ion conductivity between the lithium iron phosphate layer and the ternary material layer, and adding LAGP also improves the ion conductivity of lithium iron phosphate;

[0074] (3) The intrinsic potentials of the lithium iron phosphate material and the ternary material are different, and there will be a certain self-coupled self-discharge phenomenon. The LAGP in the buffer layer also greatly avoids the direct contact between the two materials, reducing the occurrence of self-coupled self-discharge;

[0075] (4) The outer layer of the multi-layer composite positive electrode sheet provided by the present invention is coated with a stable lithium iron phosphate material, avoiding the direct contact between the electrolyte and the relatively less stable nickel cobalt manganese ternary material, improving the safety and cycle stability of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0076] Figure 1 FIG. is a schematic structural diagram of the multi-layer composite positive electrode sheet provided in Embodiment 1 of the present invention.

[0077] Among them, 1 is the positive tab, 2 is the positive current collector, 3 is the ternary material layer, 4 is the mixed material layer, and 5 is the lithium iron phosphate layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0078] The technical solutions of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations to the present invention.

[0079] Embodiment 1

[0080] This embodiment provides a multi-layer composite positive electrode sheet as Figure 1 shown, and the multi-layer composite positive electrode sheet includes a positive current collector 2, an active material layer, and a positive tab 1;

[0081] The positive current collector 2 includes an empty foil area and a coated area, and the empty foil area is arranged on both sides of the coated area;

[0082] The positive tab 1 is fixedly arranged in the empty foil area; the active material layer is arranged in the coated area;

[0083] The active material layer includes, in the direction from the aluminum foil upwards, a ternary material layer 3, a mixed material layer 4, and a lithium iron phosphate layer 5 that are sequentially stacked and arranged;

[0084] The thickness of the ternary material layer 3 becomes thicker from the end close to the positive electrode tab towards the end far from the positive electrode tab; the thickness of the lithium iron phosphate layer 5 becomes thinner from the end close to the positive electrode tab towards the end far from the positive electrode tab; the mixed material layer has a uniform thickness.

[0085] The mass fraction of the ternary material layer in the active material layer is 40 wt%; the mass fraction of the mixed material layer in the active material layer is 20 wt%; the mass fraction of the lithium iron phosphate layer in the active material layer is 40 wt%; the thickness of the multi-layer composite positive electrode tab is 80 μm (before rolling).

[0086] The preparation method includes the following steps:

[0087] (1) Weld the positive electrode tab to the empty foil area;

[0088] (2) Then coat the ternary material layer, the mixed material layer, and the lithium iron phosphate layer in sequence in the coating area, and obtain the multi-layer composite positive electrode tab after rolling;

[0089] (2.1) Disperse a ternary material with a D50 of 12 - 16 μm, SP, and PVDF in NMP at a mass ratio of 96:2:2 to obtain a ternary material slurry with a solid content of 70% and a viscosity of 5000 mPas; coat it in the coating area, and then dry it at 100 °C for 60 min to obtain a ternary material layer with a thickness of 20 μm at the end close to the positive electrode tab and a thickness of 40 μm at the end far from the positive electrode tab;

[0090] (2.2) Disperse the mixed material, SP, LAGP, and PVDF in NMP at a mass ratio of 90:2:2:3 to obtain a mixed material slurry, then coat it on the ternary material layer obtained in step (2.1), and then dry it at 130 °C for 30 min to obtain a mixed material layer with a thickness of 20 μm;

[0091] The mixed material includes a mixture obtained by ball-milling a ternary material and lithium iron phosphate in a mass ratio of 1:1 for 4 - 6 h;

[0092] (2.3) Disperse carbon-coated lithium iron phosphate with a D50 of 2.1 μm, SP, LAGP, and PVDF in NMP at a mass ratio of 90:5:2:3 to obtain a lithium iron phosphate slurry with a solid content of 60% and a viscosity of 6500 mPas; coat it on the mixed material layer obtained in step (2.2), and then dry it at 120 °C for 50 min to obtain a lithium iron phosphate layer with a thickness of 40 μm at the end close to the positive electrode tab and a thickness of 20 μm at the end far from the positive electrode tab;

[0093] The carbon coating amount in the carbon-coated lithium iron phosphate is 1.2 wt%.

[0094] Example 2

[0095] This example provides a multi-layer composite positive electrode sheet, which is the same as that in Example 1.

[0096] The preparation method of the multi-layer composite positive electrode sheet is only different from that in Example 1 in that: in this example, the mass ratio of the ternary material and lithium iron phosphate described in step (2.2) is changed to 3:1.

[0097] Example 3

[0098] This example provides a multi-layer composite positive electrode sheet, which is the same as that in Example 1.

[0099] The preparation method of the multi-layer composite positive electrode sheet is only different from that in Example 1 in that: in this example, the coating thickness of the ternary material layer described in step (2.1) is changed to a thickness of 10 μm at the end close to the positive electrode tab and a thickness of 30 μm at the end far from the positive electrode tab;

[0100] And the coating thickness of the lithium iron phosphate layer described in step (2.3) is changed to a thickness of 30 μm at the end close to the positive electrode tab and a thickness of 10 μm at the end far from the positive electrode tab.

[0101] Example 4

[0102] This example provides a multi-layer composite positive electrode sheet, which is the same as that in Example 1.

[0103] The preparation method of the multi-layer composite positive electrode sheet is only different from that in Example 1 in that: in this example, the LAGP described in step (2.2) and step (2.3) is omitted.

[0104] Example 5

[0105] This example provides a multi-layer composite positive electrode sheet, which is only different from that in Example 1 in that: in this example, the active material layer is changed to include a lithium iron phosphate layer, a mixed material layer, and a ternary material layer stacked in sequence in the upward direction from the aluminum foil.

[0106] The preparation method of the multi-layer composite positive electrode sheet is only different from that in Example 1 in that: in this example, the order of step (2.1) and step (2.3) is swapped.

[0107] Comparative Example 1

[0108] This comparative example provides a multi-layer composite positive electrode sheet. The difference between the multi-layer composite positive electrode sheet and that of Example 1 is only that: in this example, the active material layer is changed to include a ternary material layer, a lithium iron phosphate layer, and a mixed material layer that are sequentially stacked in the upward direction from the aluminum foil.

[0109] The preparation method of the multi-layer composite positive electrode sheet is different from that of Example 1 only in that: in this comparative example, the order of steps (2.2) and (2.3) is swapped.

[0110] Comparative Example 2

[0111] This comparative example provides a multi-layer composite positive electrode sheet. The difference between the multi-layer composite positive electrode sheet and that of Example 1 is only that: in this example, the active material layer is changed to include a mixed material layer, a ternary material layer, and a lithium iron phosphate layer that are sequentially stacked in the upward direction from the aluminum foil.

[0112] The preparation method of the multi-layer composite positive electrode sheet is different from that of Example 1 only in that: in this comparative example, the coating order is changed to sequentially coat the mixed material layer, the ternary material layer, and the lithium iron phosphate layer.

[0113] Comparative Example 3

[0114] This comparative example provides a multi-layer composite positive electrode sheet. The difference between the multi-layer composite positive electrode sheet and that of Example 1 is only that: in this comparative example, the mixed material layer is omitted.

[0115] The preparation method of the multi-layer composite positive electrode sheet is different from that of Example 1 only in that: in this comparative example, step (2.2) is omitted.

[0116] Comparative Example 4

[0117] This comparative example provides a multi-layer composite positive electrode sheet. The difference between the multi-layer composite positive electrode sheet and that of Example 1 is only that: the active material layer of this comparative example only contains a mixed material layer.

[0118] The preparation method of the multi-layer composite positive electrode sheet includes:

[0119] (1) Weld the positive electrode tab to the empty foil area;

[0120] (2) Ball mill lithium iron phosphate and ternary material at a ball mill speed of 300 rpm for 5 hours to obtain a mixed material. Disperse the mixed material, SP, and PVDF in NMP in a ratio of 92:5:3 to obtain a mixed material slurry with a solid content of 60% and a viscosity of 5000 mPas. Coating the mixed slurry on a 12 μm aluminum foil, controlling the coating surface density to make the thickness 90 μm, and drying after coating.

[0121] Roll the electrode sheet to 40 μm.

[0122] Comparative Example 5

[0123] This comparative example provides a multi-layer composite positive electrode sheet. The difference between this multi-layer composite positive electrode sheet and that of Example 1 is only that: in this comparative example, the thickness of the ternary material layer is changed from getting thicker from the end close to the positive electrode tab to the end far from the positive electrode tab to getting thinner;

[0124] And the thickness of the lithium iron phosphate layer is changed from getting thinner from the end close to the positive electrode tab to the end far from the positive electrode tab to getting thicker.

[0125] The difference between the preparation method of this multi-layer composite positive electrode sheet and that of the example is only that: in this comparative example, the thickness of the ternary material layer obtained in step (2.1) is changed to a thickness of 40 μm at the end close to the positive electrode tab and a thickness of 20 μm at the end far from the positive electrode tab;

[0126] And the thickness of the lithium iron phosphate layer obtained in step (2.3) is changed to a thickness of 20 μm at the end close to the positive electrode tab and a thickness of 40 μm at the end far from the positive electrode tab.

[0127] The multi-layer composite positive electrode sheets provided in Examples 1-5 and Comparative Examples 1-5 were assembled into coin cells for electrochemical tests, and the test results are shown in Table 1.

[0128] The test contents include:

[0129] (1) Charge and discharge performance detection: Charge and discharge tests were carried out at a rate of 1C and 2.5-4.2V, and the capacity retention rate after 200 cycles was detected;

[0130] (2) Storage performance detection: Store at 25°C for 25 days, and detect the voltage drop situation.

[0131] Table 1

[0132]

[0133] In summary, the multi-layer composite positive electrode sheet provided by the present invention utilizes a three-layer structure and a gradient arrangement of thickness, so that the conductive and ion-conductive characteristics of each region of the electrode sheet are uniform, improving the cycle performance, storage performance and safety performance of the battery.

[0134] The applicant declares that the above description is only the specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A multi-layer composite positive electrode sheet, characterized in that, The multi-layer composite positive electrode sheet includes a positive electrode current collector, an active material layer, and a positive electrode tab; The positive electrode current collector includes an empty foil area and a coated area, and the empty foil area is disposed on both sides of the coated area; The positive electrode tab is fixedly disposed in the empty foil area; the active material layer is disposed in the coated area; The active material layer includes a ternary material layer, a mixed material layer, and a lithium iron phosphate layer; The mixed material layer is disposed in the middle of the ternary material layer and the lithium iron phosphate layer; The thickness of the ternary material layer becomes thicker from the end close to the positive electrode tab to the end far from the positive electrode tab; The thickness of the lithium iron phosphate layer becomes thinner from the end close to the positive electrode tab to the end far from the positive electrode tab; The mixed material layer has a uniform thickness; The coating sequence of the active material layer includes a ternary material layer, a mixed material layer, and a lithium iron phosphate layer, or a lithium iron phosphate layer, a mixed material layer, and a ternary material layer, which are sequentially stacked; The mixed material layer includes a mixture of a ternary material and a lithium iron phosphate material, and LAGP.

2. The multi-layer composite positive electrode sheet according to claim 1, wherein, The positive electrode current collector includes aluminum foil.

3. The multi-layer composite positive electrode sheet according to claim 1, characterized in that, The mass fraction of the ternary material layer in the active material layer is 30-50 wt%.

4. The multi-layer composite positive electrode sheet according to claim 1, characterized in that, The mass fraction of the mixed material layer in the active material layer is 20-30 wt%.

5. The multi-layer composite positive electrode sheet according to claim 1, characterized in that, The mass fraction of the lithium iron phosphate layer in the active material layer is 30-50 wt%.

6. The multi-layer composite positive electrode sheet according to claim 1, wherein, The total thickness of the active material layer is 60-110 μm.

7. The multi-layer composite positive electrode sheet according to claim 1, wherein The material of the ternary material layer includes any one or at least two combinations of 5-series nickel cobalt manganese ternary materials, 6-series nickel cobalt manganese ternary materials, 8-series nickel cobalt manganese ternary materials, and cobalt-free nickel manganese materials.

8. The multi-layer composite positive electrode sheet according to claim 1, wherein, The mass ratio of the ternary material to the lithium iron phosphate in the mixed material layer is (1-3):(1-3).

9. A method for preparing the multi-layer composite positive electrode sheet according to any one of claims 1-8, characterized in that, The preparation method includes: Welding the positive electrode tab to the empty foil area; then disposing a ternary material layer, a mixed material layer, and a lithium iron phosphate layer in the coated area, and obtaining the multi-layer composite positive electrode sheet after rolling.

10. The preparation method according to claim 9, characterized in that, The preparation method of the ternary material layer includes: dispersing the ternary material, SP, and PVDF in NMP to obtain a ternary material slurry; then sequentially performing coating and drying to obtain the ternary material layer.

11. The preparation method according to claim 10, characterized in that, The solid content of the ternary material slurry is 70-80%.

12. The preparation method according to claim 10, characterized in that, The mass ratio of the ternary material, SP, and PVDF is (95-97):(1-2):(1-2).

13. The preparation method according to claim 10, characterized in that, The D50 of the ternary material is 12-16 μm.

14. The preparation method according to claim 10, wherein, The thickness at the end close to the positive electrode tab during the coating is 10-30 μm.

15. The preparation method according to claim 10, characterized in that, The thickness at the end far from the positive electrode tab during the coating is 30-50 μm.

16. The preparation method according to claim 10, characterized in that, The temperature of the drying is 100-130 °C.

17. The preparation method according to claim 10, characterized in that, The time of the drying is 30-60 min.

18. The preparation method according to claim 9, characterized in that, The preparation method of the mixed material layer includes: mixing the ternary material and the lithium iron phosphate, and obtaining a mixed material after ball milling; then dispersing the mixed material, SP, LAGP, and PVDF in NMP to obtain a mixed material slurry, and then sequentially performing coating and drying to obtain the mixed material layer.

19. The preparation method according to claim 18, wherein, The mass ratio of the mixed material, SP, LAGP, and PVDF is (90-94):(2-3):(1-2):(2-3).

20. The preparation method according to claim 18, characterized in that, The thickness of the coating is 20-30 μm.

21. The preparation method according to claim 18, characterized in that, The temperature of the drying is 100-130 °C.

22. The preparation method according to claim 18, wherein, The drying time is 30 - 60 min.

23. The preparation method according to claim 18, characterized in that, The mass ratio of the ternary material to lithium iron phosphate is (1 - 3):(1 - 3).

24. The preparation method according to claim 18, wherein, The ball milling time is 4 - 6 h.

25. The preparation method according to claim 9, characterized in that, The preparation method of the lithium iron phosphate layer includes: dispersing lithium iron phosphate, SP, LAGP and PVDF into NMP to obtain a lithium iron phosphate slurry; then successively performing coating and drying to obtain the lithium iron phosphate layer.

26. The preparation method according to claim 25, wherein, The mass ratio of the lithium iron phosphate, SP, LAGP and PVDF is (90 - 95):(2 - 5):(1 - 2):(2 - 5).

27. The preparation method according to claim 25, wherein, The solid content of the lithium iron phosphate slurry is 60 - 70%.

28. The preparation method according to claim 25, wherein, The D50 of the lithium iron phosphate is 0.5 - 3 μm.

29. The preparation method according to claim 25, characterized in that, The lithium iron phosphate is carbon-coated lithium iron phosphate.

30. The preparation method according to claim 29, characterized in that, The carbon coating amount in the carbon-coated lithium iron phosphate is 1 - 2 wt%.

31. The preparation method according to claim 25, characterized in that, The thickness at one end close to the positive electrode tab during the coating is 30 - 50 μm.

32. The preparation method according to claim 25, wherein The thickness at one end far from the positive electrode tab during the coating is 10 - 30 μm.

33. The preparation method according to claim 25, characterized in that, The drying temperature is 100 - 130 °C.

34. The preparation method according to claim 25, characterized in that, The drying time is 30 - 60 min.

35. A lithium-ion battery, characterized in that, The lithium ion battery includes the multi-layer composite positive electrode sheet according to any one of claims 1 - 8.

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