A core-shell structure lithium iron phosphate positive electrode material and a preparation method thereof

Abstract

This invention relates to the field of lithium iron phosphate cathode materials, specifically to a core-shell structured lithium iron phosphate cathode material and its preparation method, which addresses the problems of low specific capacity, poor rate capability, and poor cycle stability in lithium iron phosphate cathode materials. The method involves using lithium cobalt phosphate as the main body in the shell phase, modified by nickel and magnesium doping, to form a core-shell structure with the core phase lithium iron phosphate. Carbon nanotubes and titanium dioxide form a composite coating layer, tightly adhering to the surface of the core-shell structure particles, thus obtaining the core-shell structured lithium iron phosphate cathode material. The core-shell structured lithium iron phosphate cathode material prepared by this method improves the operating voltage and energy density, enhances electronic conductivity and rate capability, and exhibits long cycle life and high safety.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode materials, specifically to a core-shell structured lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium iron phosphate (LFP) has become a key choice for cathode materials in lithium-ion batteries due to its high theoretical specific capacity, excellent safety, low cost, and environmentally friendly properties, and is widely used in new energy vehicles, energy storage devices, and other fields. However, its extremely low electronic conductivity and small lithium-ion diffusion coefficient cannot be ignored, resulting in poor rate performance. Furthermore, during long-term charge-discharge cycles, active particles are prone to agglomeration and structural collapse, leading to a significant reduction in cycle stability, which greatly limits its development in high-power, long-life applications. Summary of the Invention

[0003] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a core-shell structured lithium iron phosphate cathode material and its preparation method.

[0004] The objective of this invention can be achieved through the following technical solutions: In a first aspect, this application provides a method for preparing a core-shell structured lithium iron phosphate cathode material, comprising the following steps: Step 1: Add lithium nitrate, ferrous acetate, ammonium dihydrogen phosphate, ethylene glycol mixed solvent and citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Adjust the pH to 4-5 with ammonia water, introduce nitrogen gas for protection, and stir the reaction at 55-60℃ and 200-300 r / min for 3-4 hours to obtain the nucleus phase precursor sol. Step 2: Add lithium nitrate, cobalt acetate, nickel nitrate, magnesium nitrate, ammonium dihydrogen phosphate, deionized water and citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Purge with nitrogen for protection and stir the reaction at 45-50℃ and 200-300 r / min for 1-2 hours to obtain the shell-phase precursor solution. Step 3: Place the core-phase precursor sol in an ultrasonic disperser and ultrasonically disperse it at a temperature of 45-50℃ and a power of 350-400W. Add the shell-phase precursor solution dropwise at a rate of 1.5mL / min. After the addition is complete, continue ultrasonic dispersion for 2-3 hours. Then place it in a vacuum drying oven and dry it at a temperature of 65-70℃ for 18-20 hours. After that, place it in a tube furnace for pre-calcination. In a nitrogen atmosphere, heat the temperature to 350℃ at a rate of 3℃ / min and hold for 3 hours. Grind it through a 200-mesh sieve. Then heat the temperature to 650℃ at a rate of 5℃ / min and hold for 2 hours. Then heat the temperature to 750℃ and hold for 5 hours. After that, cool it naturally to room temperature and grind it through a 300-mesh sieve to obtain the core-shell structure powder. Step 4: Add carbon nanotubes, concentrated nitric acid, and concentrated sulfuric acid to a three-necked flask equipped with a stirrer, thermometer, and reflux condenser. Stir the reaction at 75-80℃ and a stirring rate of 250-300 r / min for 6-7 hours. After the reaction is complete, cool the reaction product to room temperature, then filter it. Wash the filter cake with deionized water 3-5 times, and then place it in a vacuum drying oven at 55-60℃ for 12-14 hours to obtain acidified carbon nanotubes. Step 5: Add acidified carbon nanotubes, anhydrous ethanol, and deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant pressure dropping funnel. Stir the reaction at a stirring rate of 250-300 r / min for 10-15 min. Then, ultrasonically disperse the mixture at a power of 350-400 W for 1-2 h. While stirring, add tetrabutyl titanate dropwise at a rate of 1-2 drops / s. Simultaneously, adjust the pH of the system to 3-4 with nitric acid. After the addition is complete, heat the mixture to 45-50℃ and continue stirring for 30-35 min to obtain a carbon nanotube-titanium dioxide composite sol. Step 6: Add the core-shell structure powder to deionized water and ultrasonically disperse it for 30-35 minutes at a power of 250-300W to obtain a suspension; Step 7: Add the carbon nanotube-titanium dioxide composite sol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under the conditions of 45-50℃ and stirring rate of 250-300 r / min, add the suspension dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, continue stirring for 2-3 hours. Then, vacuum filter, collect the solid product, wash with deionized water 3-5 times, and then place it in a vacuum drying oven and dry at 75-80℃ for 12-14 hours to obtain the precursor of carbon nanotube-titanium dioxide coated core-shell material. Step 8: Place the carbon nanotube-titanium dioxide coated core-shell material precursor in a tube furnace, heat it to 300℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, hold it at that temperature for 2h, then heat it to 500℃ at a heating rate of 5℃ / min, hold it at that temperature for 3h, then let it cool naturally to room temperature, grind it through a 300-mesh sieve to obtain the core-shell structured lithium iron phosphate cathode material.

[0005] In a preferred embodiment of the present invention, the ratio of lithium nitrate, ferrous acetate, ammonium dihydrogen phosphate, ethylene glycol mixed solvent and citric acid in step one is 1.2-1.5g: 3.5-4.0g: 2.5-2.8g: 40-60mL: 4.0-5.0g.

[0006] In a preferred embodiment of the present invention, the ethylene glycol mixed solvent in step one is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:2; the mass fraction of ammonia water is 25%.

[0007] In a preferred embodiment of the present invention, the ratio of lithium nitrate, cobalt acetate, nickel nitrate, magnesium nitrate, ammonium dihydrogen phosphate, deionized water, and citric acid in step two is 0.25-0.3g: 0.6-0.7g: 1.0-1.2g: 0.4-0.5g: 0.5-0.6g: 20-30mL: 0.8-1.0g.

[0008] In a preferred embodiment of the present invention, the ratio of the amount of nucleus-phase precursor sol to shell-phase precursor solution in step three is 10-12 mL: 2-3 mL.

[0009] In a preferred embodiment of the present invention, the ratio of carbon nanotubes, concentrated nitric acid and concentrated sulfuric acid in step four is 1-2g: 10-15mL: 30-45mL.

[0010] In a preferred embodiment of the present invention, the carbon nanotubes in step four have a diameter of 10-22 nm and a length of 5-10 μm; the mass fraction of concentrated nitric acid is 65%; and the mass fraction of concentrated sulfuric acid is 98%.

[0011] In a preferred embodiment of the present invention, the ratio of acidified carbon nanotubes, anhydrous ethanol, deionized water and tetrabutyl titanate in step five is 0.1-0.15g: 30-40mL: 10-15mL: 1.5-2.0mL.

[0012] In a preferred embodiment of the present invention, the mass fraction of nitric acid in step five is 5%.

[0013] In a preferred embodiment of the present invention, the ratio of core-shell structure powder to deionized water in step six is ​​5-6g: 100-180mL.

[0014] In a preferred embodiment of the present invention, the ratio of carbon nanotube-titanium dioxide composite sol to suspension in step seven is 1.3-1.5 mL: 4-5 mL.

[0015] Secondly, this application provides a core-shell structured lithium iron phosphate cathode material, which is prepared according to the preparation method of core-shell structured lithium iron phosphate cathode material.

[0016] The beneficial effects of this invention are: This invention discloses a core-shell structured lithium iron phosphate cathode material and its preparation method. The shell phase is primarily composed of lithium cobalt phosphate, modified by nickel and magnesium doping to form a core-shell structure with the core phase lithium iron phosphate. Carbon nanotubes and titanium dioxide form a composite coating layer, tightly adhering to the surface of the core-shell structure particles, thus obtaining the core-shell structured lithium iron phosphate cathode material. The core-shell structured lithium iron phosphate cathode material prepared by this method improves the operating voltage and energy density, enhances electronic conductivity and rate performance, and exhibits long cycle life and high safety.

[0017] The corresponding voltage of lithium cobalt phosphate is much higher than that of lithium iron phosphate. In the core-shell structure, the high voltage characteristics of the shell phase can significantly improve the overall operating voltage of the cathode material. Combined with the high capacity of the core phase, this ultimately achieves a leap in energy density. The olivine structure of lithium cobalt phosphate is a perfect match for the core phase of lithium iron phosphate, which can tightly encapsulate the core phase particles, reduce the area of ​​the core phase directly exposed to the electrolyte, and suppress Fe in the core phase. 2+ The dissolution of the cathode material simultaneously alleviates interfacial stress during charging and discharging, thereby increasing the working voltage and energy density of the cathode material.

[0018] Ni 2+ / Ni 3+ ionic radius and Co 2+ / Co 3+ Approaching, it can replace some Co sites in the lithium cobalt phosphate lattice, fine-tuning the lattice parameters, and improving Li + The diffusion coefficient in the shell phase improves high-rate performance; the redox potential of Ni is slightly higher than that of Co, which can suppress Co during charging and discharging. 3+ Excessive oxidation, while reducing Co 3+ Dissolution in the electrolyte extends cycle life.

[0019] Mg 2+ With its small ionic radius and stable valence, Mg, after doping, occupies cation vacancies or some Co sites in the lithium cobalt phosphate lattice, forming "fixed support points" that suppress lattice distortion during charging and discharging, thus improving the structural stability of the shell phase; 2+ Its electronegativity is higher than that of Co. 3+This can enhance the binding energy of O atoms in the crystal lattice, reduce the risk of O2 release under high voltage, and at the same time, Mg 2+ It can block the migration of transition metal ions into the electrolyte, prevent them from depositing on the negative electrode surface, and further improve cycle stability.

[0020] Both lithium iron phosphate and lithium cobalt phosphate have extremely low electronic conductivity. Carbon nanotubes, on the other hand, have excellent one-dimensional electronic conductivity and high specific surface area. After coating, they can form a continuous electron transport network on the surface of core-shell particles, which can significantly reduce the electronic impedance of the electrode, solve the electronic transport bottleneck, and significantly improve the high-rate discharge performance of the material. The flexible one-dimensional structure of carbon nanotubes can form a "skeleton support" during the electrode preparation process, which can alleviate the electrode cracking caused by the volume change of the particles during charging and discharging and improve the mechanical stability of the electrode.

[0021] Titanium dioxide is a chemically inert oxide that can form a dense "protective film" on the surface of core-shell particles, blocking direct contact between the electrolyte and the core-shell material and preventing the Fe atoms in the core-phase lithium iron phosphate from being released. 2+ Dissolved and shell-phase cobalt phosphate Co 3+ Dissolution reduces capacity decay caused by the loss of transition metal ions; titanium dioxide is anatase, and its lattice contains many vacancies, which can be used as Li... + The "temporary storage site" assists Li + The transport within the coating layer reduces ion impedance and further optimizes rate performance; titanium dioxide exhibits extremely high chemical stability within the battery charge / discharge voltage range of 3.0-4.8V, and will not undergo redox reactions, thus avoiding side reactions between the coating layer and the electrolyte. Detailed Implementation

[0022] The technical solutions 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.

[0023] Example 1: This embodiment describes a method for preparing a core-shell structured lithium iron phosphate cathode material, including the following steps: Step s1: Add 1.2g lithium nitrate, 3.5g ferrous acetate, 2.5g ammonium dihydrogen phosphate, 40mL ethylene glycol mixed solvent (ethylene glycol mixed solvent is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:2) and 4.0g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Adjust the pH to 4 with 25% ammonia water, purge with nitrogen for protection, and stir the reaction at 55℃ and 200r / min for 3h to obtain the nucleus phase precursor sol. Step s2: Add 0.25g lithium nitrate, 0.6g cobalt acetate, 1.0g nickel nitrate, 0.4g magnesium nitrate, 0.5g ammonium dihydrogen phosphate, 20mL deionized water and 0.8g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Purge with nitrogen for protection and stir the reaction at 45℃ and 200r / min for 1h to obtain the shell-phase precursor solution. Step s3: Place 10 mL of core-phase precursor sol in an ultrasonic disperser and ultrasonically disperse it at 45℃ and 350W. Add 2 mL of shell-phase precursor solution dropwise at a rate of 1.5 mL / min. After the addition is complete, continue ultrasonic dispersion for 2 h. Then place it in a vacuum drying oven and dry it at 65℃ for 18 h. Then place it in a tube furnace for pre-calcination. In a nitrogen atmosphere, heat it to 350℃ at a heating rate of 3℃ / min and hold it for 3 h. Grind it through a 200-mesh sieve. Then heat it to 650℃ at a heating rate of 5℃ / min and hold it for 2 h. Then heat it to 750℃ and hold it for 5 h. Then cool it naturally to room temperature and grind it through a 300-mesh sieve to obtain core-shell structure powder. Step s4: 1g of carbon nanotubes (10nm in diameter and 5μm in length), 10mL of 65% concentrated nitric acid and 30mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 75℃ and 250r / min for 6h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed three times with deionized water and then placed in a vacuum drying oven and dried at 55℃ for 12h to obtain acidified carbon nanotubes. Step s5: Add 0.1g of acidified carbon nanotubes, 30mL of anhydrous ethanol and 10mL of deionized water to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Stir the reaction at a stirring rate of 250r / min for 10min. Then, ultrasonically disperse the mixture at a power of 350W for 1h. Then, add 1.5mL of tetrabutyl titanate dropwise while stirring, controlling the dropping rate to 1 drop / s. At the same time, adjust the pH of the system to 3 with 5% nitric acid. After the addition is complete, heat the mixture to 45℃ and continue stirring for 30min to obtain carbon nanotube-titanium dioxide composite sol. Step s6: Add 5g of core-shell structure powder to 100mL of deionized water and ultrasonically disperse for 30min at a power of 250W to obtain a suspension; Step s7: Add 1.3 mL of carbon nanotube-titanium dioxide composite sol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under the conditions of 45℃ and stirring rate of 250 r / min, add 4 mL of suspension dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, continue stirring for 2 h. Then, vacuum filter, collect the solid product, wash it 3 times with deionized water, and then place it in a vacuum drying oven and dry it at 75℃ for 12 h to obtain the precursor of carbon nanotube-titanium dioxide coated core-shell material. Step s8: Place 3g of carbon nanotube-titanium dioxide coated core-shell material precursor in a tube furnace, heat to 300℃ at a heating rate of 2℃ / min under nitrogen atmosphere, hold for 2h, then heat to 500℃ at a heating rate of 5℃ / min, hold for 3h, then cool naturally to room temperature, grind through a 300-mesh sieve to obtain core-shell structured lithium iron phosphate cathode material.

[0024] Example 2: This embodiment describes a method for preparing a core-shell structured lithium iron phosphate cathode material, including the following steps: Step s1: Add 1.3g lithium nitrate, 3.7g ferrous acetate, 2.6g ammonium dihydrogen phosphate, 50mL ethylene glycol mixed solvent (ethylene glycol mixed solvent is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:2) and 4.5g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Adjust the pH to 4 with 25% ammonia water, purge with nitrogen for protection, and stir the reaction at 57℃ and a stirring rate of 250r / min for 3-4h to obtain the nucleus phase precursor sol. Step s2: Add 0.27g lithium nitrate, 0.65g cobalt acetate, 1.1g nickel nitrate, 0.45g magnesium nitrate, 0.55g ammonium dihydrogen phosphate, 25mL deionized water and 0.9g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Purge with nitrogen for protection and stir the reaction at 47℃ and 250r / min for 1.5h to obtain the shell-phase precursor solution. Step s3: Place 11 mL of core-phase precursor sol in an ultrasonic disperser and ultrasonically disperse it at 47℃ and 370W. Add 2.5 mL of shell-phase precursor solution dropwise at a rate of 1.5 mL / min. After the addition is complete, continue ultrasonic dispersion for 2.5 h. Then place it in a vacuum drying oven and dry it at 67℃ for 19 h. Then place it in a tube furnace for pre-calcination. In a nitrogen atmosphere, heat it to 350℃ at a heating rate of 3℃ / min and hold it for 3 h. Grind it through a 200-mesh sieve. Then heat it to 650℃ at a heating rate of 5℃ / min and hold it for 2 h. Then heat it to 750℃ and hold it for 5 h. Then cool it naturally to room temperature and grind it through a 300-mesh sieve to obtain core-shell structure powder. Step s4: 1.5g of carbon nanotubes (18nm in diameter and 8μm in length), 13mL of 65% concentrated nitric acid and 37mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 77℃ and 270r / min for 6.5h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed four times with deionized water and then placed in a vacuum drying oven and dried at 57℃ for 13h to obtain acidified carbon nanotubes. Step s5: Add 0.13g of acidified carbon nanotubes, 35mL of anhydrous ethanol and 13mL of deionized water to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Stir the reaction at a stirring rate of 270r / min for 13min. Then, ultrasonically disperse the mixture at a power of 370W for 1.5h. Then, add 1.7mL of tetrabutyl titanate dropwise while stirring, controlling the dropping rate to 1 drop / s. At the same time, adjust the pH of the system to 3 with 5% nitric acid. After the addition is complete, heat the mixture to 47℃ and continue stirring for 33min to obtain carbon nanotube-titanium dioxide composite sol. Step s6: Add 5.5g of core-shell structure powder to 140mL of deionized water and ultrasonically disperse for 32min at a power of 270W to obtain a suspension; Step s7: Add 1.4 mL of carbon nanotube-titanium dioxide composite sol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under the conditions of 47℃ and stirring rate of 270 r / min, add 4.5 mL of suspension dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, continue stirring for 2.5 h. Then, vacuum filter, collect the solid product, wash it 4 times with deionized water, and then place it in a vacuum drying oven and dry it at 77℃ for 13 h to obtain the precursor of carbon nanotube-titanium dioxide coated core-shell material. Step s8: Place 4g of carbon nanotube-titanium dioxide coated core-shell material precursor in a tube furnace, heat to 300℃ at a heating rate of 2℃ / min under nitrogen atmosphere, hold for 2h, then heat to 500℃ at a heating rate of 5℃ / min, hold for 3h, then cool naturally to room temperature, grind through a 300-mesh sieve to obtain core-shell structured lithium iron phosphate cathode material.

[0025] Example 3: This embodiment describes a method for preparing a core-shell structured lithium iron phosphate cathode material, including the following steps: Step s1: Add 1.5g lithium nitrate, 4.0g ferrous acetate, 2.8g ammonium dihydrogen phosphate, 60mL ethylene glycol mixed solvent (ethylene glycol mixed solvent is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:2) and 5.0g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Adjust the pH to 5 with 25% ammonia water, purge with nitrogen for protection, and stir the reaction at 60℃ and 300r / min for 4h to obtain the nucleus phase precursor sol. Step s2: Add 0.3g lithium nitrate, 0.7g cobalt acetate, 1.2g nickel nitrate, 0.5g magnesium nitrate, 0.6g ammonium dihydrogen phosphate, 30mL deionized water and 1.0g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Purge with nitrogen for protection and stir the reaction at 50℃ and 300r / min for 2h to obtain a shell-phase precursor solution. Step s3: Place 12 mL of core-phase precursor sol in an ultrasonic disperser and ultrasonically disperse it at 50 °C and 400 W. Add 3 mL of shell-phase precursor solution dropwise at a rate of 1.5 mL / min. After the addition is complete, continue ultrasonic dispersion for 3 h. Then place it in a vacuum drying oven and dry it at 70 °C for 20 h. Then place it in a tube furnace for pre-calcination. In a nitrogen atmosphere, heat it to 350 °C at a heating rate of 3 °C / min and hold it for 3 h. Grind it through a 200-mesh sieve. Then heat it to 650 °C at a heating rate of 5 °C / min and hold it for 2 h. Then heat it to 750 °C and hold it for 5 h. Then cool it naturally to room temperature and grind it through a 300-mesh sieve to obtain core-shell structure powder. Step s4: 2g of carbon nanotubes (22nm in diameter and 10μm in length), 15mL of 65% concentrated nitric acid and 45mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 80℃ and 300r / min for 7h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed 5 times with deionized water and then placed in a vacuum drying oven and dried at 60℃ for 14h to obtain acidified carbon nanotubes. Step s5: Add 0.15g of acidified carbon nanotubes, 40mL of anhydrous ethanol and 15mL of deionized water to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Stir the reaction at a stirring rate of 300r / min for 15min. Then, ultrasonically disperse the mixture at a power of 400W for 2h. Then, add 2.0mL of tetrabutyl titanate dropwise while stirring, controlling the dropping rate to 2 drops / s. At the same time, adjust the pH of the system to 4 with 5% nitric acid. After the addition is complete, heat the mixture to 50℃ and continue stirring for 35min to obtain carbon nanotube-titanium dioxide composite sol. Step s6: Add 6g of core-shell structure powder to 180mL of deionized water and ultrasonically disperse for 35min at a power of 300W to obtain a suspension; Step s7: Add 1.5 mL of carbon nanotube-titanium dioxide composite sol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under the conditions of 50℃ and stirring rate of 300 r / min, add 5 mL of suspension dropwise while stirring, controlling the dropping rate to 2 drops / s. After the addition is complete, continue stirring for 3 h. Then, vacuum filter, collect the solid product, wash it 5 times with deionized water, and then place it in a vacuum drying oven and dry it at 80℃ for 14 h to obtain the precursor of carbon nanotube-titanium dioxide coated core-shell material. Step s8: Place 5g of carbon nanotube-titanium dioxide coated core-shell material precursor in a tube furnace, heat to 300℃ at a heating rate of 2℃ / min under nitrogen atmosphere, hold for 2h, then heat to 500℃ at a heating rate of 5℃ / min, hold for 3h, then cool naturally to room temperature, grind through a 300-mesh sieve to obtain core-shell structured lithium iron phosphate cathode material.

[0026] Comparative Example 1: This comparative example illustrates a method for preparing a core-shell structured cathode material, comprising the following steps: Step s1: Add 1.5g lithium nitrate, 4.0g ferrous acetate, 2.8g ammonium dihydrogen phosphate, 60mL ethylene glycol mixed solvent (ethylene glycol mixed solvent is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:2) and 5.0g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Adjust the pH to 5 with 25% ammonia water, purge with nitrogen for protection, and stir the reaction at 60℃ and 300r / min for 4h to obtain the nucleus phase precursor sol. Step s2: Add 0.3g lithium nitrate, 0.7g cobalt acetate, 1.2g nickel nitrate, 0.5g magnesium nitrate, 0.6g ammonium dihydrogen phosphate, 30mL deionized water and 1.0g citric acid to a three-necked flask equipped with a stirrer, thermometer and gas delivery tube. Purge with nitrogen for protection and stir the reaction at 50℃ and 300r / min for 2h to obtain a shell-phase precursor solution. Step s3: Place 12 mL of core-phase precursor sol in an ultrasonic disperser and ultrasonically disperse it at 50 °C and 400 W. Add 3 mL of shell-phase precursor solution dropwise at a rate of 1.5 mL / min. After the addition is complete, continue ultrasonic dispersion for 3 h. Then place it in a vacuum drying oven and dry it at 70 °C for 20 h. After that, place it in a tube furnace for pre-calcination. In a nitrogen atmosphere, heat it to 350 °C at a heating rate of 3 °C / min and hold it for 3 h. Grind it through a 200-mesh sieve. Then heat it to 650 °C at a heating rate of 5 °C / min and hold it for 2 h. Then heat it to 750 °C and hold it for 5 h. After that, cool it naturally to room temperature and grind it through a 300-mesh sieve to obtain the core-shell structure cathode material.

[0027] Comparative Example 2: This comparative example illustrates a method for preparing a coated lithium iron phosphate cathode material, comprising the following steps: Step s1: Add 1.5g lithium nitrate, 4.0g ferrous acetate, 2.8g ammonium dihydrogen phosphate, 60mL ethylene glycol mixed solvent (ethylene glycol and deionized water mixed in a volume ratio of 1:2), and 5.0g citric acid to a three-necked flask equipped with a stirrer, thermometer, and gas delivery tube. Adjust the pH to 5 with 25% ammonia solution, purge with nitrogen for protection, and stir at 60℃ and a stirring speed of 300r / min. The mixture was stirred and reacted for 4 hours, then placed in a vacuum drying oven and dried at 70°C for 20 hours. After that, it was placed in a tube furnace for pre-calcination. Under a nitrogen atmosphere, the temperature was increased to 350°C at a rate of 3°C / min and held for 3 hours. The mixture was then ground through a 200-mesh sieve. The temperature was then increased to 650°C at a rate of 5°C / min and held for 2 hours. The temperature was then increased to 750°C and held for 5 hours. After that, the mixture was allowed to cool naturally to room temperature and ground through a 300-mesh sieve to obtain lithium iron phosphate powder. Step s2: 2g of carbon nanotubes (22nm in diameter and 10μm in length), 15mL of 65% concentrated nitric acid and 45mL of 98% concentrated sulfuric acid were added to a three-necked flask equipped with a stirrer, thermometer and reflux condenser. The mixture was stirred at 80℃ and 300r / min for 7h. After the reaction was completed, the reaction product was cooled to room temperature and then filtered. The filter cake was washed 5 times with deionized water and then placed in a vacuum drying oven and dried at 60℃ for 14h to obtain acidified carbon nanotubes. Step s3: Add 0.15g of acidified carbon nanotubes, 40mL of anhydrous ethanol and 15mL of deionized water to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Stir the reaction at a stirring rate of 300r / min for 15min. Then, ultrasonically disperse the mixture at a power of 400W for 2h. Then, add 2.0mL of tetrabutyl titanate dropwise while stirring, controlling the dropping rate to 2 drops / s. At the same time, adjust the pH of the system to 4 with 5% nitric acid. After the addition is complete, heat the mixture to 50℃ and continue stirring for 35min to obtain carbon nanotube-titanium dioxide composite sol. Step s4: Add 6g of lithium iron phosphate powder to 180mL of deionized water and ultrasonically disperse for 35min at a power of 300W to obtain a suspension; Step s5: Add 1.5 mL of carbon nanotube-titanium dioxide composite sol to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under the conditions of 50℃ and stirring rate of 300 r / min, add 5 mL of suspension dropwise while stirring, controlling the dropping rate to 2 drops / s. After the addition is complete, continue stirring for 3 h. Then, vacuum filter, collect the solid product, wash it 5 times with deionized water, and then place it in a vacuum drying oven and dry it at 80℃ for 14 h to obtain carbon nanotube-titanium dioxide coated lithium iron phosphate powder precursor. Step s6: Place 5g of carbon nanotube-titanium dioxide coated lithium iron phosphate powder precursor in a tube furnace, heat it to 300℃ at a heating rate of 2℃ / min under a nitrogen atmosphere, hold it at that temperature for 2h, then heat it to 500℃ at a heating rate of 5℃ / min, hold it at that temperature for 3h, then cool it naturally to room temperature, grind it through a 300-mesh sieve to obtain coated lithium iron phosphate cathode material.

[0028] Comparative Example 3: This comparative example illustrates a method for preparing a lithium iron phosphate cathode material, comprising the following steps: Step s1: Add 1.5g lithium nitrate, 4.0g ferrous acetate, 2.8g ammonium dihydrogen phosphate, 60mL ethylene glycol mixed solvent (ethylene glycol and deionized water mixed in a volume ratio of 1:2), and 5.0g citric acid to a three-necked flask equipped with a stirrer, thermometer, and gas delivery tube. Adjust the pH to 5 with 25% ammonia solution, purge with nitrogen for protection, and maintain the mixture at 60℃ and a stirring rate of 300r / min. The mixture was stirred and reacted for 4 hours, then placed in a vacuum drying oven and dried at 70°C for 20 hours. After that, it was placed in a tube furnace for pre-calcination. Under a nitrogen atmosphere, the temperature was increased to 350°C at a rate of 3°C / min and held for 3 hours. The mixture was then ground through a 200-mesh sieve. The temperature was then increased to 650°C at a rate of 5°C / min and held for 2 hours. The temperature was then increased to 750°C and held for 5 hours. After that, the mixture was allowed to cool naturally to room temperature and ground through a 300-mesh sieve to obtain the lithium iron phosphate cathode material.

[0029] Cathode preparation: 79g of positive electrode material (including the core-shell structure lithium iron phosphate positive electrode material in Examples 1-3, the core-shell structure positive electrode material in Comparative Example 1, the coated lithium iron phosphate positive electrode material in Comparative Example 2, and the lithium iron phosphate positive electrode material in Comparative Example 3), 9g of conductive carbon black, 9g of polyvinylidene fluoride, and 160mL of N-methylpyrrolidone were added to a beaker and stirred at 300r / min for 3h using a magnetic stirrer to prepare a slurry. The slurry was coated on aluminum foil and then transferred to a vacuum drying oven and dried at 60°C for 4h. The electrode sheet was then rolled using a roller press with the rolling pressure controlled at 8MPa. After rolling, the electrode sheet was cut into positive electrode discs with a diameter of 10mm to obtain the positive electrode. Battery manufacturing: Using graphite as the negative electrode and Celgard 2400 polypropylene microporous membrane as the separator, and 1 mol / L LiPF6 (EC:DEC=1:1, v / v) as the electrolyte, the positive electrode, negative electrode, separator and electrolyte were assembled in an argon glove box with water and oxygen content of less than 1 ppm. After assembly, the cells were left to stand for 24 hours to obtain the cells.

[0030] Performance testing: The batteries of Examples 1-3 and Comparative Examples 1-3 were tested for discharge specific capacity at 0.5C and 5C rates under charge / discharge cutoff voltages of 3.0-4.3V; and the capacity retention rate was tested after 500 cycles at 1C and 5C charge / discharge rates.

[0031] The test results are shown in Table 1: Table 1: Test Results Summary Table

[0032] Referring to Table 1, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that the core-shell structure lithium iron phosphate cathode material of the present invention has excellent rate performance and cycle stability.

[0033] Based on the comparison between Example 3 and Comparative Example 1, it can be seen that the battery prepared by using lithium cobalt phosphate with doped and modified nickel and magnesium as the shell phase and lithium iron phosphate as the core phase to form a core-shell structure, with carbon nanotubes and titanium dioxide forming a composite coating layer attached to the surface of the core-shell structure, has better performance than the battery prepared by using lithium cobalt phosphate with doped and modified nickel and magnesium as the shell phase and lithium iron phosphate as the core phase to form a core-shell structure. Based on the comparison between Example 3 and Comparative Example 2, it can be seen that the battery prepared by using lithium cobalt phosphate with modified nickel and magnesium as the shell phase and lithium iron phosphate as the core phase to form a core-shell structure, and carbon nanotubes and titanium dioxide forming a composite coating layer attached to the surface of the core-shell structure, has better performance than the battery prepared by using carbon nanotubes and titanium dioxide forming a composite coating layer attached to the surface of lithium iron phosphate. Based on the comparison between Example 3 and Comparative Example 3, it can be seen that the battery prepared by using lithium cobalt phosphate with modified nickel and magnesium as the shell phase and lithium iron phosphate as the core phase to form a core-shell structure, with carbon nanotubes and titanium dioxide forming a composite coating layer attached to the surface of the core-shell structure, has better performance than the battery prepared by unmodified lithium iron phosphate.

[0034] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0035] The above description is merely an example and illustration of the present invention. Those skilled in the art can make various modifications or additions to the specific embodiments described or use similar methods to replace them, as long as they do not deviate from the invention or exceed the scope defined in this application, they should all fall within the protection scope of the present invention.

Claims

1. A method for preparing a core-shell structured lithium iron phosphate cathode material, characterized in that, Includes the following steps: Step 1: Add lithium nitrate, ferrous acetate, ammonium dihydrogen phosphate, ethylene glycol mixed solvent and citric acid to a three-necked flask, adjust the pH, stir the reaction to obtain the nucleus phase precursor sol; Step 2: Lithium nitrate, cobalt acetate, nickel nitrate, magnesium nitrate, ammonium dihydrogen phosphate, deionized water, and citric acid are stirred and reacted to obtain a shell-phase precursor solution; Step 3: Disperse the core phase precursor sol with ultrasound, add the shell phase precursor solution dropwise, continue to disperse with ultrasound, dry, then pre-calcine, cool, grind and sieve to obtain core-shell structure powder; Step 4: Stir carbon nanotubes, concentrated nitric acid, and concentrated sulfuric acid to react and obtain acidified carbon nanotubes; Step 5: Stir the acidified carbon nanotubes, anhydrous ethanol and deionized water, disperse by ultrasonication, add tetrabutyl titanate, adjust the pH, stir, and obtain carbon nanotube-titanium dioxide composite sol. Step 6: Add the core-shell structure powder to deionized water and disperse it by ultrasonication to obtain a suspension; Step 7: Add the carbon nanotube-titanium dioxide composite sol to the suspension, stir, filter, wash, and dry to obtain the precursor of the carbon nanotube-titanium dioxide coated core-shell material; Step 8: Sinter the precursor of the carbon nanotube-titanium dioxide coated core-shell material to obtain the core-shell structured lithium iron phosphate cathode material.

2. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, The subsequent steps for the stirring reaction in step four are: cooling, followed by filtration, washing, and drying.

3. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step one, the ratio of lithium nitrate, ferrous acetate, ammonium dihydrogen phosphate, ethylene glycol mixed solvent, and citric acid is 1.2-1.5g: 3.5-4.0g: 2.5-2.8g: 40-60mL: 4.0-5.0g; the ethylene glycol mixed solvent in step one is a solution of ethylene glycol and deionized water mixed in a volume ratio of 1:

2.

4. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step two, the ratio of lithium nitrate, cobalt acetate, nickel nitrate, magnesium nitrate, ammonium dihydrogen phosphate, deionized water, and citric acid is 0.25-0.3g: 0.6-0.7g: 1.0-1.2g: 0.4-0.5g: 0.5-0.6g: 20-30mL: 0.8-1.0g.

5. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step three, the ratio of the volume of the nucleus-phase precursor sol to the shell-phase precursor solution is 10-12 mL: 2-3 mL.

6. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step four, the ratio of carbon nanotubes, concentrated nitric acid, and concentrated sulfuric acid is 1-2g: 10-15mL: 30-45mL; the mass fraction of concentrated nitric acid is 65%; and the mass fraction of concentrated sulfuric acid is 98%.

7. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, The carbon nanotubes in step four have a diameter of 10-22 nm and a length of 5-10 μm.

8. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step five, the ratio of acidified carbon nanotubes, anhydrous ethanol, deionized water, and tetrabutyl titanate is 0.1-0.15g: 30-40mL: 10-15mL: 1.5-2.0mL.

9. The method for preparing a core-shell structured lithium iron phosphate cathode material according to claim 1, characterized in that, In step six, the ratio of core-shell structure powder to deionized water is 5-6g: 100-180mL; in step seven, the ratio of carbon nanotube-titanium dioxide composite sol to suspension is 1.3-1.5mL: 4-5mL.

10. A core-shell structured lithium iron phosphate cathode material, characterized in that, The core-shell structured lithium iron phosphate cathode material is prepared according to any one of claims 1-9.

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