High-capacity lithium iron phosphate cathode material based on quantum dot doping and preparation process thereof

Abstract

The application relates to a lithium iron phosphate positive electrode material, in particular to a high-capacity lithium iron phosphate positive electrode material based on quantum dot doping and a preparation process thereof, and is used for solving the problems of low specific capacity of lithium iron phosphate and poor high-rate charge-discharge performance; quantum dot doped lithium iron phosphate powder is obtained by uniformly stirring lithium hydroxide, iron phosphate, ammonium dihydrogen phosphate, carbon quantum dots and zirconium oxide quantum dots and then sintering, and the quantum dot doped lithium iron phosphate powder is coated with a conductive polymer to obtain the high-capacity lithium iron phosphate positive electrode material doped with quantum dots; the preparation process takes the quantum dot doped lithium iron phosphate powder as a main raw material; the quantum dot doped lithium iron phosphate powder can construct an efficient electron transmission network, strengthen structural stability, inhibit volume expansion, the lithium iron phosphate is coated with a conductive polymer, the conductive performance is improved, the low-temperature performance is improved, the cycle stability is enhanced, and the high-power and high-energy characteristics are possessed, so that the obtained positive electrode material has high specific capacity, high-rate performance and cycle stability.

Description

Technical Field

[0001] This invention relates to the field of lithium iron phosphate cathode materials, specifically to high-capacity lithium iron phosphate cathode materials based on quantum dot doping and their preparation process. Background Technology

[0002] Currently, with the booming demand in the new energy vehicle market, the industry is placing higher demands on key dimensions of power batteries, such as energy density, cycle life, and safety performance, striving to adapt to the new development trend of the industry. Regarding the selection of cathode materials, while lithium cobalt oxide has good performance, its high cost, narrow temperature characteristics, and the radioactivity of cobalt make it unsuitable as an ideal cathode material. Ternary materials are known for their high energy density, but their shortcomings in cycle life and safety performance make them less than ideal. Lithium iron phosphate (LFP) has been widely used in power batteries due to its low cost, long lifespan, high stability, and safety. However, LFP suffers from low electronic conductivity and a small lithium-ion diffusion coefficient, resulting in an actual specific capacity far lower than the theoretical capacity, and poor high-rate charge / discharge performance. Summary of the Invention

[0003] In order to overcome the above-mentioned technical problems, the purpose of this invention is to provide a high-capacity lithium iron phosphate cathode material based on quantum dot doping and its preparation process.

[0004] The objective of this invention can be achieved through the following technical solutions:

[0005] In a first aspect, this application provides a process for preparing high-capacity lithium iron phosphate cathode materials based on quantum dot doping, including the following steps:

[0006] Step 1: Add carbon quantum dots and deionized water to a beaker and sonicate for 25-35 minutes at a power of 400-500W to form the first solution;

[0007] Step 2: Add zirconium oxide quantum dots, deionized water and sodium dodecylbenzenesulfonate to a beaker, sonicate for 40-50 minutes at a power of 450-500W, and then let stand for 10-12 minutes to form a second solution.

[0008] Step 3: Add the first solution, the second solution, and polyvinylpyrrolidone to a three-necked flask equipped with a stirrer and a thermometer. Stir magnetically for 80-90 minutes at a temperature of 25-30℃ and a stirring rate of 900-1000 r / min to obtain a sol.

[0009] Step 4: Add lithium hydroxide, ferric phosphate, ammonium dihydrogen phosphate, and deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant-pressure dropping funnel. Adjust the pH to 3.8-4.8 with citric acid. Stir the reaction mixture for 10-20 minutes at 80-85℃ and a stirring rate of 500-600 rpm. Then, add the sol dropwise while stirring, controlling the dropping rate to 1-2 drops / s. After the addition is complete, continue stirring for 2-3 hours. Then, place the mixture in a spray dryer with a feed rate of 4... The inlet air temperature is 180-220℃, and the outlet air temperature is 80-100℃. After drying, the powder is placed in a tube furnace and heated to 380-480℃ at a rate of 5-8℃ / min under a nitrogen atmosphere with a flow rate of 80mL / min. The temperature is held for 2.5-3.5h, and then heated to 700-850℃ at a rate of 7-10℃ / min. The temperature is held for 6-8h, and then cooled to room temperature naturally. The powder is then pulverized through a 200-300 mesh sieve to obtain quantum dot-doped lithium iron phosphate powder.

[0010] Step 5: Add 1,5-diaminonaphthalene and anhydrous ethanol to a beaker, and stir magnetically for 30-35 minutes at a temperature of 35-40℃ and a stirring rate of 200-300 r / min to obtain the third solution.

[0011] Step 6: Add terephthalaldehyde and anhydrous ethanol to a beaker, and stir magnetically for 20-23 minutes at a temperature of 55-60℃ and a stirring rate of 200-300 r / min to obtain the fourth solution;

[0012] Step 7: Add chromium nitrate nonahydrate and anhydrous ethanol to a beaker, and stir in a water bath for 10-12 minutes at a temperature of 45-50℃ and a stirring rate of 200-300 r / min to obtain the fifth solution.

[0013] Step 8: Add the third solution to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 55-60℃ and a stirring rate of 350-400 r / min, add the fourth solution dropwise while stirring, controlling the dropping rate at 1-2 drops / s. After the addition is complete, continue stirring for 30-35 min. Then, add the fifth solution dropwise while stirring, controlling the dropping rate at 1-2 drops / s. Continue stirring for 3-4 h. Cool to room temperature, then filter to collect the solid and wash it 3-4 times with deionized water and then 3-4 times with anhydrous ethanol. Place it in a vacuum drying oven and dry it for 12-14 h at 55-60℃ and 0.08 MPa. Grind it through a 200-mesh sieve to obtain the conductive polymer.

[0014] Step 9: Add the conductive polymer to anhydrous ethanol and sonicate it for 30 minutes at a power of 200-300W. Then add quantum dot-doped lithium iron phosphate powder and stir for 3-4 hours at a temperature of 55-60℃ and a stirring rate of 270-300r / min. Then place it in a vacuum drying oven and dry it for 4-5 hours at a temperature of 75-80℃. Then place it in a tube furnace and heat it to 350℃ at a heating rate of 5℃ / min under a nitrogen atmosphere. Hold it at this temperature for 2 hours and then let it cool naturally to room temperature. Grind it through a 200-mesh sieve to obtain the quantum dot-doped high-capacity lithium iron phosphate cathode material.

[0015] In a preferred embodiment of the present invention, the ratio of carbon quantum dots to deionized water in step one is 1.2-1.4g:36-42mL.

[0016] In a preferred embodiment of the present invention, the carbon quantum dot in step one is the product model CQD-420 from Bohr Technology.

[0017] In a preferred embodiment of the present invention, the ratio of zirconium oxide quantum dots, deionized water and sodium dodecylbenzenesulfonate in step two is 1.8-2g: 45-50mL: 0.45-0.5g.

[0018] In a preferred embodiment of the present invention, the zirconium quantum dots in step two are zirconium dioxide quantum dot dispersions from Guangzhou Hongwu Materials Technology Co., Ltd.

[0019] In a preferred embodiment of the present invention, the ratio of the first solution, the second solution, and polyvinylpyrrolidone used in step three is 36-37 mL: 45-46 mL: 1.8-1.9 g.

[0020] In a preferred embodiment of the present invention, the CAS number of the polyvinylpyrrolidone in step three is 9003-39-8.

[0021] In a preferred embodiment of the present invention, the ratio of lithium hydroxide, iron phosphate, ammonium dihydrogen phosphate, deionized water and sol in step four is 2.6-2.8g: 15-17g: 11-13g: 180-210mL: 12-15mL.

[0022] In a preferred embodiment of the present invention, the ratio of 1,5-diaminonaphthalene to anhydrous ethanol in step five is 0.79-0.81 g: 25-27 mL.

[0023] In a preferred embodiment of the present invention, the ratio of terephthalaldehyde to anhydrous ethanol in step six is ​​0.67-0.69 g: 25-27 mL.

[0024] In a preferred embodiment of the present invention, the ratio of chromium nitrate nonahydrate to anhydrous ethanol in step seven is 2-3g: 20-30mL.

[0025] In a preferred embodiment of the present invention, the ratio of the amount of the third solution, the fourth solution and the fifth solution used in step eight is 25-27 mL: 25-27 mL: 21-23 mL.

[0026] In a preferred embodiment of the present invention, the ratio of the conductive polymer, anhydrous ethanol and quantum dot-doped lithium iron phosphate powder in step nine is 0.2-0.4g: 50-100mL: 10-11g.

[0027] Secondly, the present invention provides a high-capacity lithium iron phosphate cathode material based on quantum dot doping, which is prepared using the preparation process of the high-capacity lithium iron phosphate cathode material based on quantum dot doping described in the first aspect above.

[0028] The beneficial effects of this invention are:

[0029] This invention relates to a quantum dot-doped high-capacity lithium iron phosphate cathode material and its preparation process. The process involves uniformly mixing lithium hydroxide, iron phosphate, ammonium dihydrogen phosphate, carbon quantum dots, and zirconium oxide quantum dots, followed by sintering to obtain quantum dot-doped lithium iron phosphate powder. This powder is then coated with a conductive polymer to obtain the quantum dot-doped high-capacity lithium iron phosphate cathode material. This preparation process uses quantum dot-doped lithium iron phosphate powder as the main raw material. The quantum dot-doped lithium iron phosphate powder can construct an efficient electron transport network, enhance structural stability, and suppress volume expansion. Coating the lithium iron phosphate with a conductive polymer improves conductivity, low-temperature performance, and cycle stability, while also exhibiting high power and high energy characteristics. This results in a quantum dot-doped high-capacity lithium iron phosphate cathode material with high rate performance and cycle stability.

[0030] Carbon quantum dots can fill the gaps between lithium iron phosphate particles or adsorb onto the particle surface, forming a three-dimensional electron transport channel with "point-to-surface" contact. This significantly reduces the resistance to electron transport between lithium iron phosphate particles and improves conductivity. The surface of carbon quantum dots is rich in functional groups such as hydroxyl and carboxyl groups, which can combine with lithium iron phosphate through hydrogen bonds. This not only prevents their own aggregation but also "anchors" zirconium oxide quantum dots, ensuring that the two types of quantum dots are evenly dispersed.

[0031] The ionic radius of zirconium ions in zirconium oxide quantum dots is close to that of iron ions in the lithium iron phosphate lattice. They can partially embed into the interstices of the olivine-type lattice of lithium iron phosphate or replace a small amount of iron ions, forming a "solid solution strengthening" effect and suppressing the lattice distortion caused by lithium ion insertion / extraction during charging and discharging. Zirconia quantum dots have extremely strong chemical stability, are resistant to electrolyte corrosion and high temperature, and can form a protective layer on the surface of lithium iron phosphate particles, preventing the erosion of lithium iron phosphate by electrolyte decomposition products. During sintering, zirconium oxide quantum dots can act as "heterogeneous nucleation sites," hindering the excessive growth of lithium iron phosphate particles and shortening the lithium ion diffusion distance.

[0032] Carbon quantum dots and zirconia quantum dots complement each other in terms of performance. Carbon quantum dots compensate for the poor conductivity of zirconia quantum dots, while zirconia quantum dots solve the problem of "conductive channel breakage" caused by volume expansion of carbon quantum dots during long-term cycling, thus forming a balance between high conductivity and high stability. The surface functional groups of carbon quantum dots assist the dispersion of zirconia quantum dots through hydrogen bonds, while the rigid structure of zirconia quantum dots provides a "support framework" for carbon quantum dots, preventing the aggregation of carbon quantum dots due to thermal contraction during high-temperature sintering.

[0033] The synthesis of conductive polymers involves two steps. The first step is the formation of Schiff base ligands: the nitrogen atom of the amino group in 1,5-diaminonaphthalene attacks the electrophilic carbon of the terephthalaldehyde aldehyde group, resulting in nucleophilic addition and forming an α-hydroxyamine intermediate. The intermediate undergoes dehydration elimination under heating conditions, and the hydroxyl group combines with the proton of the adjacent NH group to release water, transforming the CN single bond into a C=N double bond, ultimately forming a double Schiff base ligand. The naphthalene ring is connected to the benzene ring of terephthalaldehyde through two -C=N- groups, exhibiting a symmetrical conjugated structure. The second step is the coordination reaction between chromium ions and Schiff base ligands: the nitrogen atoms of the two imine groups in the Schiff base ligand fill the empty orbitals of the chromium ion with electrons, forming a coordinate bond. Due to the spatial matching of the two N atoms, they combine with the chromium ion through a "bidentate chelate" mode, forming a stable six-membered ring structure, ultimately generating a metal ion-Schiff base ligand coordination compound, which is the conductive polymer.

[0034] Conductive polymers, containing conjugated large π bonds and electron transport sites formed by metal-ligand coordination, can construct a continuous conductive network on the surface of lithium iron phosphate after coating. This effectively reduces the contact resistance between electrode materials, accelerates the electron transport rate, and significantly improves the electrochemical rate performance of lithium iron phosphate. The polymer coating layer optimizes the ion transport environment on the lithium iron phosphate surface, reduces interfacial impedance at low temperatures, and enhances the lithium-ion diffusion rate, enabling the battery to maintain good charge and discharge efficiency even at low temperatures. Compared with traditional carbon coating, the density of Schiff base polymers is more suitable for lithium iron phosphate, and the coating layer structure is loose and uniform. It does not significantly reduce the tap density of the material, thus ensuring that the volumetric energy and gravimetric energy of lithium iron phosphate do not decrease while improving conductivity, thus balancing high power and high energy characteristics. The polymer and the surface of lithium iron phosphate form a strong bond through coordination, which on the one hand inhibits particle agglomeration and reduces the erosion of active materials by the electrolyte, and on the other hand alleviates the volume change of lithium iron phosphate during charging and discharging, effectively extending the battery cycle life. The conductive polymer coating of lithium iron phosphate improves conductivity, improves low-temperature performance, enhances cycle stability, and has high power and high energy characteristics. Detailed Implementation

[0035] 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.

[0036] Example 1:

[0037] This embodiment describes the fabrication process of high-capacity lithium iron phosphate cathode material based on quantum dot doping, including the following steps:

[0038] Step s1: Add 1.2g of carbon quantum dots (the carbon quantum dots are the product model CQD-420 from Bohr Technology) and 36mL of deionized water to a beaker, and sonicate for 25min at a power of 400W to form the first solution;

[0039] Step s2: Add 1.8g of zirconia quantum dots (zirconia quantum dots are zirconia quantum dot dispersion from Guangzhou Hongwu Materials Technology Co., Ltd.), 45mL of deionized water and 0.45g of sodium dodecylbenzenesulfonate to a beaker, sonicate for 40min at a power of 450W, and then let stand for 10min to form the second solution.

[0040] Step s3: Add 36 mL of the first solution, 45 mL of the second solution and 1.8 g of polyvinylpyrrolidone (CAS number of polyvinylpyrrolidone is 9003-39-8) to a three-necked flask equipped with a stirrer and a thermometer. Stir magnetically for 80 min at a temperature of 25 °C and a stirring rate of 900 r / min to obtain a sol.

[0041] Step s4: Add 2.6g lithium hydroxide, 15g ferric phosphate, 11g ammonium dihydrogen phosphate, and 180mL deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant-pressure dropping funnel. Adjust the pH to 3.8 with citric acid. Stir the reaction at 80℃ and a stirring rate of 500r / min for 10min. Then, while stirring, add 12mL of sol dropwise at a rate of 1 drop / s. After the addition is complete, continue stirring and reacting for 2h. The material was then placed in a spray dryer with a feed rate of 43 mL / h, an inlet air temperature of 180℃, and an outlet air temperature of 80℃. After drying, it was placed in a tube furnace and heated to 380℃ at a rate of 5℃ / min under a nitrogen atmosphere with a flow rate of 80 mL / min. The temperature was held for 2.5 h, and then heated to 700℃ at a rate of 7℃ / min. The temperature was held for 6 h, and then the material was allowed to cool naturally to room temperature. It was then pulverized through a 200-mesh sieve to obtain quantum dot-doped lithium iron phosphate powder.

[0042] Step s5: Add 0.79g of 1,5-diaminonaphthalene and 25mL of anhydrous ethanol to a beaker, and stir magnetically for 30min at a temperature of 35℃ and a stirring rate of 200r / min to obtain the third solution.

[0043] Step s6: Add 0.67g of terephthalaldehyde and 25mL of anhydrous ethanol to a beaker, and stir magnetically for 20min at a temperature of 55℃ and a stirring rate of 200r / min to obtain the fourth solution.

[0044] Step s7: Add 2g of chromium nitrate nonahydrate and 20mL of anhydrous ethanol to a beaker, and stir in a water bath for 10min at a temperature of 45℃ and a stirring rate of 200r / min to obtain the fifth solution;

[0045] Step s8: Add 25 mL of the third solution to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 55℃ and a stirring rate of 350 r / min, add 25 mL of the fourth solution dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, continue stirring and react for 30 min. Then, add 21 mL of the fifth solution dropwise while stirring, controlling the dropping rate to 1 drop / s. Continue stirring and react for 3 h. Cool to room temperature, then filter and collect the solid. Wash it three times with deionized water and then three times with anhydrous ethanol. Place it in a vacuum drying oven and dry it for 12 h at 55℃ and 0.08 MPa. Grind it through a 200-mesh sieve to obtain the conductive polymer.

[0046] Step s9: Add 0.2g of conductive polymer to 50mL of anhydrous ethanol and sonicate for 30min at 200W. Then add 10g of quantum dot-doped lithium iron phosphate powder and stir for 3h at 55℃ and 270r / min. Then place it in a vacuum drying oven and dry at 75℃ for 4h. Then place it in a tube furnace and heat to 350℃ at a rate of 5℃ / min under nitrogen atmosphere and hold for 2h. Then cool naturally to room temperature and grind through a 200-mesh sieve to obtain quantum dot-doped high-capacity lithium iron phosphate cathode material.

[0047] Example 2:

[0048] This embodiment describes the fabrication process of high-capacity lithium iron phosphate cathode material based on quantum dot doping, including the following steps:

[0049] Step s1: Add 1.3g of carbon quantum dots (the carbon quantum dots are the product model CQD-420 from Bohr Technology) and 39mL of deionized water to a beaker, and sonicate for 30min at a power of 450W to form the first solution;

[0050] Step s2: Add 1.9g of zirconia quantum dots (zirconia quantum dots are zirconia quantum dot dispersion from Guangzhou Hongwu Materials Technology Co., Ltd.), 47mL of deionized water and 0.47g of sodium dodecylbenzenesulfonate to a beaker, sonicate for 45min at a power of 470W, and then let stand for 11min to form the second solution.

[0051] Step s3: Add 36.5 mL of the first solution, 45.5 mL of the second solution, and 1.85 g of polyvinylpyrrolidone (CAS number of polyvinylpyrrolidone is 9003-39-8) to a three-necked flask equipped with a stirrer and a thermometer. Stir magnetically for 85 min at a temperature of 27 °C and a stirring rate of 950 r / min to obtain a sol.

[0052] Step s4: Add 2.7g lithium hydroxide, 16g ferric phosphate, 12g ammonium dihydrogen phosphate, and 190mL deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant pressure dropping funnel. Adjust the pH to 4.2 with citric acid. Stir the reaction at 83℃ and a stirring rate of 550r / min for 15min. Then, while stirring, add 13mL of sol dropwise at a rate of 2 drops / s. After the addition is complete, continue stirring for 2 minutes. After 5 hours, it was placed in a spray dryer with a feed rate of 44 mL / h, an inlet air temperature of 200℃, and an outlet air temperature of 90℃. After drying, it was placed in a tube furnace and heated to 430℃ at a heating rate of 6℃ / min under a nitrogen atmosphere with a flow rate of 80 mL / min. It was held at this temperature for 3 hours, and then heated to 780℃ at a heating rate of 8℃ / min. It was held at this temperature for 7 hours, and then naturally cooled to room temperature. It was then pulverized through a 250-mesh sieve to obtain quantum dot-doped lithium iron phosphate powder.

[0053] Step s5: Add 0.80g of 1,5-diaminonaphthalene and 26mL of anhydrous ethanol to a beaker, and stir magnetically for 33min at a temperature of 37℃ and a stirring rate of 250r / min to obtain the third solution;

[0054] Step s6: Add 0.68g of terephthalaldehyde and 26mL of anhydrous ethanol to a beaker, and stir magnetically for 22min at a temperature of 57℃ and a stirring rate of 250r / min to obtain the fourth solution;

[0055] Step s7: Add 2.5g of chromium nitrate nonahydrate and 25mL of anhydrous ethanol to a beaker, and stir in a water bath for 11min at a temperature of 47℃ and a stirring rate of 250r / min to obtain the fifth solution;

[0056] Step s8: Add 26 mL of the third solution to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 57°C and a stirring rate of 370 r / min, add 26 mL of the fourth solution dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, continue stirring for 33 min. Then, add 22 mL of the fifth solution dropwise while stirring, controlling the dropping rate to 2 drops / s. Continue stirring for 3.5 h. Cool to room temperature, then filter to collect the solid and wash it three times with deionized water and four times with anhydrous ethanol. Place it in a vacuum drying oven and dry it for 13 h at 57°C and 0.08 MPa. Grind it through a 200-mesh sieve to obtain the conductive polymer.

[0057] Step s9: Add 0.3g of conductive polymer to 80mL of anhydrous ethanol and sonicate for 30min at 250W. Then add 10.5g of quantum dot-doped lithium iron phosphate powder and stir for 3.5h at 57℃ and 290r / min. Then place it in a vacuum drying oven and dry at 77℃ for 4.5h. Then place it in a tube furnace and heat to 350℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, hold for 2h, and then cool naturally to room temperature. Grind through a 200-mesh sieve to obtain quantum dot-doped high-capacity lithium iron phosphate cathode material.

[0058] Example 3:

[0059] This embodiment describes the fabrication process of high-capacity lithium iron phosphate cathode material based on quantum dot doping, including the following steps:

[0060] Step s1: Add 1.4g of carbon quantum dots (the carbon quantum dots are the product model CQD-420 from Bohr Technology) and 42mL of deionized water to a beaker, and sonicate for 35min at a power of 500W to form the first solution;

[0061] Step s2: Add 2g of zirconia quantum dots (zirconia quantum dots are zirconia quantum dot dispersion from Guangzhou Hongwu Materials Technology Co., Ltd.), 50mL of deionized water and 0.5g of sodium dodecylbenzenesulfonate to a beaker, sonicate for 50min at 500W, and then let stand for 12min to form the second solution.

[0062] Step s3: Add 37 mL of the first solution, 46 mL of the second solution and 1.9 g of polyvinylpyrrolidone (CAS number of polyvinylpyrrolidone is 9003-39-8) to a three-necked flask equipped with a stirrer and a thermometer. Stir magnetically for 90 min at a temperature of 30 °C and a stirring rate of 1000 r / min to obtain a sol.

[0063] Step s4: Add 2.8g lithium hydroxide, 17g ferric phosphate, 13g ammonium dihydrogen phosphate, and 210mL deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant-pressure dropping funnel. Adjust the pH to 4.8 with citric acid. Stir the reaction at 85℃ and a stirring rate of 600r / min for 20min. Then, while stirring, add 15mL of sol dropwise at a rate of 2 drops / s. After the addition is complete, continue stirring and reacting for 3h. The material was then placed in a spray dryer with a feed rate of 45 mL / h, an inlet air temperature of 220℃, and an outlet air temperature of 100℃. After drying, it was placed in a tube furnace and heated to 480℃ at a rate of 8℃ / min under a nitrogen atmosphere with a flow rate of 80 mL / min. The temperature was held for 3.5 h, and then heated to 850℃ at a rate of 10℃ / min. The temperature was held for 8 h, and then the material was allowed to cool naturally to room temperature. The powder was then pulverized through a 300-mesh sieve to obtain quantum dot-doped lithium iron phosphate powder.

[0064] Step s5: Add 0.81g of 1,5-diaminonaphthalene and 27mL of anhydrous ethanol to a beaker, and stir magnetically for 35min at a temperature of 40℃ and a stirring rate of 300r / min to obtain the third solution.

[0065] Step s6: Add 0.69g of terephthalaldehyde and 27mL of anhydrous ethanol to a beaker, and stir magnetically for 23min at a temperature of 60℃ and a stirring rate of 300r / min to obtain the fourth solution;

[0066] Step s7: Add 3g of chromium nitrate nonahydrate and 30mL of anhydrous ethanol to a beaker, and stir in a water bath for 12min at a temperature of 50℃ and a stirring rate of 300r / min to obtain the fifth solution;

[0067] Step s8: Add 27 mL of the third solution to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 60 °C and a stirring rate of 400 r / min, add 27 mL of the fourth solution dropwise while stirring, controlling the dropping rate at 2 drops / s. After the addition is complete, continue stirring and react for 35 min. Then, add 23 mL of the fifth solution dropwise while stirring, controlling the dropping rate at 2 drops / s. Continue stirring and react for 4 h. Cool to room temperature, then filter and collect the solid. Wash it 4 times with deionized water and then 4 times with anhydrous ethanol. Place it in a vacuum drying oven and dry it for 14 h at 60 °C and 0.08 MPa. Grind it through a 200-mesh sieve to obtain the conductive polymer.

[0068] Step s9: Add 0.4g of conductive polymer to 100mL of anhydrous ethanol and sonicate for 30min at 300W. Then add 11g of quantum dot-doped lithium iron phosphate powder and stir for 4h at 60℃ and 300r / min. Then place it in a vacuum drying oven and dry for 5h at 80℃. Then place it in a tube furnace and heat to 350℃ at a rate of 5℃ / min under nitrogen atmosphere and hold for 2h. Then cool naturally to room temperature and grind through a 200-mesh sieve to obtain quantum dot-doped high-capacity lithium iron phosphate cathode material.

[0069] Comparative Example 1:

[0070] This comparative example demonstrates the fabrication process of quantum dot-doped lithium iron phosphate cathode material, including the following steps:

[0071] Step s1: Add 1.2g of carbon quantum dots (the carbon quantum dots are the product model CQD-420 from Bohr Technology) and 36mL of deionized water to a beaker, and sonicate for 25min at a power of 400W to form the first solution;

[0072] Step s2: Add 1.8g of zirconia quantum dots (zirconia quantum dots are zirconia quantum dot dispersion from Guangzhou Hongwu Materials Technology Co., Ltd.), 45mL of deionized water and 0.45g of sodium dodecylbenzenesulfonate to a beaker, sonicate for 40min at a power of 450W, and then let stand for 10min to form the second solution.

[0073] Step s3: Add 36 mL of the first solution, 45 mL of the second solution and 1.8 g of polyvinylpyrrolidone (CAS number of polyvinylpyrrolidone is 9003-39-8) to a three-necked flask equipped with a stirrer and a thermometer. Stir magnetically for 80 min at a temperature of 25 °C and a stirring rate of 900 r / min to obtain a sol.

[0074] Step s4: Add 2.6g lithium hydroxide, 15g ferric phosphate, 11g ammonium dihydrogen phosphate, and 180mL deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant-pressure dropping funnel. Adjust the pH to 3.8 with citric acid. Stir the reaction at 80℃ and a stirring rate of 500r / min for 10min. Then, while stirring, add 12mL of sol dropwise at a rate of 1 drop / s. After the addition is complete, continue stirring and reacting for 2 hours. The material was then placed in a spray dryer with a feed rate of 43 mL / h, an inlet air temperature of 180℃, and an outlet air temperature of 80℃. After drying, it was placed in a tube furnace and heated to 380℃ at a rate of 5℃ / min under a nitrogen atmosphere with a flow rate of 80 mL / min. The temperature was held for 2.5 h, and then heated to 700℃ at a rate of 7℃ / min. The temperature was held for 6 h, and then the material was allowed to cool naturally to room temperature. It was then pulverized through a 200-mesh sieve to obtain quantum dot-doped lithium iron phosphate cathode material.

[0075] Comparative Example 2:

[0076] This comparative example demonstrates the preparation process of conductive polymer-coated lithium iron phosphate cathode material, including the following steps:

[0077] Step s1: Add 2.6g lithium hydroxide, 15g iron phosphate, 11g ammonium dihydrogen phosphate and 180mL deionized water to a three-necked flask equipped with a stirrer, thermometer and constant pressure dropping funnel. Adjust the pH to 3.8 with citric acid. Stir and react for 2 hours at 80℃ and a stirring rate of 500r / min. Then place it in a spray dryer with a feed rate of 43mL / h, an inlet air temperature of 180℃ and an outlet air temperature of 80℃. After drying, place it in a tube furnace and heat it to 380℃ at a rate of 5℃ / min under a nitrogen atmosphere with a flow rate of 80mL / min. Hold it at this temperature for 2.5 hours. Then heat it to 700℃ at a rate of 7℃ / min and hold it for 6 hours. After cooling to room temperature, pulverize it through a 200-mesh sieve to obtain lithium iron phosphate powder.

[0078] Step s2: Add 0.79g of 1,5-diaminonaphthalene and 25mL of anhydrous ethanol to a beaker, and stir magnetically for 30min at a temperature of 35℃ and a stirring rate of 200r / min to obtain the third solution.

[0079] Step s3: Add 0.67g of terephthalaldehyde and 25mL of anhydrous ethanol to a beaker, and stir magnetically for 20min at a temperature of 55℃ and a stirring rate of 200r / min to obtain the fourth solution.

[0080] Step s4: Add 2g of chromium nitrate nonahydrate and 20mL of anhydrous ethanol to a beaker, and stir in a water bath for 10min at a temperature of 45℃ and a stirring rate of 200r / min to obtain the fifth solution;

[0081] Step s5: Add 25 mL of the third solution to a three-necked flask equipped with a stirrer, thermometer, gas delivery tube, and constant pressure dropping funnel. Purge with nitrogen for protection. Under conditions of 55℃ and a stirring rate of 350 r / min, add 25 mL of the fourth solution dropwise while stirring, controlling the dropping rate to 1 drop / s. After the addition is complete, continue stirring and react for 30 min. Then, add 21 mL of the fifth solution dropwise while stirring, controlling the dropping rate to 1 drop / s. Continue stirring and react for 3 h. Cool to room temperature, then filter and collect the solid. Wash it three times with deionized water and then three times with anhydrous ethanol. Place it in a vacuum drying oven and dry it for 12 h at 55℃ and 0.08 MPa. Grind it through a 200-mesh sieve to obtain the conductive polymer.

[0082] Step s6: Add 0.2g of conductive polymer to 50mL of anhydrous ethanol and sonicate for 30min at 200W. Then add 10g of lithium iron phosphate powder and stir for 3h at 55℃ and 270r / min. Then place it in a vacuum drying oven and dry at 75℃ for 4h. Then place it in a tube furnace and heat to 350℃ at a rate of 5℃ / min under nitrogen atmosphere and hold for 2h. Then cool naturally to room temperature and grind through a 200-mesh sieve to obtain conductive polymer-coated lithium iron phosphate cathode material.

[0083] Comparative Example 3:

[0084] This comparative example demonstrates the preparation process of lithium iron phosphate cathode material, including the following steps:

[0085] Step s1: Add 2.6g lithium hydroxide, 15g iron phosphate, 11g ammonium dihydrogen phosphate, and 180mL deionized water to a three-necked flask equipped with a stirrer, thermometer, and constant pressure dropping funnel. Adjust the pH to 3.8 with citric acid. Stir and react for 2 hours at 80℃ and a stirring rate of 500r / min. Then place it in a spray dryer with a feed rate of 43mL / h, an inlet air temperature of 180℃, and an outlet air temperature of 80℃. After drying, place it in a tube furnace and heat it to 380℃ at a rate of 5℃ / min under a nitrogen atmosphere with a flow rate of 80mL / min. Hold it at this temperature for 2.5 hours, then heat it to 700℃ at a rate of 7℃ / min and hold it for 6 hours. After that, allow it to cool naturally to room temperature and pulverize it through a 200-mesh sieve to obtain lithium iron phosphate cathode material.

[0086] Preparation of the positive electrode:

[0087] 39g of positive electrode material (including the quantum dot-doped high-capacity lithium iron phosphate positive electrode material in Examples 1-3, the quantum dot-doped lithium iron phosphate positive electrode material in Comparative Example 1, the conductive polymer-coated lithium iron phosphate positive electrode material in Comparative Example 2, and the lithium iron phosphate positive electrode material in Comparative Example 3), 4g of conductive carbon black, 5g of polyvinylidene fluoride, and 100mL 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 7MPa. After rolling, the electrode sheet was cut into positive electrode discs with a diameter of 13mm to obtain the positive electrode.

[0088] Battery manufacturing:

[0089] 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.

[0090] Performance testing:

[0091] The batteries of Examples 1-3 and Comparative Examples 1-3 were tested at 25°C and with a charge / discharge cutoff voltage of 3.0-4.1V. The discharge specific capacity at 1C rate was tested; the capacity retention rate at 5C high rate was tested; the capacity retention rate after 500 cycles at 1C charge / discharge rate was tested; and the electronic conductivity was tested.

[0092] The test results are shown in Table 1:

[0093] Table 1. Battery performance test results

[0094]

[0095] Referring to Table 1, based on the comparison between Examples 1-3 and Comparative Examples 1-3, it can be seen that the quantum dot-doped high-capacity lithium iron phosphate cathode material of the present invention has excellent high specific capacity, high rate performance and cycle stability.

[0096] Based on the comparison between Example 1 and Comparative Example 1, it can be seen that lithium iron phosphate powder doped with carbon quantum dots and zirconium oxide quantum dots can improve electronic conductivity and discharge specific capacity, giving it excellent high-rate performance and cycle stability. The battery prepared by coating lithium iron phosphate powder doped with carbon quantum dots and zirconium oxide quantum dots with a conductive polymer has better performance than the battery prepared by lithium iron phosphate powder doped with carbon quantum dots and zirconium oxide quantum dots.

[0097] Based on the comparison between Example 1 and Comparative Example 2, it can be seen that coating lithium iron phosphate powder with conductive polymer can improve the electrochemical rate performance of lithium iron phosphate and enhance cycle stability. The battery prepared by coating lithium iron phosphate powder doped with carbon quantum dots and zirconium oxide quantum dots with conductive polymer has better performance than the battery prepared by coating lithium iron phosphate powder with conductive polymer.

[0098] Based on the comparison between Example 1 and Comparative Example 3, it can be seen that the battery prepared by coating lithium iron phosphate powder doped with carbon quantum dots and zirconium oxide quantum dots with a conductive polymer has better performance than the battery prepared by lithium iron phosphate powder.

[0099] 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.

[0100] 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 process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping, characterized by, Includes the following steps: Step 1: Sonicate carbon quantum dots and deionized water to form the first solution; Step 2: Sonicate the zirconium oxide quantum dots, deionized water, and sodium dodecylbenzenesulfonate, then let them stand to form a second solution; Step 3: Magnetically stir the first solution, the second solution, and polyvinylpyrrolidone to obtain a sol; Step 4: Add lithium hydroxide, iron phosphate, ammonium dihydrogen phosphate and deionized water to a three-necked flask, adjust the pH with citric acid, stir, then add sol, continue stirring, dry, sinter and cool, pulverize and sieve to obtain quantum dot-doped lithium iron phosphate powder. Step 5: Stir 1,5-diaminonaphthalene and anhydrous ethanol magnetically to obtain the third solution; Step 6: Stir terephthalaldehyde and anhydrous ethanol magnetically to obtain the fourth solution; Step 7: Stir chromium nitrate nonahydrate and anhydrous ethanol in a water bath to obtain the fifth solution; Step 8: Add the third solution to the three-necked flask, add the fourth solution, then continue stirring, then add the fifth solution, continue stirring, cool, then filter to collect the solid and wash, then dry, grind and sieve to obtain the conductive polymer; Step 9: Add the conductive polymer to anhydrous ethanol, sonicate, then add quantum dot-doped lithium iron phosphate powder, stir, dry, then sinter, cool, grind and sieve to obtain quantum dot-doped high-capacity lithium iron phosphate cathode material.

2. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, The ratio of carbon quantum dots to deionized water used in step one is 1.2-1.4g: 36-42mL.

3. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step two, the ratio of zirconium oxide quantum dots, deionized water, and sodium dodecylbenzenesulfonate is 1.8-2g: 45-50mL: 0.45-0.5g.

4. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step three, the ratio of the first solution, the second solution, and polyvinylpyrrolidone used is 36-37 mL: 45-46 mL: 1.8-1.9 g.

5. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, The ratio of lithium hydroxide, iron phosphate, ammonium dihydrogen phosphate, deionized water, and sol in step four is 2.6-2.8g: 15-17g: 11-13g: 180-210mL: 12-15mL.

6. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step five, the ratio of 1,5-diaminonaphthalene to anhydrous ethanol is 0.79-0.81 g: 25-27 mL.

7. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step six, the ratio of terephthalaldehyde to anhydrous ethanol is 0.67-0.69 g: 25-27 mL.

8. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step seven, the ratio of chromium nitrate nonahydrate to anhydrous ethanol is 2-3g: 20-30mL.

9. The process for the preparation of high capacity lithium iron phosphate cathode material based on quantum dot doping as claimed in claim 1 wherein, In step eight, the ratio of the third, fourth, and fifth solutions is 25-27 mL: 25-27 mL: 21-23 mL; in step nine, the ratio of the conductive polymer, anhydrous ethanol, and quantum dot-doped lithium iron phosphate powder is 0.2-0.4 g: 50-100 mL: 10-11 g.

10. A high capacity lithium iron phosphate cathode material based on quantum dot doping, characterized in that, It is prepared using the preparation process of high-capacity lithium iron phosphate cathode material based on quantum dot doping as described in any one of claims 1-9.

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