Gradient positive plate of lithium ion battery and preparation method of gradient positive plate

Abstract

The invention provides a lithium ion battery gradient positive electrode plate and a preparation method thereof. The lithium ion battery gradient positive electrode plate comprises a current collector, and a first positive electrode layer and a second positive electrode layer which are sequentially laminated on the surface of at least one side of the current collector, the first positive electrode layer comprises a first positive electrode active material; the second positive electrode layer comprises a second positive electrode active material and a lithium supplement agent; the mass content of nickel in the first positive electrode active material is greater than the mass content of nickel in the second positive electrode active material. The first positive electrode layer with high nickel content and the second positive electrode layer with relatively low nickel content are sequentially stacked on the surface of one side of the current collector, and the lithium supplement agent is introduced into the second positive electrode layer, so that the first-week coulombic efficiency and the cycle performance of the battery are remarkably improved, and the actually available energy density of the whole battery is further improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology and relates to a gradient positive electrode sheet for lithium-ion batteries and its preparation method. Background Technology

[0002] With the widespread application of lithium-ion batteries in power and energy storage, the market has placed higher demands on their energy density and safety, with improving battery energy density being the most urgent. Under existing lithium-ion battery systems, the mainstream approaches to improving energy density include increasing operating voltage, lithium replenishment, and increasing the nickel content in the cathode material.

[0003] Increasing the operating voltage not only requires maintaining the structural stability of the cathode material but also poses challenges to the electrolyte's tolerance. Currently, the main bottleneck of this technology is that electrolyte components are prone to violent interfacial side reactions with the cathode active material under high voltage, leading to gas accumulation and potentially thermal runaway, posing safety hazards. While developing ultra-high nickel ternary cathode materials (Ni≥90mol%) can effectively improve capacity density, their highly active surfaces are prone to side reactions with the electrolyte, causing the active material to dissolve and forming a rock salt phase on the particle surface that hinders lithium-ion migration, thus reducing the battery's cycle life and rate performance. In contrast, high-nickel ternary materials (Ni≥80mol%) maintain higher capacity while exhibiting greater structural stability, achieving a better balance between cycle performance and safety, but their energy density is relatively lower.

[0004] On the other hand, when a battery first begins a charge-discharge cycle, it consumes active lithium to form a solid electrolyte interphase (SEI) film on the negative electrode surface, resulting in low coulombic efficiency and capacity loss. Replenishing the lost lithium ions through technical means can improve the battery's energy density. Currently, lithium replenishment technologies include positive electrode lithium replenishment, negative electrode lithium replenishment, separator lithium replenishment, and electrolyte lithium replenishment.

[0005] In summary, providing a high-performance lithium-ion battery cathode and its preparation method has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a gradient cathode sheet for lithium-ion batteries and its preparation method. By sequentially stacking a first cathode layer with a high nickel content and a second cathode layer with a relatively low nickel content on the surface of the current collector side, and by introducing a lithium replenishing agent in the second cathode layer, the first-cycle coulombic efficiency and cycle performance of the battery are significantly improved, thereby further improving the actual usable energy density of the whole battery.

[0007] To achieve this objective, the present invention employs the following technical solution:

[0008] In a first aspect, the present invention provides a lithium-ion battery gradient cathode sheet, the lithium-ion battery gradient cathode sheet comprising a current collector and a first cathode layer and a second cathode layer sequentially stacked on at least one side surface of the current collector;

[0009] The first positive electrode layer includes a first positive electrode active material;

[0010] The second positive electrode layer includes a second positive electrode active material and a lithium replenishing agent;

[0011] The nickel content in the first positive electrode active material is greater than the nickel content in the second positive electrode active material.

[0012] In this invention, the current collector includes aluminum foil and / or carbon-coated aluminum foil; the thickness of the current collector is 15μm-25μm, for example, it can be 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm or 25μm, etc.

[0013] The lithium-ion battery gradient cathode sheet provided by this invention adopts a gradient double-layer electrode structure. A first cathode layer with a high nickel content and a second cathode layer with a relatively low nickel content are sequentially stacked on at least one side of the current collector. Through the synergistic effect of these two layers, while maintaining the high capacity characteristics of the ultra-high nickel ternary cathode material, the energy density of the electrode is effectively improved, and it exhibits excellent structural stability, ensuring the mechanical integrity of the electrode during long-cycle operation, suppressing particle cracking and interfacial side reactions, and also contributing to improving the battery's cycle life and capacity retention. Simultaneously, the introduction of a lithium replenishing agent in the second cathode layer releases active lithium during the battery activation stage, directly compensating for the irreversible lithium loss caused by the formation of the SEI in the negative electrode, significantly improving the battery's first-cycle coulombic efficiency, thereby further enhancing the actual usable energy density of the entire battery.

[0014] It should be noted that by placing the second cathode layer of the second cathode active material, which has a relatively low nickel content, strong structural stability, and weak surface reactivity, on the surface of the first cathode active material, which has a high charge and discharge capacity, weak structural stability, strong surface reactivity, and high nickel content, the first cathode active material is protected. This improves the charge and discharge cycle stability of lithium-ion batteries, alleviates capacity decay, and extends service life. Compared with traditional inactive coatings, the second cathode active material containing redox couples performs better in suppressing specific capacity loss.

[0015] Preferably, in the lithium-ion battery gradient cathode sheet, the mass content of the first cathode active material is greater than or equal to the mass content of the second cathode active material.

[0016] In this invention, the energy density of the electrode is further improved by controlling the mass content of the first positive electrode active material to be greater than or equal to the mass content of the second positive electrode active material. Specifically, the total mass content of the first and second positive electrode active materials in the lithium-ion battery gradient positive electrode sheet is 100%.

[0017] Preferably, the mass content of the first positive electrode active material in the lithium-ion battery gradient positive electrode sheet is 20%-80%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%, etc.

[0018] Preferably, the mass content of the second positive electrode active material in the lithium-ion battery gradient positive electrode sheet is 20%-80%, for example, it can be 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%, etc.

[0019] Preferably, the molecular formula of the first positive electrode active material is LiNi. x1 Co y1 Mn z1 In the formula, 0.99≥x1≥0.80, 0.10≥y1≥0, 0.10≥z1≥0, and x1+y1+z1=1.

[0020] In this invention, 0.99≥x1≥0.80, for example, can be 0.99, 0.98, 0.96, 0.90, 0.88, 0.86, 0.85, 0.84, 0.82, or 0.80, etc.; 0.10≥y1≥0, for example, can be 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01, etc.; 0.10≥z1≥0, for example, can be 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01, etc.

[0021] Preferably, the molecular formula of the second positive electrode active material is LiNi. x2 Co y2 Mn z2 In the formula, 0.90≥x2≥0.60, 0.20≥y2≥0, 0.20≥z2≥0, and x2+y2+z2=1.

[0022] In this invention, 0.90 ≥ x² ≥ 0.60, for example, can be 0.90, 0.88, 0.86, 0.85, 0.84, 0.82, 0.80, 0.78, 0.76, 0.75, 0.74, 0.72, 0.70, 0.68, 0.66, 0.65, 0.64, 0.62, or 0.60, etc.; 0.20 ≥ y² ≥ 0, for example, can be 0.20, 0.19, 0.18, 0.17, 0.16, 0.15, 0.14, 0.13, 0, etc. 0.12, 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01, etc.; 0.20≥z2≥0, for example, it can be 0.20, 0.19, 0.18, 0.17, 0.16, 0.15, 0.14, 0.13, 0.12, 0.11, 0.10, 0.09, 0.08, 0.07, 0.06, 0.05, 0.04, 0.03, 0.02, or 0.01, etc.

[0023] Preferably, the mass of the lithium replenishing agent is 0.005-0.1 of the total mass of the first positive electrode active material and the second positive electrode active material, for example, it can be 0.005, 0.006, 0.008, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09 or 0.1, and preferably 0.02-0.1.

[0024] It should be noted that by adjusting the range of lithium supplementation agent addition, the problem of reduced coulombic efficiency in the first week can be improved, thereby increasing the capacity, cycle life, and energy density of lithium batteries. If too much lithium supplementation agent is added, it may lead to over-lithiation and the formation of lithium dendrites. When lithium dendrites grow to a certain extent, they will pierce the separator, causing an internal short circuit in the lithium battery and affecting its safety performance. It may also result in incomplete decomposition of the lithium supplementation agent during delithiation, leaving inactive residues in the cathode material and affecting the battery's specific energy density. If too little lithium supplementation agent is added, it will not effectively improve the battery's coulombic efficiency in the first week.

[0025] Preferably, the lithium supplement includes any one or a combination of at least two of lithium oxide, aryl lithium compound, or lithium naphthalene.

[0026] Preferably, the lithium oxide includes any one or a combination of at least two of Li2NiO2, Li2CO3, Li2O, Li2O2, Li6CoO4 or Li5FeO4.

[0027] Preferably, the first positive electrode layer further includes a first conductive agent and a first adhesive.

[0028] Preferably, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder in the first positive electrode layer is (80-99):(0.5-10):(0.5-10), for example, it can be 80:10:10, 82:9:9, 85:7:8, 88:6:6, 90:5:5, 92:4:4, 95:2:3, 98:1:1, or 99:0.5:0.5, etc.

[0029] Preferably, the second positive electrode layer further includes a second conductive agent and a second adhesive.

[0030] In this invention, no specific requirements or limitations are placed on the conductive agent and adhesive in the first and second positive electrode layers. Those skilled in the art can make reasonable selections according to actual requirements. The conductive agent and adhesive in the first and second positive electrode layers can be the same material or different materials. Specifically, both the first and second conductive agents can be any one or a combination of at least two of conductive carbon black, Ketjen black, graphite, graphene, carbon nanotubes, or carbon fibers. Similarly, both the first and second adhesives can be any one or a combination of at least two of polyvinylidene fluoride, polyaniline, sodium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylic acid, polyvinylpyrrolidone, polysiloxane, or resins.

[0031] Preferably, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder in the second positive electrode layer is (80-99):(0.5-10):(0.5-10), for example, it can be 80:10:10, 82:9:9, 85:7:8, 88:6:6, 90:5:5, 92:4:4, 95:2:3, 98:1:1, or 99:0.5:0.5, etc.

[0032] Preferably, the thickness of the first positive electrode layer is 20μm-200μm, for example, it can be 20μm, 30μm, 40μm, 50μm, 60μm, 80μm, 100μm, 120μm, 140μm, 150μm, 160μm, 180μm or 200μm.

[0033] Preferably, the thickness of the second positive electrode layer is 20μm-200μm, for example, it can be 20μm, 30μm, 40μm, 50μm, 60μm, 80μm, 100μm, 120μm, 140μm, 150μm, 160μm, 180μm or 200μm.

[0034] In a second aspect, the present invention provides a method for preparing a lithium-ion battery gradient cathode sheet as described in the first aspect, the method comprising:

[0035] A first positive electrode layer is prepared on at least one side surface of the current collector using a first positive electrode slurry containing a first positive electrode active material;

[0036] A second positive electrode slurry containing a second positive electrode active material and a lithium replenishing agent is coated onto the surface of the first positive electrode layer, and then subjected to a first drying, a rolling press, and a second drying in sequence to obtain a lithium-ion battery gradient positive electrode sheet.

[0037] In this invention, the preparation method of the first positive electrode layer includes coating and drying.

[0038] The preparation method provided by this invention involves sequentially coating a first positive electrode slurry and a second positive electrode slurry with lithium replenishment function onto at least one side of a current collector to obtain a gradient positive electrode sheet for a lithium-ion battery. This method can be achieved by adapting existing coating equipment, without requiring large-scale modifications to the production line. It has strong process compatibility and significant advantages for rapid scaling up to mass production.

[0039] Preferably, the preparation method of the first positive electrode slurry includes: mixing a first binder and a first organic solvent, then adding a first positive electrode material and a first conductive agent thereto, and obtaining the first positive electrode slurry after vacuum degassing.

[0040] Preferably, the mass content of the first organic solvent in the first positive electrode slurry is 30%-60%, for example, it can be 30%, 32%, 35%, 36%, 38%, 40%, 42%, 45%, 46%, 48%, 50%, 52%, 55%, 56%, 58% or 60%, etc.

[0041] Preferably, the preparation method of the second positive electrode slurry includes: mixing a second binder and a second organic solvent, then adding a second positive electrode material, a lithium supplement agent, and a second conductive agent thereto, and obtaining the second positive electrode slurry after vacuum degassing.

[0042] Preferably, the mass content of the second organic solvent in the second positive electrode slurry is 30%-60%, for example, it can be 30%, 32%, 35%, 36%, 38%, 40%, 42%, 45%, 46%, 48%, 50%, 52%, 55%, 56%, 58% or 60%, etc.

[0043] In this invention, no specific requirements or special limitations are made for the first and second organic solvents. Those skilled in the art can choose them reasonably according to actual requirements. They can be the same material or different materials. For example, any one or a combination of at least two of the following can be selected: N-methylpyrrolidone, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethylene carbonate, 1,2-difluoroethylene carbonate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran.

[0044] Preferably, the temperature of the first drying is 85℃-95℃, for example, it can be 85℃, 86℃, 87℃, 88℃, 89℃, 90℃, 91℃, 92℃, 93℃, 94℃ or 95℃, etc.

[0045] Preferably, the temperature of the second drying is 100℃-110℃, for example, it can be 100℃, 101℃, 102℃, 103℃, 104℃, 105℃, 106℃, 107℃, 108℃, 109℃ or 110℃, etc.

[0046] In this invention, the second drying process also includes sequential slicing and weighing.

[0047] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0048] Compared with the prior art, the present invention has the following beneficial effects:

[0049] (1) The lithium-ion battery gradient cathode sheet provided by the present invention adopts a gradient double-layer electrode structure. A first cathode layer with high nickel content and a second cathode layer with relatively low nickel content are sequentially stacked on at least one side of the current collector. Through the synergistic effect of the two, while maintaining the high capacity characteristics of the ultra-high nickel ternary cathode material, the energy density of the electrode is effectively improved, and it has excellent structural stability. It not only ensures the mechanical integrity of the electrode in long-cycle cycling and suppresses the generation of particle cracks and interface side reactions, but also helps to improve the cycle life and capacity retention of the battery. At the same time, the lithium replenishing agent introduced into the second cathode layer can release active lithium during the battery activation stage, directly compensating for the irreversible lithium loss caused by the formation of the solid electrolyte interface film of the negative electrode, significantly improving the first-cycle coulombic efficiency of the battery, thereby further improving the actual usable energy density of the whole battery.

[0050] (2) The preparation method provided by the present invention involves sequentially coating a first positive electrode slurry and a second positive electrode slurry with lithium replenishment function onto at least one side of the current collector to obtain a lithium-ion battery gradient positive electrode sheet. This method can be achieved by adapting existing coating equipment, without the need for large-scale modification of the production line, and has strong process compatibility, giving it a significant advantage in rapidly scaling up to mass production. Attached Figure Description

[0051] Figure 1 This is a schematic diagram of the structure of the gradient positive electrode sheet for a lithium-ion battery provided by the present invention;

[0052] Wherein, 1-current collector, 2-first positive electrode layer, 3-second positive electrode layer. Detailed Implementation

[0053] To facilitate understanding, the technical solutions and implementation methods of this study are further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented based on the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0054] Unless otherwise specified, the experimental methods used in the following specific embodiments are conventional methods; the materials and reagents used in the specific embodiments are commercially available unless otherwise specified.

[0055] This invention provides a gradient positive electrode for lithium-ion batteries, wherein the gradient positive electrode (e.g.) Figure 1 (As shown) includes a current collector 1 and a first positive electrode layer 2 and a second positive electrode layer 3 sequentially stacked on one side surface of the current collector 1; the first positive electrode layer 2 includes a first positive electrode active material; the second positive electrode layer 3 includes a second positive electrode active material and a lithium supplement; the nickel mass content in the first positive electrode active material is greater than the nickel mass content in the second positive electrode active material.

[0056] Example 1

[0057] This embodiment provides a lithium-ion battery gradient cathode sheet and its preparation method. The lithium-ion battery gradient cathode sheet includes a current collector and a first cathode layer and a second cathode layer sequentially stacked on one side surface of the current collector.

[0058] The first positive electrode layer comprises a first positive electrode active material, a first conductive agent, and a first binder; the molecular formula of the first positive electrode active material is LiNi. 0.96 Co 0.02 Mn 0.02 The first conductive agent is conductive carbon black; the first adhesive is polyvinylidene fluoride.

[0059] The second positive electrode layer comprises a second positive electrode active material, a second conductive agent, a second binder, and a lithium replenishing agent; the molecular formula of the second positive electrode active material is LiNi. 0.80 Co 0.10 Mn 0.10 The second conductive agent is conductive carbon black; the second adhesive is polyvinylidene fluoride; the lithium supplement is Li2NiO2.

[0060] In the lithium-ion battery gradient cathode sheet, the mass content of the first cathode active material is 60%, the mass content of the second cathode active material is 40%, and the mass of the lithium replenishing agent is 0.05 of the total mass of the first cathode active material and the second cathode active material.

[0061] In the first positive electrode layer, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is 90:5:5;

[0062] In the second positive electrode layer, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is 90:5:5;

[0063] The areal density of the first positive electrode layer is 300 g / m³. 2 The thickness is 80 μm; the areal density of the second positive electrode layer is 240 g / m³. 2 The thickness is 80μm.

[0064] The preparation method includes the following steps:

[0065] (1) Mix the first binder and the first organic solvent, then add the first positive electrode material and the first conductive agent and mix them evenly. After vacuum degassing, the first positive electrode slurry is obtained.

[0066] Wherein, the first organic solvent is N-methylpyrrolidone, and the mass content of the first organic solvent in the first positive electrode slurry is 40%;

[0067] (2) Mix the second binder and the second organic solvent, then add the second positive electrode material, lithium supplementer and the second conductive agent and mix them evenly. After vacuum degassing, the second positive electrode slurry is obtained.

[0068] The second organic solvent is N-methylpyrrolidone, and the mass content of the second organic solvent in the second positive electrode slurry is 40%.

[0069] (3) The first positive electrode slurry is coated on the surface of the current collector with a thickness of 20 μm, and the first drying is carried out at a temperature of 90°C to form the first positive electrode layer. Then the second positive electrode slurry is coated on the surface of the first positive electrode layer, and the second drying is carried out at a temperature of 90°C. After rolling, the third drying is carried out at a temperature of 105°C. Finally, the lithium-ion battery gradient positive electrode sheet is obtained after slicing and weighing.

[0070] The current collector is an aluminum foil.

[0071] Example 2

[0072] This embodiment provides a lithium-ion battery gradient cathode sheet and its preparation method. The lithium-ion battery gradient cathode sheet includes a current collector and a first cathode layer and a second cathode layer sequentially stacked on one side surface of the current collector.

[0073] The first positive electrode layer comprises a first positive electrode active material, a first conductive agent, and a first binder; the molecular formula of the first positive electrode active material is LiNi. 0.90 Co 0.05 Mn 0.05 The first conductive agent is graphite; the first adhesive is polyvinylpyrrolidone.

[0074] The second positive electrode layer comprises a second positive electrode active material, a second conductive agent, a second binder, and a lithium replenishing agent; the molecular formula of the second positive electrode active material is LiNi. 0.75 Co 0.15 Mn 0.10 The second conductive agent is graphite; the second adhesive is polyvinylpyrrolidone; the lithium supplement is Li₂O.

[0075] In the lithium-ion battery gradient cathode sheet, the mass content of the first cathode active material is 70%, the mass content of the second cathode active material is 30%, and the mass of the lithium replenishing agent is 0.04% of the total mass of the first and second cathode active materials.

[0076] In the first positive electrode layer, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is 92:4:4;

[0077] In the second positive electrode layer, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is 92:4:4;

[0078] The areal density of the first positive electrode layer is 280 g / m³. 2 The thickness is 80 μm; the areal density of the second positive electrode layer is 240 g / m³. 2 The thickness is 40μm.

[0079] The preparation method includes the following steps:

[0080] (1) Mix the first binder and the first organic solvent, then add the first positive electrode material and the first conductive agent and mix them evenly. After vacuum degassing, the first positive electrode slurry is obtained.

[0081] Wherein, the first organic solvent is N-methylpyrrolidone, and the mass content of the first organic solvent in the first positive electrode slurry is 50%;

[0082] (2) Mix the second binder and the second organic solvent, then add the second positive electrode material, lithium supplementer and the second conductive agent and mix them evenly. After vacuum degassing, the second positive electrode slurry is obtained.

[0083] The second organic solvent is N-methylpyrrolidone, and the mass content of the second organic solvent in the second positive electrode slurry is 50%.

[0084] (3) The first positive electrode slurry is coated on the surface of the current collector with a thickness of 20 μm, and the first drying is carried out at a temperature of 85°C to form a first positive electrode layer. Then the second positive electrode slurry is coated on the surface of the first positive electrode layer, and the second drying is carried out at a temperature of 85°C. After rolling, the third drying is carried out at a temperature of 100°C. Finally, the lithium-ion battery gradient positive electrode sheet is obtained after slicing and weighing.

[0085] The current collector is an aluminum foil.

[0086] Example 3

[0087] This embodiment provides a lithium-ion battery gradient cathode sheet and its preparation method. The lithium-ion battery gradient cathode sheet includes a current collector and a first cathode layer and a second cathode layer sequentially stacked on one side surface of the current collector.

[0088] The first positive electrode layer comprises a first positive electrode active material, a first conductive agent, and a first binder; the molecular formula of the first positive electrode active material is LiNi. 0.95 Co 0.03 Mn 0.02 The first conductive agent is Ketjen Black; the first adhesive is polyvinylidene fluoride.

[0089] The second positive electrode layer comprises a second positive electrode active material, a second conductive agent, a second binder, and a lithium replenishing agent; the molecular formula of the second positive electrode active material is LiNi. 0.75 Co 0.15 Mn 0.10 The second conductive agent is Ketjen Black; the second adhesive is polyvinylidene fluoride; the lithium supplement is Li₂CO₃.

[0090] In the lithium-ion battery gradient cathode sheet, the mass content of the first cathode active material is 50%, the mass content of the second cathode active material is 50%, and the mass of the lithium replenishing agent is 0.06 of the total mass of the first cathode active material and the second cathode active material.

[0091] In the first positive electrode layer, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is 88:6:6;

[0092] In the second positive electrode layer, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is 88:6:6;

[0093] The areal density of the first positive electrode layer is 300 g / m³. 2 The thickness is 80 μm; the areal density of the second positive electrode layer is 300 g / m³. 2 The thickness is 80μm.

[0094] The preparation method includes the following steps:

[0095] (1) Mix the first binder and the first organic solvent, then add the first positive electrode material and the first conductive agent and mix them evenly. After vacuum degassing, the first positive electrode slurry is obtained.

[0096] Wherein, the first organic solvent is N-methylpyrrolidone, and the mass content of the first organic solvent in the first positive electrode slurry is 60%;

[0097] (2) Mix the second binder and the second organic solvent, then add the second positive electrode material, lithium supplementer and the second conductive agent and mix them evenly. After vacuum degassing, the second positive electrode slurry is obtained.

[0098] The second organic solvent is N-methylpyrrolidone, and the mass content of the second organic solvent in the second positive electrode slurry is 60%.

[0099] (3) The first positive electrode slurry is coated on the surface of the current collector with a thickness of 20 μm, and the first drying is carried out at a temperature of 95°C to form the first positive electrode layer. Then the second positive electrode slurry is coated on the surface of the first positive electrode layer, and the second drying is carried out at a temperature of 95°C. After rolling, the third drying is carried out at a temperature of 110°C. Finally, the lithium-ion battery gradient positive electrode sheet is obtained after slicing and weighing.

[0100] The current collector is an aluminum foil.

[0101] Example 4

[0102] This embodiment provides a graded cathode sheet for a lithium-ion battery and its preparation method, except that the molecular formula of the first cathode active material is LiNi. 0.75 Co0.15 Mn 0.10 The molecular formula of the second positive electrode active material is LiNi. 0.60 Co 0.20 Mn 0.20 Except for the above, all other conditions are the same as in Example 1.

[0103] Example 5

[0104] This embodiment provides a lithium-ion battery gradient cathode sheet and its preparation method, except that the molecular formula of the second cathode active material is LiNi. 0.50 Co 0.30 Mn 0.20 Except for the above, all other conditions are the same as in Example 1.

[0105] Example 6

[0106] This embodiment provides a gradient cathode sheet for a lithium-ion battery and its preparation method. Except that the mass content of the first cathode active material in the gradient cathode sheet is 90% and the mass content of the second cathode active material is 10%, all other conditions are the same as in Example 1.

[0107] Example 7

[0108] This embodiment provides a gradient cathode sheet for a lithium-ion battery and its preparation method. Except that the mass content of the first cathode active material in the gradient cathode sheet is 40% and the mass content of the second cathode active material is 60%, all other conditions are the same as in Example 1.

[0109] Example 8

[0110] This embodiment provides a gradient cathode sheet for a lithium-ion battery and its preparation method. Except that the mass of the lithium replenishing agent is 0.2 times the total mass of the first cathode active material and the second cathode active material, all other conditions are the same as in Example 1.

[0111] Comparative Example 1

[0112] This comparative example provides a lithium-ion battery gradient cathode sheet and its preparation method, except that the molecular formula of the first cathode active material is LiNi. 0.80 Co 0.10 Mn 0.10 The molecular formula of the second positive electrode active material is LiNi. 0.96 Co 0.02 Mn 0.02 Except for the above, all other conditions are the same as in Example 1.

[0113] Comparative Example 2

[0114] This comparative example provides a lithium-ion battery cathode sheet and its preparation method, except that the molecular formula of both the first and second cathode active materials is LiNi. 0.90 Co 0.05 Mn 0.05 Except for the above, all other conditions are the same as in Example 1.

[0115] Comparative Example 3

[0116] This comparative example provides a lithium-ion battery positive electrode sheet and its preparation method. Except for the absence of a second positive electrode layer, all other conditions are the same as in Example 1.

[0117] Comparative Example 4

[0118] This comparative example provides a lithium-ion battery positive electrode sheet and its preparation method. Except that no lithium replenishing agent is added to the second positive electrode layer, all other conditions are the same as in Example 1.

[0119] Inside an argon-protected glove box, using the lithium-ion battery positive electrode sheets obtained in the above examples and comparative examples as the positive electrode, a polyethylene separator, a lithium metal sheet as the negative electrode, and a 1 mol / L lithium hexafluorophosphate EC / DEC (volume ratio 1:1) solution as the electrolyte, a button cell was assembled. The electrochemical performance of the obtained battery was tested using a Blue Electric testing system. At a temperature of 25°C and a voltage range of 2.5V-4.25V, the battery was first charged and discharged for 3 cycles at a 0.1C rate, followed by 100 cycles at a 1C rate.

[0120] The test results are shown in Table 1.

[0121] Table 1

[0122]

[0123] "-" indicates that no corresponding substance was added.

[0124] As shown in Table 1:

[0125] The lithium-ion battery gradient cathode sheet and its preparation method provided in Examples 1-3 of this invention, by sequentially stacking a first cathode layer with a high nickel content and a second cathode layer with a relatively low nickel content on the surface of the current collector side, and by introducing a lithium replenishing agent in the second cathode layer, significantly improves the first-cycle coulombic efficiency and cycle performance of the battery, thereby further improving the actual usable energy density of the entire battery. Specifically, the battery's first-cycle efficiency is >90%, the first-cycle discharge specific capacity is >199 mAh / g, and the capacity retention rate after 100 cycles is >94%.

[0126] A comparison of Examples 1 and 4-5 shows that if the nickel content of the first positive electrode active material and / or the second positive electrode active material is low, the average nickel content of the active material in the overall positive electrode sheet will be reduced, resulting in a lower discharge capacity. This deviates from the original intention of preparing a gradient positive electrode sheet for lithium-ion batteries, which is to provide better capacity retention through the gradient design of nickel content while maintaining a high discharge capacity.

[0127] A comparison of Examples 1 and 6-7 reveals that if the content of the first positive electrode active material in the gradient positive electrode sheet of a lithium-ion battery is too high, the protection of the first positive electrode layer with a higher capacity will be reduced because the content of the second positive electrode active material with a lower nickel content in the second positive electrode layer is too low. Although the specific capacity of the first discharge cycle is improved, the capacity decay during long-term charge-discharge cycles is only partially mitigated due to the propagation of microcracks caused by interfacial side reactions and internal strain accumulation. If the mass content of the first positive electrode active material in the gradient positive electrode sheet of a lithium-ion battery is less than the mass content of the second positive electrode active material, the first positive electrode active material with a higher nickel content mainly provides a higher charge-discharge capacity for the positive electrode sheet. Therefore, when the mass content of the second positive electrode active material is high, the charge-discharge capacity of the lithium-ion battery will decrease.

[0128] A comparison of Examples 1 and 8, and Comparative Example 4, reveals that if too much lithium replenishing agent is added, over-lithiation may occur, leading to lithium dendrites. When these dendrites grow to a certain extent, they can pierce the separator, causing an internal short circuit in the lithium battery and affecting its safety performance. Furthermore, if the lithium replenishing agent is not completely decomposed during delithiation, inactive residues may remain in the positive electrode material, affecting the battery's specific energy density. If no lithium replenishing agent is added to the second positive electrode layer, it cannot effectively compensate for the lithium ions lost during the first charge-discharge cycle due to the formation of an SEI film on the negative electrode surface, resulting in minimal improvement in the battery's first-cycle discharge specific capacity and cycle performance.

[0129] A comparison of Example 1 and Comparative Examples 1-2 shows that if the nickel content in the first positive electrode active material is less than or equal to the nickel content in the second positive electrode active material, i.e., the gradient design of the nickel content is contrary to the original intention, the positive electrode active material with higher nickel content has a more reactive surface, which makes it easy to have interfacial side reactions with the electrolyte. In addition, it is prone to irreversible phase transitions in the high delithiation state, which causes strain accumulation, leading to microcracks and propagation. The exposed fresh interface continues to have interfacial side reactions with the electrolyte, resulting in the dissolution of active materials and the formation of inactive phases such as surface salt rock layers. As a result, the assembled lithium-ion battery suffers severe capacity decay during long-term charge-discharge cycles, and the battery life is greatly shortened.

[0130] A comparison of Example 1 and Comparative Example 3 shows that although the cathode sheet prepared using a single ultra-high nickel cathode active material can achieve a high energy density, its cycle performance is poor.

[0131] The present invention has been illustrated with the above embodiments to illustrate its detailed structural features. However, the present invention is not limited to the above detailed structural features, that is, it does not mean that the present invention must rely on the above detailed structural features to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions for the components used in the present invention, additions of auxiliary components, and selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A gradient cathode sheet for lithium-ion batteries, characterized in that, The lithium-ion battery gradient cathode includes a current collector and a first cathode layer and a second cathode layer sequentially stacked on at least one side surface of the current collector; The first positive electrode layer includes a first positive electrode active material; The second positive electrode layer includes a second positive electrode active material and a lithium replenishing agent; The nickel content in the first positive electrode active material is greater than the nickel content in the second positive electrode active material.

2. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, In the lithium-ion battery gradient cathode sheet, the mass content of the first cathode active material is greater than or equal to the mass content of the second cathode active material. Preferably, the mass content of the first positive electrode active material in the lithium-ion battery gradient positive electrode sheet is 20%-80%; Preferably, the mass content of the second positive electrode active material in the lithium-ion battery gradient positive electrode sheet is 20%-80%.

3. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The molecular formula of the first positive electrode active material is LiNi x1 Co y1 Mn z1 In the formula, 0.99≥x1≥0.80, 0.10≥y1≥0, 0.10≥z1≥0, and x1+y1+z1=1; Preferably, the molecular formula of the second positive electrode active material is LiNi. x2 Co y2 Mn z2 In the formula, 0.90≥x2≥0.60, 0.20≥y2≥0, 0.20≥z2≥0, and x2+y2+z2=1.

4. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The mass of the lithium replenishing agent is 0.005-0.1 of the total mass of the first positive electrode active material and the second positive electrode active material, preferably 0.02-0.

1. Preferably, the lithium supplement includes any one or a combination of at least two of lithium oxide, aryl lithium compound, or lithium naphthalene; Preferably, the lithium oxide includes any one or a combination of at least two of Li2NiO2, Li2CO3, Li2O, Li2O2, Li6CoO4 or Li5FeO4.

5. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The first positive electrode layer further includes a first conductive agent and a first adhesive; Preferably, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder in the first positive electrode layer is (80-99):(0.5-10):(0.5-10).

6. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The second positive electrode layer also includes a second conductive agent and a second adhesive; Preferably, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder in the second positive electrode layer is (80-99):(0.5-10):(0.5-10).

7. The lithium-ion battery gradient cathode sheet according to claim 1, characterized in that, The thickness of the first positive electrode layer is 20μm-200μm; Preferably, the thickness of the second positive electrode layer is 20μm-200μm.

8. A method for preparing a lithium-ion battery gradient cathode sheet as described in any one of claims 1-7, characterized in that, The preparation method includes: A first positive electrode layer is prepared on the surface of the current collector using a first positive electrode slurry containing a first positive electrode active material; A second positive electrode slurry containing a second positive electrode active material and a lithium replenishing agent is coated onto the surface of the first positive electrode layer, and then subjected to a first drying, a rolling press, and a second drying in sequence to obtain a lithium-ion battery gradient positive electrode sheet.

9. The preparation method according to claim 8, characterized in that, The preparation method of the first positive electrode slurry includes: mixing a first binder and a first organic solvent, then adding a first positive electrode material and a first conductive agent thereto, and obtaining the first positive electrode slurry after vacuum degassing; Preferably, the mass content of the first organic solvent in the first positive electrode slurry is 30%-60%.

10. The preparation method according to claim 8, characterized in that, The preparation method of the second positive electrode slurry includes: mixing a second binder and a second organic solvent, then adding a second positive electrode material, a lithium supplement agent and a second conductive agent thereto, and obtaining the second positive electrode slurry after vacuum degassing; Preferably, the mass content of the second organic solvent in the second positive electrode slurry is 30%-60%; Preferably, the temperature of the first drying is 85℃-95℃; Preferably, the temperature for the second drying is 100℃-110℃.

Images