A lithium battery cathode sheet, its preparation method and application

Abstract

This invention discloses a lithium-ion battery positive electrode sheet, its preparation method, and its application, belonging to the field of lithium-ion battery technology. It includes a current collector and a multi-layered positive electrode structure attached to the current collector. The multi-layered positive electrode structure, from top to bottom, consists of a lithium manganese iron phosphate layer, a polycrystalline ternary material-doped lithium manganese iron phosphate layer, and a lithium manganese oxide layer, with the lithium manganese oxide layer attached to the surface of the current collector. The polycrystalline ternary material is lithium nickel cobalt manganese oxide. The polycrystalline ternary material is compounded with nano-lithium manganese iron phosphate. The nano-lithium manganese iron phosphate particles are finer and can fill the gaps between the ternary positive electrode active material and the lithium manganese oxide positive electrode material, improving the compaction density of the composite material and the contact between particles, thus reducing the internal resistance of the electrode sheet. The multi-layered active material improves the battery's rate charging and discharging capabilities, and enhances battery safety and low-temperature resistance.

Description

Technical Field

[0001] This invention belongs to the field of lithium battery technology, specifically relating to a lithium battery positive electrode sheet, its preparation method, and its application. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] With the continuous development of the electric vehicle market, higher demands are being placed on the energy density and cycle performance of lithium-ion batteries to achieve longer lifespans and driving ranges. As a new energy storage device, high energy density and long cycle life lithium-ion batteries have become performance indicators of urgent concern to consumers. Positive and negative electrode materials, as important components of lithium-ion batteries, directly affect the battery's capacity, voltage plateau, and energy density. Inconsistent ion diffusion rates on the electrodes severely impact battery performance, thus attracting significant attention from researchers.

[0004] Current mainstream cathode materials include lithium iron phosphate (LFP), ternary cathodes, and lithium manganese iron phosphate (LFP). While LFP cathodes offer structural stability and superior safety, they suffer from low specific capacity and poor low-temperature performance. Ternary cathodes boast high reversible capacity, but their safety performance needs improvement. LFP, on the other hand, has poor processing performance and is unsuitable for use alone. Furthermore, direct blending of LFP with other cathode materials presents several drawbacks: spinel-structured lithium manganese oxide has a three-dimensional structure, while olivine-structured LFP has a one-dimensional structure. This results in a faster lithium-ion diffusion rate on spinel-structured lithium manganese oxide compared to LFP, leading to different lithium insertion / extraction rates. Consequently, battery safety and performance cannot be guaranteed, and the different ion diffusion rates within the cell further contribute to battery performance degradation.

[0005] Therefore, there is an urgent need to develop a cathode material and its preparation method that meets the requirements of cell safety and electrochemical performance, while also taking into account excellent low-temperature performance and cycle performance, and meeting the requirements of production and processing. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a lithium battery positive electrode sheet, its preparation method, and its application. The lithium battery positive electrode sheet provided by the present invention can improve battery safety performance, rate performance, and battery cycle performance.

[0007] To achieve the above objectives, the technical solution of the present invention is as follows:

[0008] In a first aspect, the present invention provides a lithium battery positive electrode sheet, comprising a current collector and a positive electrode multilayer structure attached to the current collector;

[0009] The positive electrode multilayer structure consists of a lithium manganese iron phosphate layer, a polycrystalline ternary material mixed lithium manganese iron phosphate layer, and a lithium manganese oxide layer from top to bottom, with the lithium manganese oxide layer attached to the surface of the current collector.

[0010] The polycrystalline ternary material is lithium nickel cobalt manganese oxide.

[0011] A second aspect of the present invention provides a method for preparing the above-mentioned lithium battery positive electrode sheet, comprising the following steps:

[0012] Prepare lithium manganese iron phosphate slurry; prepare lithium manganese iron phosphate slurry mixed with polycrystalline ternary materials; prepare lithium manganese oxide slurry;

[0013] Lithium manganese iron phosphate slurry, polycrystalline ternary material mixed with lithium manganese iron phosphate slurry and lithium manganese oxide slurry are sequentially coated on the surface of the current collector, and then dried, rolled and die-cut to obtain the positive electrode sheet of lithium battery.

[0014] A third aspect of the present invention provides a lithium battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte; wherein the positive electrode is the lithium battery positive electrode described above or a lithium battery positive electrode prepared by the preparation method described above.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention provides a lithium-ion battery cathode by optimizing the multilayer structure of the cathode attached to the current collector. With the direction closest to the current collector as the lower layer and the direction furthest from the current collector as the upper layer, the multilayer structure of the cathode attached to the current collector is, from bottom to top, configured as a lithium manganese oxide layer, a polycrystalline ternary material-doped lithium manganese iron phosphate layer, and a lithium manganese iron phosphate layer. Because the upper layer has a high current, it is made of lithium manganese iron phosphate, which promotes faster lithium-ion diffusion and improves battery safety. A blend of polycrystalline ternary cathode active material and nano-sized lithium manganese iron phosphate forms the middle layer. Due to the small particle size and olivine structure of the nano-sized lithium manganese iron phosphate, the excellent safety and cycle performance of the lithium manganese iron phosphate cathode material are leveraged to improve the cycle, rate, and safety performance of the polycrystalline ternary cathode active material. The upper layer is made of nano-sized lithium manganese iron phosphate, and the middle layer uses a blend of polycrystalline ternary material and nano-sized lithium manganese iron phosphate, which improves both compaction and interfacial impedance, further enhancing cycle performance while improving safety. Since lithium manganese oxide has better conductivity than polycrystalline ternary material, the lower layer is made of lithium manganese oxide cathode material. Furthermore, because spinel lithium manganese oxide has poor high-temperature performance, placing it in the lower layer reduces direct contact with the electrolyte, minimizing side reactions and promoting long battery cycles. Therefore, the multi-layer structure plays a significant role in improving safety performance and battery cycle performance. Meanwhile, the middle layer is a polycrystalline ternary material mixed with lithium manganese iron phosphate material, which can improve battery safety performance while also taking into account the energy density of the cell. Detailed Implementation

[0017] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0018] A first typical embodiment of the present invention provides a lithium battery positive electrode sheet, including a current collector and a positive electrode multilayer structure attached to the current collector;

[0019] The positive electrode multilayer structure consists of a lithium manganese iron phosphate layer, a polycrystalline ternary material mixed lithium manganese iron phosphate layer, and a lithium manganese oxide layer from top to bottom, with the lithium manganese oxide layer attached to the surface of the current collector.

[0020] The polycrystalline ternary material is lithium nickel cobalt manganese oxide.

[0021] It should be noted that the thickness of the positive electrode multilayer structure is determined based on the cell capacity and energy density, and should not be too thick or too thin. The total thickness of the positive electrode multilayer structure and the current collector is generally between 80mm and 200mm.

[0022] Because the upper layer has a high current, it is made of lithium manganese iron phosphate (LMP), which accelerates lithium-ion diffusion and improves battery safety. A blend of polycrystalline ternary cathode active material and nano-sized LMP serves as the intermediate layer. Due to the small particle size and olivine structure of LMP, the excellent safety and cycle performance of the LMP cathode material improve the cycle life, rate capability, and safety of the polycrystalline ternary cathode active material. The use of a blend of polycrystalline ternary material and LMP in the intermediate layer improves both compaction and interfacial impedance, further enhancing cycle performance while improving safety. Since lithium manganese oxide has better conductivity than polycrystalline ternary material, the lower layer is also lithium manganese oxide cathode material. Furthermore, because spinel lithium manganese oxide has poor high-temperature performance, placing it in the lower layer reduces direct contact with the electrolyte, minimizing side reactions and promoting long battery cycles. Therefore, the multi-layer structure plays a significant role in improving safety performance and battery cycle performance. Meanwhile, the middle layer is a polycrystalline ternary material mixed with lithium manganese iron phosphate material, which can improve battery safety performance while also taking into account the energy density of the cell.

[0023] In some embodiments of this implementation, the current collector includes, but is not limited to, aluminum foil, copper foil, and titanium mesh. Because aluminum foil has advantages such as high strength, good conductivity, easy surface treatment, and low cost, the current collector is preferably aluminum foil.

[0024] In some embodiments of this implementation, the median particle size D50 of the lithium manganese iron phosphate is preferably 200-1500 nm, more preferably 500-1200 nm, and even more preferably 800-1000 nm.

[0025] In some embodiments of this implementation, the median particle size D50 of the polycrystalline ternary material is preferably 6-12 μm, and more preferably 8-10 μm.

[0026] In some embodiments of this implementation, the median particle size D50 of the lithium manganese oxide is preferably 8-15 μm, more preferably 10-13 μm.

[0027] This invention employs nano-scale lithium manganese iron phosphate material. On one hand, its doping with micron-scale polycrystalline ternary materials improves compaction and interfacial impedance, thereby enhancing both safety and the cycle performance of the lithium battery. The intermediate layer cathode active material features a composite of particles of varying sizes, combining polycrystalline ternary cathode active material with nano-scale lithium manganese iron phosphate. The finer nano-scale lithium manganese iron phosphate particles can fill the gaps between the polycrystalline ternary cathode active material and the lithium manganese oxide cathode material, increasing the compaction density of the composite material and improving interparticle contact, thus reducing the internal resistance of the electrode. This multi-layered active material structure enhances the battery's rate charging and discharging capabilities, while the olivine-structured lithium manganese iron phosphate material improves battery safety.

[0028] In some embodiments of this implementation, the lithium nickel cobalt manganese oxide has the chemical formula LiNi. x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, x + y < 1.

[0029] In some embodiments of this implementation, the lithium manganese iron phosphate has an orthorhombic olivine-type crystal structure and the chemical formula LiMn. x Fe 1-x PO4, wherein 0.5 ≤ x < 1.0, preferably 0.6 ≤ x ≤ 0.8.

[0030] The olivine-type crystal structure of lithium manganese iron phosphate (LFP) endows it with high stability and good safety. A key characteristic of olivine-type LFP is that even during charging, when all lithium ions are released, structural collapse does not occur, thus ensuring battery safety. Furthermore, the crystal structure of LFP gives it the potential for high energy density, which is its greatest advantage over lithium iron phosphate (LFP).

[0031] In some embodiments of this implementation, the lithium manganese oxide has a spinel-type structure and its chemical formula is LiMn2O4.

[0032] In some embodiments of this implementation, the mass ratio of the polycrystalline ternary material to the nano-manganese iron lithium phosphate layer is preferably 50:50-99:2, and more preferably 70:30-80:20.

[0033] In some embodiments of this implementation, the coating weight ratio of the lithium manganese iron phosphate layer, the polycrystalline ternary material mixed nano-lithium manganese iron phosphate layer, and the lithium manganese oxide layer is preferably (1-10):(1-10):1, more preferably (1-7):(2-8):1.

[0034] A second typical embodiment of the present invention provides a method for preparing the above-mentioned lithium battery positive electrode sheet, comprising the following steps:

[0035] Prepare lithium manganese iron phosphate slurry; prepare lithium manganese iron phosphate slurry mixed with polycrystalline ternary materials; prepare lithium manganese oxide slurry;

[0036] Lithium manganese iron phosphate slurry, polycrystalline ternary material mixed with lithium manganese iron phosphate slurry and lithium manganese oxide slurry are sequentially coated on the surface of the current collector, and then dried, rolled and die-cut to obtain the positive electrode sheet of lithium battery.

[0037] In some embodiments of this implementation, the coating is performed by applying multiple layers of slurry through an extrusion coating method.

[0038] A third typical embodiment of the present invention provides a lithium battery, the lithium battery comprising a positive electrode, a negative electrode, a separator and an electrolyte; the positive electrode is the lithium battery positive electrode described above or a lithium battery positive electrode prepared by the preparation method described above.

[0039] In some embodiments of this implementation, the negative electrode sheet is a negative electrode active composite material, including one or more of graphite negative electrode materials, silicon-oxygen negative electrode materials, and modified silicon-oxygen negative electrode materials. The negative electrode sheet is an existing product; for example, it can be prepared by referring to a method for preparing a negative electrode material for lithium batteries disclosed in patent CN117105231A.

[0040] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0041] In the following embodiments, the modified silicon-oxygen anode is prepared using a method for preparing anode materials for lithium batteries disclosed in patent CN117105231A. The specific preparation method is as follows: 100g of silicon-oxygen compound SiO2 is prepared... x The particles were placed in a 1000 mL mixture of hydrogen peroxide and sulfuric acid (sulfuric acid concentration 10% and hydrogen peroxide concentration 15%) and reacted thoroughly to obtain SiO₂ containing hydroxyl groups. xParticles, wherein the mass ratio of hydrogen peroxide to sulfuric acid is 2:1; treated SiO x The particle surface has a large number of hydroxyl groups attached, which are hydroxyl-containing silicon oxide compounds SiO x Add granules to 100g containing titanium tert-butoxide C 16 H 36 In an ethanol solution of O4Ti (where the mass concentration of titanium tert-butoxide is 10%), it is chemically bonded to C of titanium tert-butoxide. 16 H 36 The O4Ti structure is more tightly bonded. After uniform stirring, filtration, and vacuum drying, it is calcined at 600℃ for 4 hours in a tube furnace filled with inert gas to obtain TiO2 / SiO2. x Particles.

[0042] 100g of the prepared TiO2 / SiO x The particles were ultrasonically dispersed in deionized water, and hydrochloric acid (10% by mass) was added to adjust the acidity for 1 hour until the pH value reached approximately 3. Then, 20 g of 3-aminopropyltrimethoxysilane was added dropwise to the acidic solution using a peristaltic pump, and the reaction was carried out for 12 hours after the addition. The mixture was then washed and dried by centrifugation with water and ethanol to obtain amino-containing TiO2 / SiO2. x Particles, namely: modified silicon-oxygen anodes.

[0043] In the following examples, the preparation method of the carboxyl-containing adhesive CMC is as follows: at room temperature, vinyl ethylene carbonate is added to a styrene and butadiene polymerization system (styrene and butadiene are mixed in a mass ratio of 1:1) for reaction. The amount of vinyl ethylene carbonate added is 50% of the mass of the styrene and butadiene polymerization system. After the reaction is completed, the mixture is diluted to a concentration of 3% and then the pH is adjusted to neutral to obtain the carboxyl-containing adhesive.

[0044] Example 1

[0045] A method for preparing a lithium-ion battery includes the following steps:

[0046] (a) Preparation of lithium-ion battery cathode

[0047] Preparation of lower layer spinel lithium manganese oxide slurry ①, with a concentration of 60 wt%, D 50 It is 13±1μm;

[0048] A mixed slurry was prepared by combining intermediate polycrystalline ternary cathode material and nano-manganese iron lithium phosphate cathode material at a mass ratio of 70:30. The chemical formula of the ternary cathode material is LiNi. 0.8 Co 0.1 Mn0 .1 O2, particle size: 8~12μm; nano-lithium manganese iron phosphate particle size D 50The thickness is 1±0.5μm, solvent type: NMP, and dosage: 45% of the total slurry volume;

[0049] Preparation of upper layer lithium manganese iron phosphate slurry ③, with a concentration of 50 wt%, solvent NMP, particle size: D 50 It is 1±0.5μm.

[0050] Three types of positive electrode slurry were coated in multiple layers by extrusion coating, with the weight ratio of the lower, middle and upper layers being 1:5:4. After drying, rolling and die cutting, positive electrode sheets were obtained.

[0051] (b) Preparation of lithium-ion battery anode

[0052] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0053] (c) Preparation of lithium-ion batteries

[0054] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0055] Example 2

[0056] A method for preparing a lithium-ion battery includes the following steps:

[0057] (a) Preparation of lithium-ion battery cathode

[0058] A lower layer of spinel lithium manganese oxide slurry ① was prepared, which had the same particle size, slurry solvent and concentration as in Example 1.

[0059] A mixed slurry ② was prepared by combining the intermediate layer polycrystalline ternary cathode material and the nano-manganese iron lithium cathode material at a mass ratio of 80:20. Except for the above ratio, the other parameters (particle size, slurry solvent and concentration) were the same as those in Example 1.

[0060] The upper layer of lithium manganese iron phosphate slurry ③ has the same particle size, slurry solvent and concentration as in Example 1.

[0061] Three types of positive electrode slurry were coated in multiple layers by extrusion coating, with the weight ratio of the lower, middle and upper layers being 2:5:3. After drying, rolling and die cutting, the positive electrode sheet was obtained.

[0062] (b) Preparation of lithium-ion battery anode

[0063] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0064] (c) Preparation of lithium-ion batteries

[0065] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0066] Example 3

[0067] A method for preparing a lithium-ion battery includes the following steps:

[0068] (a) Preparation of lithium-ion battery cathode

[0069] A lower layer of spinel lithium manganese oxide slurry ① was prepared, which had the same particle size, slurry solvent and concentration as in Example 1.

[0070] A mixed slurry ② was prepared by combining the intermediate layer polycrystalline ternary cathode material and the nano-manganese iron lithium cathode material at a mass ratio of 70:30. Except for the above ratio, the other parameters (particle size, slurry solvent and concentration) were the same as those in Example 1.

[0071] The upper layer of lithium manganese iron phosphate slurry ③ has the same particle size, slurry solvent and concentration as in Example 1.

[0072] Three types of positive electrode slurry were coated in multiple layers by extrusion coating, with the weight ratio of the lower, middle and upper layers being 1:7:2. After drying, rolling and die cutting, the positive electrode sheet was obtained.

[0073] (b) Preparation of lithium-ion battery anode

[0074] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0075] (c) Preparation of lithium-ion batteries

[0076] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0077] Comparative Example 1

[0078] A method for preparing a lithium-ion battery includes the following steps:

[0079] (a) Preparation of lithium-ion battery cathode

[0080] A composite active cathode material is prepared by blending polycrystalline ternary materials, lithium manganese iron phosphate materials, and lithium manganese oxide cathode materials in a ratio of 5:3:2. The polycrystalline ternary material has the chemical formula LiNi. 0.8 Co 0.1 Mn0 .1 O2, particle size: D 50 : 8~12μm; Lithium manganese iron phosphate particle size: D 50 : 1±0.5μm; Lithium manganese oxide particle size: D 50 13±1μm. The positive electrode composite active material, conductive agent Super-P, carbon nanotube (CNT) slurry, and binder PVDF are mixed in a mass ratio of 96.5:1:0.5:2 and added to the solvent NMP through a multi-step process. The mixture is stirred thoroughly to obtain a positive electrode slurry. The positive electrode slurry is then coated onto a 12μm thick aluminum foil, and after drying, rolling, and die-cutting, a positive electrode sheet is obtained.

[0081] (b) Preparation of lithium-ion battery anode

[0082] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0083] (c) Preparation of lithium-ion batteries

[0084] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0085] Comparative Example 2

[0086] A method for preparing a lithium-ion battery includes the following steps:

[0087] (a) Preparation of lithium-ion battery cathode

[0088] Ternary materials and lithium manganese iron phosphate cathode materials were mixed in a 5:5 ratio to form a cathode composite active material (the ternary materials and lithium manganese iron phosphate materials were the same as those in Comparative Example 1). The cathode composite active material, conductive agent Super-P, carbon nanotube (CNT) slurry, and binder PVDF were mixed in a mass ratio of 96.5:1:0.5:2 and added to solvent NMP through a multi-step method. After thorough stirring, a cathode mixed slurry was obtained. The cathode mixed slurry was then coated onto a 12μm thick aluminum foil, and after drying, rolling, and die-cutting, a cathode sheet was obtained.

[0089] (b) Preparation of lithium-ion battery anode

[0090] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0091] (c) Preparation of lithium-ion batteries

[0092] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0093] Comparative Example 3

[0094] A method for preparing a lithium-ion battery includes the following steps:

[0095] (a) Preparation of lithium-ion battery cathode

[0096] Using a ternary material as the positive electrode active material (the ternary material is the same as that in Comparative Example 1), the positive electrode active material, conductive agent Super-P, carbon nanotube (CNT) slurry, and binder PVDF are mixed in a mass ratio of 96.5:1:0.5:2. The mixture is added to the solvent NMP through a multi-step process and stirred thoroughly to obtain a positive electrode mixed slurry. The positive electrode mixed slurry is then coated onto a 12μm thick aluminum foil, and after drying, rolling, and die-cutting, a positive electrode sheet is obtained.

[0097] (b) Preparation of lithium-ion battery anode

[0098] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0099] (c) Preparation of lithium-ion batteries

[0100] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0101] Comparative Example 4

[0102] A method for preparing a lithium-ion battery includes the following steps:

[0103] (a) Preparation of lithium-ion battery cathode

[0104] Preparation of lower layer spinel lithium manganese oxide slurry, spinel lithium manganese oxide particle size: D 50 13±1um, concentration: 60%, solvent: NMP.

[0105] Preparation of upper layer lithium manganese iron phosphate slurry, lithium manganese iron phosphate particle size: D 50 1±0.5um, concentration: 50%, solvent: NMP.

[0106] Two types of positive electrode slurry are coated in multiple layers by extrusion coating, with the weight ratio of the lower layer to the upper layer being 3:7. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0107] (b) Preparation of lithium-ion battery anode

[0108] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0109] (c) Preparation of lithium-ion batteries

[0110] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0111] Comparative Example 5

[0112] A method for preparing a lithium-ion battery includes the following steps:

[0113] (a) Preparation of lithium-ion battery cathode

[0114] A mixed slurry was prepared by combining a lower layer of polycrystalline ternary cathode material and nano-lithium manganese iron phosphate cathode material at a mass ratio of 70:30; an upper layer of lithium manganese iron phosphate slurry was then prepared. The lower layer consists of a polycrystalline ternary cathode material with the chemical formula LiNi. 0.8 Co 0.1 Mn0 .1 O2, particle size: 8–12 μm; nano-lithium manganese iron phosphate particle size: D 50 1±0.5μm, solvent type: NMP, concentration: 55%. Upper layer lithium manganese iron phosphate slurry: lithium manganese iron phosphate particle size: D 50 1±0.5μm, solvent type: NMP, concentration: 50%.

[0115] Two types of positive electrode slurry are coated in multiple layers by extrusion coating, with the weight ratio of the lower layer to the upper layer being 5:5. After drying, rolling, and die cutting, a positive electrode sheet is obtained.

[0116] (b) Preparation of lithium-ion battery anode

[0117] A graphite anode and a modified silicon-oxygen anode were composited at a mass ratio of 95:5 to form a composite active material for the anode. The composite active material for the anode was mixed with carbon black Super-P, a carboxyl-containing binder CMC, and SBR at a mass ratio of 96:1:1.3:1.7. The mixture was then added to pure water and stirred until fully dissolved to obtain a negative electrode slurry. The negative electrode slurry was coated onto a 6μm thick copper foil, and after drying, rolling, and cutting, a negative electrode sheet was obtained.

[0118] (c) Preparation of lithium-ion batteries

[0119] The positive and negative electrode sheets prepared in (a) and (b) are assembled with the separator, packaged and injected with electrolyte, and subjected to formation and capacity testing to obtain the desired lithium-ion battery and charge-discharge test.

[0120] Lithium-ion battery safety testing:

[0121] Capacity test: The battery was charged and discharged at 0.33C and 1C respectively to obtain the battery capacity data at 0.33C and 1C.

[0122] Internal resistance test: Use a DC internal resistance tester to test the battery's internal resistance.

[0123] Hot box test: Place the fully charged battery in an explosion-proof oven, start at 25°C, increase the temperature to 130°C at a rate of 5°C / min, maintain the temperature for 60 minutes, and record the temperature rise and voltage change of the battery.

[0124] Needle penetration test: At 25°C, a 5mm steel needle is used to penetrate the geometric center of a fully charged battery at a speed of 25mm / s and held for 30 minutes. The temperature rise and voltage change of the battery are recorded.

[0125] Table 1. Battery internal resistance, battery capacity, and maximum battery temperature at 130°C in the hot box.

[0126]

[0127] As can be seen from the comparison of Examples and Comparative Examples 1, 2, and 3, coating the current collector surface with a mixture of various positive electrode active materials results in a multi-layer positive electrode structure. The middle layer of positive electrode active material features a mix of large and small particles, combining polycrystalline ternary positive electrode active material with nano-lithium manganese iron phosphate. The finer nano-lithium manganese iron phosphate particles can fill the gaps between the ternary positive electrode active material and the lithium manganese oxide positive electrode material, improving the compaction density of the composite material and the contact between particles, thus reducing the internal resistance of the electrode. The multi-layered active material structure improves the battery's rate charging and discharging capabilities, while the olivine-structured lithium manganese iron phosphate material enhances battery safety.

[0128] As can be seen from the comparison of Example 1, Comparative Examples 4 and 5, lithium manganese oxide can effectively improve the battery's capacity ratio and reduce its internal resistance. However, since lithium manganese iron phosphate has a significant impact on internal resistance, although it can improve the battery's safety performance, it will reduce the battery's internal resistance and capacity ratio. It should also be noted that the extensive use of lithium manganese iron phosphate in the upper layer will also result in a loss of battery capacity and compaction density.

[0129] The lithium-ion batteries in the examples and comparative examples were tested for high and low temperature performance (1C constant current and constant voltage charging to 3.65V, 1C discharge to 2.0V at different temperatures (25℃, 0℃), and 1C discharge to 1.6V at different temperatures (-10℃, -20℃, -30℃), and the results are shown in Table 2.

[0130] Table 2. High and Low Temperature Performance Test Results

[0131] Group 25℃ 0℃ -10℃ -20℃ -30℃ Example 1 100％ 98.8％ 95.1％ 89.7％ 84.3％ Example 2 100％ 95.8％ 92.3％ 88.5％ 81.9％ Example 3 100％ 92.4％ 88.5％ 85.2％ 78.6％ Comparative Example 1 100％ 88.2％ 81.9％ 79.5％ 67.1％ Comparative Example 2 100％ 82.3％ 77.2％ 73.2％ 62.5％ Comparative Example 3 100％ 78.3％ 74.2％ 65.2％ 57.5％ Comparative Example 4 100％ 89.1％ 83.6％ 80.4％ 68.9％ Comparative Example 5 100％ 90.2％ 85.8％ 82.7％ 71.1％

[0132] As can be seen from the comparison between the examples and the comparative examples in Table 2, coating the current collector with a multilayer positive electrode structure greatly improves the low-temperature performance of the lithium battery.

[0133] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A lithium battery positive electrode sheet, characterized in that, It includes a current collector and a multilayer positive electrode structure attached to the current collector; The positive electrode multilayer structure consists of a lithium manganese iron phosphate layer, a polycrystalline ternary material mixed lithium manganese iron phosphate layer, and a lithium manganese oxide layer from top to bottom, with the lithium manganese oxide layer attached to the surface of the current collector. The polycrystalline ternary material is lithium nickel cobalt manganese oxide; In the polycrystalline ternary material mixed with nano-manganese iron lithium phosphate layer, the mass ratio of polycrystalline ternary material to manganese iron lithium phosphate is 50:50-99:

2. The coating weight ratio of the lithium manganese iron phosphate layer, the polycrystalline ternary material mixed lithium manganese iron phosphate layer, and the lithium manganese oxide layer is (1-10):(1-10):

1.

2. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The current collector is aluminum foil.

3. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The median particle size D50 of the lithium manganese iron phosphate is 200-1500 nm.

4. The lithium battery positive electrode sheet as described in claim 3, characterized in that, The median particle size D50 of the lithium manganese iron phosphate is 500-1200 nm.

5. The lithium battery positive electrode sheet as described in claim 3, characterized in that, The median particle size D50 of the lithium manganese iron phosphate is 800-1000 nm.

6. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The median particle size D50 of the polycrystalline ternary material is 6-12 μm.

7. The lithium battery positive electrode sheet as described in claim 6, characterized in that, The median particle size D50 of the polycrystalline ternary material is 8-10 μm.

8. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The median particle size D50 of the lithium manganese oxide is 8-15 μm.

9. The lithium battery positive electrode sheet as described in claim 8, characterized in that, The median particle size D50 of the lithium manganese oxide is 10-13 μm.

10. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The chemical formula of the lithium nickel cobalt manganese oxide is LiNi x Co y Mn 1-x-y O2, where 0 < x < 1, 0 < y < 1, x + y < 1.

11. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The lithium manganese iron phosphate has an orthorhombic olivine-type crystal structure and the chemical formula LiMn. x Fe 1-x PO4, where 0.5 ≤ x < 1.

0.

12. The lithium battery positive electrode sheet as described in claim 11, characterized in that, 0.6≤x≤0.8。 13. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The lithium manganese oxide has a spinel-type structure.

14. The lithium battery positive electrode sheet as described in claim 1, characterized in that, In the polycrystalline ternary material mixed with nano-manganese iron lithium phosphate layer, the mass ratio of polycrystalline ternary material to manganese iron lithium phosphate is 70:30-80:

20.

15. The lithium battery positive electrode sheet as described in claim 1, characterized in that, The coating weight ratio of the lithium manganese iron phosphate layer, the polycrystalline ternary material mixed lithium manganese iron phosphate layer, and the lithium manganese oxide layer is (1-7):(2-8):

1.

16. A method for preparing a lithium battery positive electrode sheet according to any one of claims 1-15, characterized in that, Includes the following steps: Prepare lithium manganese iron phosphate slurry; prepare lithium manganese iron phosphate slurry mixed with polycrystalline ternary materials; prepare lithium manganese oxide slurry; Lithium manganese iron phosphate slurry, polycrystalline ternary material mixed with lithium manganese iron phosphate slurry and lithium manganese oxide slurry are sequentially coated on the surface of the current collector, and then dried, rolled and die-cut to obtain the positive electrode sheet of lithium battery.

17. The preparation method according to claim 16, characterized in that, The coating process involves applying multiple layers of slurry using an extrusion coating method.

18. A lithium battery, characterized in that, The lithium battery includes a positive electrode, a negative electrode, a separator, and an electrolyte; the positive electrode is the lithium battery positive electrode according to any one of claims 1-15 or the lithium battery positive electrode prepared by the preparation method according to claim 16 or 17.

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