A positive electrode material for improving cycle performance of a lithium battery and a preparation method thereof

Abstract

This invention discloses a cathode material and its preparation method for improving the cycle performance of lithium-ion batteries, relating to the technical field of lithium-ion battery cathode materials. The invention involves adding aluminum isopropoxide to anhydrous ethanol, ultrasonicating, stirring, adding an ethanol-water solution, stirring, adding silicon-coated cathode material, stirring, reacting, drying, grinding, and calcining under a nitrogen atmosphere to obtain the cathode material. The cathode material prepared by this invention exhibits a decreasing concentration of nickel and zirconium from the inside out, while a increasing concentration of cobalt and manganese from the inside out. This results in: firstly, reducing the nickel content on the surface of the cathode material, suppressing surface side reactions, and improving the electrochemical performance of the cathode material; secondly, using equivalent cation doping, such as zirconium doping in the cathode material, which helps reduce Li-. + / Ni 2+ Cation mixing enhances lattice oxygen stability, thereby improving the structural stability, rate performance, and cycle performance of the cathode material.

Description

Technical Field

[0001] This invention relates to the field of lithium battery cathode material technology, specifically a cathode material for improving the cycle performance of lithium batteries and its preparation method. Background Technology

[0002] As an indispensable raw material for lithium-ion batteries, cathode materials have a lower specific capacity compared to anode materials. Developing cathode materials with high energy density and high cycle performance is an effective way to solve this problem.

[0003] Among many cathode materials, high-nickel ternary cathode materials have excellent electrochemical performance. However, due to surface side reactions and cation mixing and microcrack generation during charging and discharging, their cycle performance decreases as the nickel content increases.

[0004] To address the aforementioned problems, this invention provides a cathode material and its preparation method for improving the cycle performance of lithium batteries. Summary of the Invention

[0005] The purpose of this invention is to provide a cathode material and its preparation method for improving the cycle performance of lithium batteries, so as to solve the problems raised in the prior art.

[0006] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a cathode material to improve the cycle performance of lithium batteries includes the following steps: Step 1: Take the precursor and lithium carbonate, grind, heat, and keep warm to obtain a concentration gradient cathode material; take the concentration gradient cathode material and add it to anhydrous ethanol, sonicate, add modified phenolic resin, mix evenly, dry, calcine, quench, stand, and dry to obtain a single-layer coated cathode material. Step 2: Take a single-layer coated positive electrode material, add it to a vinyltrimethoxysilane ethanol solution, stir, filter, and dry; add it to anhydrous ethanol, add tetraethyl orthosilicate and deionized water, adjust the pH value with ammonia, react, filter, wash, dry, and calcine under a nitrogen atmosphere to obtain silicon-coated positive electrode material. Step 3: Add aluminum isopropoxide to anhydrous ethanol, sonicate, stir, add ethanol aqueous solution, stir, add silicon-coated cathode material, stir, react, dry, grind, and calcine under nitrogen atmosphere to obtain cathode material.

[0007] A more optimized method for preparing the precursor is as follows: Take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C; Under a nitrogen atmosphere, pump metal salt solutions A, B, and C, along with sodium hydroxide and ammonia, into a reactor, react, age, filter, wash, and dry to obtain the precursor.

[0008] The optimal stoichiometric ratio of nickel, cobalt, manganese, and zirconium in metal salt solution A is (5.5-6.5):(1.5-2.5):(1.5-2.5):(0.05-0.07).

[0009] The optimal stoichiometric ratio of nickel, cobalt, manganese and zirconium in metal salt solution B is (4.5-5.5):(1.5-2.5):(2.5-3.5):(0.01-0.05).

[0010] The optimal stoichiometric ratio of nickel, cobalt, manganese, and zirconium in metal salt solution C is (3.5-4.5):(2-3):(3-4):0.

[0011] The optimized reaction parameters are: reaction temperature of 55-65℃, reaction pH of 10.45-10.55, and reaction time of 10-14h.

[0012] A more optimized stoichiometric ratio of precursor to lithium carbonate is 1:(1.0-1.2).

[0013] A more optimized method for preparing modified phenolic resin is as follows: add resorcinol and urea to formaldehyde, sonicate until completely dissolved, and react at 75-85℃ for 3-4 hours to obtain modified phenolic resin.

[0014] In a more optimized approach, the calcination temperature in step one is 695-705℃.

[0015] Compared with the prior art, the beneficial effects of the present invention are: 1. Preparation of Concentration Gradient Cathode Material: This invention employs a staged co-precipitation method, i.e., staged feeding, to prepare a precursor with uniform particle size and spherical shape. The prepared precursor is then calcined at high temperature to obtain a high-nickel ternary cathode material with a gradient distribution of metal elements, i.e., a cathode material with a concentration gradient structure. Compared with traditional ternary cathode materials prepared by feeding nickel, cobalt, and manganese metal salts in a fixed proportion, the cathode material prepared by this invention uses nickel, cobalt, manganese, and zirconium metal salts in different proportions in a staged feeding method. The concentration of nickel and zirconium in the prepared cathode material decreases from the inside to the outside, while the concentration of cobalt and manganese increases from the inside to the outside. Therefore: First, it reduces the nickel content on the surface of the cathode material, suppresses surface side reactions, and improves the electrochemical performance of the cathode material; Second, it uses equivalent cation doping to dope zirconium in the cathode material, which on the one hand helps to reduce Li + / Ni 2+The mixing of cations enhances the oxygen stability of the lattice, thereby improving the structural stability, rate performance, and cycle performance of the cathode material. On the other hand, zirconium is calcined with residual lithium salt at high temperature to generate lithium zirconate. As a fast ion conductor, lithium zirconate is beneficial to improving the rate performance and cycle performance of the cathode material.

[0016] 2. Preparation of a single-layer coated cathode material: Using urea as a nitrogen source, nitrogen-doped modified phenolic resin is prepared by reacting resorcinol and formaldehyde. Using the modified phenolic resin as a raw material, poly(phenylene oxide) coated cathode material is prepared by high-temperature pyrolysis. Its function is as follows: On the one hand, poly(phenylene oxide) is a conductive polymer that improves the electronic conductivity of the cathode material, and nitrogen doping is beneficial to further improve the electron transfer rate and electronic conductivity. On the other hand, the poly(phenylene oxide) layer has excellent flexibility, elasticity and chemical stability, which is beneficial to improving the cycle performance, electrochemical stability and structural stability of the cathode material.

[0017] 3. Preparation of cathode material: The cathode material is coated with a single layer and treated with vinyltrimethoxysilane. Tetraethyl orthosilicate is hydrolyzed to form silicon dioxide. The silicon dioxide layer is formed by the reaction of silanol groups with the silanol groups of vinyltrimethoxysilane. An aluminum isopropoxide layer is formed on the outside of the silicon dioxide layer by hydrolysis and calcination. The silicon dioxide layer and the polyphenylene layer are bonded by vinyltrimethoxysilane to improve the interfacial bonding force between the two. The prepared cathode material has better flexibility than alumina due to its silicon dioxide content, while alumina has stable chemical properties, which is beneficial to improving the cycle performance, electrochemical stability and structural stability of the cathode material. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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 skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Example 1: A method for preparing a cathode material to improve the cycle performance of lithium batteries; Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.4g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.05, 5:2:3:0.01, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.0, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 695℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0020] Example 2: A method for preparing a cathode material to improve the cycle performance of lithium batteries; Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0021] Example 3: A method for preparing a cathode material to improve the cycle performance of lithium batteries; Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.6g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.07, 5:2:3:0.02, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.2, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 705℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0022] Comparative Example 1: No urea was added; otherwise, refer to Example 2. Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0023] Comparative Example 2: Feeding at a fixed ratio, with the rest following the same procedure as in Example 2; Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratio of nickel:cobalt:manganese:zirconium = 5:2.15:2.85:0.025, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solution A, metal salt solution B, and metal salt solution C, with a concentration of 2 mol / L. S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0024] Comparative Example 3: The proportions were changed to graded feeding at different proportions, and the rest were the same as in Example 2; Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 8:2:2:0.2, 7:2:3:0.1, 6:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solution A, metal salt solution B, and metal salt solution C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0025] Comparative Example 4: Zirconium sulfate tetrahydrate was not added; otherwise, refer to Example 2. Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratio of nickel:cobalt:manganese = 6:2:2, 5:2:3, 4:2.5:3.5, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, and manganese sulfate monohydrate and add them to deionized water to prepare metal salt solution A, metal salt solution B, and metal salt solution C, with a concentration of 2mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0026] Comparative Example 5: No modified phenolic resin was added; all other aspects are the same as in Example 2. Step 1: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 2: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0027] Comparative Example 6: Vinyltrimethoxysilane was not added; otherwise, refer to Example 2. Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of anhydrous ethanol, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ for 4h under nitrogen atmosphere to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0028] Comparative Example 7: No tetraethyl orthosilicate was added; otherwise, refer to Example 2. Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add it to 150mL of anhydrous ethanol, add 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain silicon coated positive electrode material; S3: Add 0.2g of aluminum isopropoxide to 60mL of anhydrous ethanol, sonicate for 1h, stir for 0.5h at 80℃, add 50mL of 10% ethanol aqueous solution, stir for 4h, add 5g of silicon-coated cathode material, stir for 0.5h at 80℃, place in a reaction vessel, react for 20h at 80℃, dry, grind, and calcine at 500℃ for 4h under nitrogen atmosphere to obtain the cathode material.

[0029] Comparative Example 8: Aluminum isopropoxide was not added; otherwise, refer to Example 2. Step 1: Preparation of modified phenolic resin Add 5.5g of resorcinol and 1.5g of urea to 5mL of formaldehyde, sonicate until completely dissolved, and react at 80℃ for 4h to obtain modified phenolic resin. Step 2: Preparation of concentration gradient cathode materials S1: According to the stoichiometric ratios of nickel:cobalt:manganese:zirconium = 6:2:2:0.06, 5:2:3:0.015, 4:2.5:3.5:0, take nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate and add them to deionized water to prepare metal salt solutions A, B, and C, with a concentration of 2 mol / L; S2: Under a nitrogen atmosphere, metal salt solutions A, B, and C, along with 6 mol / L sodium hydroxide and 3 mol / L ammonia, were pumped into a reactor. The reaction was carried out at 60°C with the pH controlled at 10.5 for 12 hours, followed by aging for 10 hours. The mixture was then filtered, washed, and dried to obtain the precursor. S3: Take the precursor and lithium carbonate in a stoichiometric ratio of 1:1.1, grind them, heat them to 450℃ at a heating rate of 3℃ / min, hold them at 450℃ for 5h, heat them to 800℃ at a heating rate of 2℃ / min, hold them at 800℃ for 15h, and obtain the concentration gradient cathode material. Step 3: Preparation of cathode material S1: Take 5g of concentration gradient cathode material and add it to 50mL of anhydrous ethanol. Sonicate for 1h, add 0.1g of modified phenolic resin, mix evenly, dry at 80℃ for 6h, calcine at 700℃ for 10min, quench, stand, and dry at 80℃ for 6h to obtain a single-layer coated cathode material. S2: Take 5g of monolayer coated positive electrode material, add it to 100mL of 5% vinyltrimethoxysilane ethanol solution, stir for 1h, filter, and dry; add 150mL of anhydrous ethanol, add 0.5mL of tetraethyl orthosilicate and 0.15mL of deionized water, adjust the pH to 8.0 with 0.1mol / L ammonia water, react at 40℃ for 4h, filter, wash, dry, calcine at 500℃ under nitrogen atmosphere for 4h to obtain silicon coated positive electrode material; S3: Take 60 mL of anhydrous ethanol, sonicate for 1 h, stir for 0.5 h at 80 °C, add 50 mL of 10% ethanol aqueous solution, stir for 4 h, add 5 g of silicon-coated positive electrode material, stir for 0.5 h at 80 °C, place in a reaction vessel, react for 20 h at 80 °C, dry, grind, and calcine for 4 h at 500 °C under a nitrogen atmosphere to obtain the positive electrode material.

[0030] experiment: The cathode materials prepared in Examples 1-3 and Comparative Examples 1-8 were tested. The positive electrode materials prepared in Examples 1-3 and Comparative Examples 1-8, acetylene black, and polyvinylidene fluoride were mixed evenly according to a mass ratio of 8:1:1. N-methylpyrrolidone was added, and the mixture was evenly coated onto aluminum foil. The mixture was dried at 100°C for 12 hours to obtain a positive electrode sheet as the positive electrode. A 1 mol / L lithium hexafluorophosphate solution was prepared by mixing ethyl carbonate and dimethyl carbonate according to a volume ratio of 1:1 as the electrolyte. A lithium metal sheet was used as the negative electrode, and a Celgard 2400 membrane was used as the separator. The mixture was assembled to obtain a CR2032 coin cell. A BTS-5V / 10mA charge-discharge tester was used, with the voltage window set to 2.8-4.3V and the temperature set to 25℃. A constant current charge-discharge test was performed at a rate of 0.5C, and the discharge specific capacity and capacity retention rate after 200 cycles were calculated. The above experiments were completed, and the results are shown in Table 1 below: Table 1

[0031] Conclusion: Based on the analysis of the above experimental data, the batteries assembled with the positive electrode sheets prepared in Examples 1-3 of this invention have higher discharge specific capacity and capacity retention rate, and excellent electrochemical performance; while the batteries assembled with the positive electrode sheets prepared in Examples 1-8 have lower discharge specific capacity and capacity retention rate, and poorer electrochemical performance.

[0032] Comparative analysis of Comparative Example 1 (without urea) and Example 2 shows that nitrogen-doped modified phenolic resin was prepared by reacting resorcinol and formaldehyde with urea as the nitrogen source. Polyphenylene oxide-coated cathode material was prepared by high-temperature pyrolysis using the modified phenolic resin as the raw material. Polyphenylene oxide, as a conductive polymer, improves the electronic conductivity of the cathode material, while nitrogen doping is beneficial to further improve the electron transfer rate and electronic conductivity.

[0033] Comparative analysis of Comparative Example 2 and Example 2, which uses a fixed feed ratio, reveals that, compared to the traditional ternary cathode material prepared by using a fixed feed ratio of corresponding metal salts of nickel, cobalt, and manganese, the cathode material prepared by this invention uses a graded feed ratio of corresponding metal salts of nickel, cobalt, manganese, and zirconium. The resulting cathode material exhibits a decreasing concentration of nickel and zirconium from the inside out, while the concentration of cobalt and manganese increases from the inside out. Therefore: First, it reduces the nickel content on the surface of the cathode material, suppresses surface side reactions, and improves the electrochemical performance of the cathode material; Second, it uses equivalent cation doping to dope zirconium in the cathode material, which, on the one hand, helps reduce Li... + / Ni 2+ The mixing of cations enhances the oxygen stability of the lattice, thereby improving the structural stability, rate performance, and cycle performance of the cathode material. On the other hand, zirconium is calcined with residual lithium salt at high temperature to generate lithium zirconate. As a fast ion conductor, lithium zirconate is beneficial to improving the rate performance and cycle performance of the cathode material.

[0034] By comparing Comparative Example 3, which uses different proportions of graded feeding, with Example 2, it can be found that, under a specific proportion, doping zirconium into the cathode material is beneficial for stable operation under charge balance.

[0035] Comparative analysis of Comparative Example 4 (without zirconium sulfate tetrahydrate) and Example 2 shows that the doping of zirconium cannot suppress cation mixing, nor can it form lithium zirconate to further improve rate performance and cycle performance.

[0036] Comparative analysis of Comparative Example 5 (without modified phenolic resin) and Example 2 shows that the polyphenylene layer has excellent flexibility, elasticity and chemical stability, which is beneficial to improving the cycle performance, electrochemical stability and structural stability of the cathode material.

[0037] Comparative analysis of Comparative Example 6 (without vinyltrimethoxysilane) and Example 2 shows that when the monolayer coated positive electrode material is treated with vinyltrimethoxysilane, tetraethyl orthosilicate undergoes hydrolysis to form silicon dioxide. The silicon dioxide layer is formed by the reaction of silanol groups with the silanol groups of vinyltrimethoxysilane. The silicon dioxide layer and the polyphenylene layer are bonded by vinyltrimethoxysilane, which improves the interfacial bonding between the two.

[0038] Comparative analysis of Comparative Example 7 (without tetraethyl orthosilicate) and Example 2 shows that no silicon dioxide layer can be formed. Silicon dioxide has better flexibility than alumina, which is beneficial to improving the electrochemical stability and structural stability of the cathode material.

[0039] Comparative analysis of Comparative Example 8 (without aluminum isopropoxide) and Example 2 shows that aluminum isopropoxide hydrolyzes and calcines to form an aluminum oxide layer on the outside of the silicon dioxide layer. The aluminum oxide has stable chemical properties, which is beneficial to improving the cycle performance, electrochemical stability and structural stability of the cathode material.

[0040] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

Claims

1. A method for preparing a cathode material for improving the cycle performance of a lithium battery, characterized in that: Includes the following steps: ​ Step 1: Take the precursor and lithium carbonate, grind, heat, and keep warm to obtain a concentration gradient cathode material; take the concentration gradient cathode material and add it to anhydrous ethanol, sonicate, add modified phenolic resin, mix evenly, dry, calcine, quench, stand, and dry to obtain a single-layer coated cathode material. Step 2: Take a single-layer coated positive electrode material, add it to a vinyltrimethoxysilane ethanol solution, stir, filter, and dry; add it to anhydrous ethanol, add tetraethyl orthosilicate and deionized water, adjust the pH value with ammonia, react, filter, wash, dry, and calcine under a nitrogen atmosphere to obtain silicon-coated positive electrode material. Step 3: Add aluminum isopropoxide to anhydrous ethanol, sonicate, stir, add ethanol aqueous solution, stir, add silicon-coated cathode material, stir, react, dry, grind, and calcine under nitrogen atmosphere to obtain cathode material.

2. The method for preparing a cathode material for improving the cycle performance of a lithium battery according to claim 1, characterized in that: The precursor is prepared as follows: nickel sulfate hexahydrate, cobalt sulfate heptahydrate, manganese sulfate monohydrate, and zirconium sulfate tetrahydrate are added to deionized water to prepare metal salt solutions A, B, and C; under a nitrogen atmosphere, metal salt solutions A, B, and C are pumped into a reactor with sodium hydroxide and ammonia, reacted, aged, filtered, washed, and dried to obtain the precursor.

3. The method for preparing a positive electrode material to improve the cycle performance of a lithium battery according to claim 2, characterized in that: The stoichiometric ratio of nickel, cobalt, manganese and zirconium in the metal salt solution A is (5.5-6.5):(1.5-2.5):(1.5-2.5):(0.05-0.07). 4.The method for preparing a cathode material for improving the cycle performance of a lithium battery of claim 2, characterized by: The stoichiometric ratio of nickel, cobalt, manganese and zirconium in the metal salt solution B is (4.5-5.5):(1.5-2.5):(2.5-3.5):(0.01-0.05).

5. The method for preparing a positive electrode material to improve the cycle performance of a lithium battery according to claim 2, characterized in that: The stoichiometric ratio of nickel, cobalt, manganese and zirconium in the metal salt solution C is (3.5-4.5):(2-3):(3-4):

0.

6. The method of claim 2, wherein the method further comprises: adding a lithium source to the mixture of the lithium transition metal oxide and the lithium source to form a mixture; and heating the mixture to form the lithium transition metal oxide. The reaction parameters are as follows: reaction temperature is 55-65℃, reaction pH is controlled at 10.45-10.55, and reaction time is 10-14h.

7. The method of claim 1, wherein the method further comprises: adding a lithium source to the mixture of the lithium transition metal oxide and the lithium source to form a mixture; and heating the mixture to form the lithium transition metal oxide. The stoichiometric ratio of the precursor to lithium carbonate is 1:(1.0-1.2). 8.The method for preparing a cathode material for improving cycle performance of a lithium battery of claim 1, characterized by: The modified phenolic resin is prepared by adding resorcinol and urea to formaldehyde, sonicating until completely dissolved, and reacting at 75-85℃ for 3-4 hours to obtain the modified phenolic resin.

9. The method for preparing a positive electrode material to improve the cycle performance of a lithium battery according to claim 1, characterized in that: In step one, the calcination temperature is 695-705℃.

10. A cathode material with improved cycle performance of a lithium battery prepared by a method for preparing a cathode material with improved cycle performance according to any one of claims 1-9.

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