A Lithium Iron Phosphate Material, Its Preparation Method and Application

Abstract

The present disclosure relates to an iron phosphate material, a preparation method thereof and an application thereof, belonging to the technical field of batteries. The iron phosphate material includes a carbon quantum dot core located inside and an iron phosphate shell formed on the surface of the carbon quantum dot core, and the surface of the iron phosphate shell has a carbon coating layer formed by a carbon-containing dispersant. The iron phosphate material is nano-scale iron phosphate with uniform crystal size. The preparation method includes the following steps: mixing and reacting a first mixed solution containing a divalent iron salt and carbon quantum dots, a second mixed solution containing a phosphate and a dispersant, and an oxidant to obtain iron phosphate dihydrate; annealing and calcining the iron phosphate dihydrate. The iron phosphate material can be further used for preparing a lithium iron phosphate cathode material and a battery.

Description

Technical Field

[0001] The present disclosure relates to the technical field of batteries, and more particularly, to a lithium iron phosphate material, a preparation method thereof, and an application thereof. Background Art

[0002] At present, the cathode materials of lithium batteries mainly include lithium cobaltate, lithium iron phosphate, lithium manganate, and ternary materials. Compared with ternary cathode materials, lithium iron phosphate cathode materials have the advantages of high safety performance, long cycle life, low pollution, and low cost, and have become one of the key materials for research. However, the lithium iron phosphate cathode material itself has poor conductivity and low lithium ion diffusion coefficient. Therefore, processes such as doping and coating are the main development directions of lithium iron phosphate cathode materials.

[0003] The existing production processes of lithium iron phosphate cathode materials mainly include the ferrous oxalate process, the iron oxide red process, the all-wet process, and the iron phosphate process. When synthesizing the precursor of lithium iron phosphate cathode material (i.e., iron phosphate), the above production processes have the disadvantages of difficult process control, irregular iron phosphate, large particle size, and serious agglomeration, which in turn lead to poor performance of lithium iron phosphate prepared from the precursor.

[0004] In view of this, the present disclosure is particularly proposed. Summary of the Invention

[0005] The purpose of the present disclosure is to provide a lithium iron phosphate material, a preparation method thereof, and an application thereof to solve or improve at least one of the above technical problems.

[0006] The present disclosure can be implemented as follows:

[0007] In a first aspect, the present disclosure provides a lithium iron phosphate material, which includes a carbon quantum dot core located inside and a lithium iron phosphate shell formed on the surface of the carbon quantum dot core, and the surface of the lithium iron phosphate shell has a carbon coating layer formed by a carbon-containing dispersant.

[0008] In an optional embodiment, the lithium iron phosphate material further has at least one of the following characteristics:

[0009] Characteristic 1: The D of the lithium iron phosphate material 50 does not exceed 200 nm;

[0010] Characteristic 2: The D of the carbon quantum dot core 50 is 10 nm to 20 nm;

[0011] Characteristic 3: The lithium iron phosphate shell is formed in situ on the surface of the carbon quantum dot core;

[0012] Characteristic 4: The thickness of the carbon coating layer is 1 - 3 nm;

[0013] Characteristic 5: The mass of the carbon coating layer does not exceed 5 wt% of the lithium iron phosphate material.

[0014] In an optional embodiment, the D of the iron phosphate material is from 20 nm to 200 nm; and / or, the mass of the carbon coating layer is 1 wt% - 5 wt% of the iron phosphate material. 50 For the second aspect, the present disclosure provides a method for preparing an iron phosphate material according to any one of the foregoing embodiments, comprising the following steps: mixing and reacting a first mixed solution, a second mixed solution, and an oxidizing agent to obtain iron phosphate dihydrate; annealing and calcining the iron phosphate dihydrate to obtain an iron phosphate material;

[0015] wherein the first mixed solution includes a divalent iron salt and carbon quantum dots; the second mixed solution includes a phosphate and a carbon-containing dispersant.

[0016] In an optional embodiment, the preparation of the carbon quantum dots includes: modifying citric acid with an organic amine in a solution environment to obtain a modified solution; heating the modified solution to cause the cracking of citric acid to obtain a carbon-containing solution; mixing the carbon-containing solution with an aqueous sodium hydroxide solution, performing solid-liquid separation, dialyzing the separated liquid phase, and drying the dialysate obtained by dialysis.

[0017] In an optional embodiment, the mass ratio of citric acid to the organic amine is 1:0.1 - 1:0.5.

[0018] In an optional embodiment, the mass fraction of citric acid in the modified solution is not less than 99.5%.

[0019] In an optional embodiment, the organic amine includes at least one of ethanolamine, ethylenediamine, and polyethylene diamine.

[0020] In an optional embodiment, the heating temperature is 180°C - 280°C, and / or, the heating time is 4 h - 8 h.

[0021] In an optional embodiment, the concentration of the aqueous sodium hydroxide solution is 10 g / L - 20 g / L, and the volume ratio of the carbon-containing solution to the aqueous sodium hydroxide solution is 1:1 - 1:2.

[0022] In an optional embodiment, the cut-off molecular weight of the dialysis bag used for dialysis is 500 D - 1500 D.

[0023] In an optional embodiment, the drying temperature is -20°C to -60°C, and the drying time is 10 h - 14 h.

[0024] In an optional embodiment, the mass ratio of the carbon quantum dots to the divalent iron salt is 5:100 to 20:100.

[0025] In an optional embodiment, the divalent iron salt includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride.

[0026] In an optional embodiment, the divalent iron salt includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride.

[0027] In an alternative embodiment, the molar ratio of iron element in the ferrous salt to phosphorus element in the phosphate is from 1.1:1 to 1.5:1.

[0028] In an alternative embodiment, the phosphate includes at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate.

[0029] In an alternative embodiment, the carbon-containing dispersant includes acrylic acid.

[0030] In an alternative embodiment, the dosage of the carbon-containing dispersant is 3 wt% - 10 wt% of the ferrous salt.

[0031] In an alternative embodiment, the oxidant includes hydrogen peroxide.

[0032] In an alternative embodiment, in terms of molar amount, the addition amount of the oxidant is 1 time - 1.2 times that of the iron element in the ferrous salt.

[0033] In an alternative embodiment, the reaction temperature is 100°C - 130°C; and / or, the reaction time is 2 h - 6 h.

[0034] In an alternative embodiment, the annealing calcination includes at least one of the following features:

[0035] Feature 1: The temperature of the annealing calcination is 400°C - 600°C;

[0036] Feature 2: The time of the annealing calcination is 3 h - 8 h;

[0037] Feature 3: The annealing calcination is carried out under vacuum conditions.

[0038] In a third aspect, the present disclosure provides a lithium iron phosphate cathode material, and its preparation raw materials include the iron phosphate material of any one of the foregoing embodiments.

[0039] In a fourth aspect, the present disclosure provides a battery, which contains the lithium iron phosphate cathode material of the foregoing embodiment.

[0040] The beneficial effects of the present disclosure include:

[0041] The iron phosphate material provided by the present disclosure includes a carbon quantum dot core located inside and an iron phosphate shell formed on the surface of the carbon quantum dot core. The surface of the iron phosphate shell has a carbon coating layer. Among them, the carbon quantum dot core can play a role in increasing the formation rate of the iron phosphate shell and refining the particles, which is beneficial to controlling the particle size of the iron phosphate material. The carbon-containing dispersant can not only inhibit the agglomeration between grains, but also the carbon coating layer formed by it can further limit the growth of iron phosphate particles. By combining the carbon quantum dot core with the carbon-containing dispersant, nano-level iron phosphate with regular morphology, uniform particles and small particle size can be obtained. This iron phosphate material can be used as a precursor to prepare nano-level lithium iron phosphate cathode materials. The preparation method of the above iron phosphate material is simple, easy to operate, easy to control, and the product has good stability, which is suitable for industrial production. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the drawings required for use in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present disclosure, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0043] Figure 1 It is the SEM image of the iron phosphate material prepared in Example 1 of the present disclosure. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] To make the objectives, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions in the embodiments of the present disclosure will be clearly and completely described below. Those not specified in the embodiments are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or instruments not specified by the manufacturer are all conventional products that can be obtained through commercial purchase.

[0045] The iron phosphate material provided by the present disclosure, its preparation method and application will be specifically described below.

[0046] The present disclosure provides an iron phosphate material, which includes a carbon quantum dot core located inside and an iron phosphate shell formed on the surface of the carbon quantum dot core. The surface of the iron phosphate shell has a carbon coating layer formed by a carbon-containing dispersant.

[0047] The above carbon quantum dot core can play a role in increasing the formation rate of the iron phosphate shell and refining the particles, which is beneficial to controlling the particle size of the iron phosphate material, so as to obtain nano iron phosphate with regular morphology, uniform particles and small particle size. This nano iron phosphate can be used as a precursor to prepare nano-level lithium iron phosphate cathode materials. It should be noted that nano-level lithium iron phosphate cathode materials can shorten the electron and ion transport distances, have the ability of high-current charge and discharge, and have better electrochemical performance.

[0048] The carbon-containing dispersant can not only reduce the surface energy of the nanocrystals, change the thickness of the double electric layer and increase the steric hindrance effect between the grains, so as to play a role in inhibiting the agglomeration of the nanocrystals; but also the carbon coating layer formed by it coats on the surface of iron phosphate dihydrate, which can further limit the growth of iron phosphate dihydrate particles.

[0049] In the present disclosure, the D of the iron phosphate material 50 does not exceed 200 nm, such as 200 nm, 180 nm, 150 nm, 120 nm, 100 nm, 80 nm, 50 nm, 20 nm or 10 nm, etc., and can also be any other value within the range not exceeding 200 nm. In some alternative embodiments, the D of the iron phosphate material 50 is 20 nm to 200 nm, for example, it can be 120 nm to 185 nm.

[0050] In the present disclosure, the carbon quantum dot core is at the nanoscale. In some embodiments, the D of the carbon quantum dot core 50 can be 10 nm to 20 nm, such as 10 nm, 12 nm, 15 nm, 18 nm or 20 nm, etc., and can also be any other value within the range of 10 nm to 20 nm.

[0051] The above-mentioned nanoscale carbon quantum dots, as ultra-fine spherical particles, can be used as a structure-directing agent for shell growth during the synthesis of iron phosphate, realizing the in-situ growth of iron phosphate on the nanocarbon quantum dots. Moreover, while the iron phosphate grows on the surface of the carbon quantum dot core, it will also nucleate itself. The nanoscale carbon quantum dots are beneficial to increasing the nucleation rate of iron phosphate and significantly refining the crystal grains, thereby facilitating the obtaining of nanoscale iron phosphate materials.

[0052] It should be noted that if the D of the carbon quantum dot core 50 is too small, it is easy to agglomerate in the solution, which is not conducive to controlling the particle size of the iron phosphate material; if the D of the carbon quantum dot core 50 is too large, it will cause the overall particle size of the iron phosphate material to be too large.

[0053] In the present disclosure, the thickness of the carbon coating layer can be 1 - 3 nm, such as 1 nm, 1.5 nm, 2 nm, 2.5 nm or 3 nm, etc.

[0054] The mass of the carbon coating layer does not exceed 5 wt% of the iron phosphate material, such as 5 wt%, 4 wt%, 3 wt%, 2 wt%, 1 wt% or 0.5 wt% of the iron phosphate material, etc., and can also be any other value within the range not exceeding 5 wt%. In some alternative embodiments, the mass of the carbon coating layer is 1 wt% - 5 wt% of the iron phosphate material.

[0055] Correspondingly, the present disclosure also provides a method for preparing the above-mentioned iron phosphate material, which may include the following steps: mixing and reacting a first mixed solution, a second mixed solution, and an oxidant to obtain iron phosphate dihydrate; annealing and calcining the iron phosphate dihydrate to obtain the iron phosphate material;

[0056] Wherein, the first mixed solution includes a divalent iron salt and carbon quantum dots; the second mixed solution includes a phosphate and a carbon-containing dispersant.

[0057] As a reference, the preparation of carbon quantum dots may include: modifying citric acid with an organic amine in a solution environment to obtain a modified solution; heating the modified solution to cause the cleavage of citric acid to obtain a carbon-containing solution; mixing the carbon-containing solution with an aqueous sodium hydroxide solution, performing solid-liquid separation, dialyzing the separated liquid phase, and drying the dialysis obtained dialysate.

[0058] Wherein, the solution environment may be an aqueous environment. The modification of citric acid by the above-mentioned organic amine is mainly to increase the hydrophilicity of citric acid by providing an N source, thereby facilitating more uniform dispersion of the particles in water.

[0059] Exemplarily, the mass ratio of citric acid to the organic amine may be 1:0.1 - 1:0.5, such as 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, 1:0.35, 1:0.4, 1:0.45, or 1:0.5, etc., or any other value within the range of 1:0.1 - 1:0.5. Among them, the organic amine may exemplarily but not limitedly include at least one of ethanolamine, ethylenediamine, and polyethylene diamine. In some embodiments, the mass fraction of citric acid in the modified solution is not less than 99.5%.

[0060] In the present disclosure, the temperature for heating the modified solution may be 180°C - 280°C, such as 180°C, 200°C, 220°C, 250°C, or 280°C, etc., or any other value within the range of 180°C - 280°C. The heating time may be 4h - 8h, such as 4h, 5h, 6h, 7h, or 8h, etc., or any other value within the range of 4h - 8h.

[0061] By heating treatment, the cleavage of citric acid can be enabled. In the present disclosure, heating the modified solution at the above temperature and time is beneficial to controlling the size of the carbon quantum dots formed by cleavage.

[0062] If the heating temperature is lower than 180°C, it is likely to result in too few carbon quantum dots formed and too low a yield; if the heating temperature is higher than 280°C, it is likely to result in too large a particle size of the carbon quantum dots formed. If the heating time is shorter than 4h, it is likely to result in poor dispersibility of the carbon quantum dots formed and low yield; if the heating time is longer than 8h, it is likely to result in non-uniform particle size distribution of the carbon quantum dots formed.

[0063] Continuing from the above, the surface of the nano-carbon quantum dots obtained by pyrolyzing citric acid at high temperature contains a large number of oxygen-containing functional groups, which can effectively adsorb iron ions and improve the synthesis rate of iron phosphate. The nano-carbon quantum dots obtained by pyrolysis under the above conditions are ultra-fine spherical particles and can serve as a structure-directing agent for nucleation and growth during the synthesis of iron phosphate, enabling in-situ nucleation and growth of iron phosphate on the nano-carbon quantum dots, increasing the nucleation rate and thus significantly refining the crystal grains, which is beneficial to the formation of nano-scale iron phosphate.

[0064] For reference, the concentration of the aqueous sodium hydroxide solution used for mixing with the carbon-containing solution can be 10 g / L - 20 g / L, such as 10 g / L, 12 g / L, 15 g / L, 18 g / L or 20 g / L, etc., or any other arbitrary value within the range of 10 g / L - 20 g / L. The volume ratio of the carbon-containing solution to the aqueous sodium hydroxide solution can be 1:1 - 1:2, such as 1:1, 1:1.5 or 1:2, etc., or any other arbitrary value within the range of 1:1 - 1:2.

[0065] The functions of the above-mentioned aqueous sodium hydroxide solution include further impurity removal and purification of the carbon quantum dot solution.

[0066] Exemplarily, the mixed solution after mixing the carbon-containing solution and the aqueous sodium hydroxide solution can be subjected to solid-liquid separation by centrifugation. The centrifugation speed can be 7000 r / min - 9000 r / min, such as 7000 r / min, 8000 r / min or 9000 r / min, etc.

[0067] After centrifugation, the supernatant is collected and transferred to a dialysis bag for dialysis. The cut-off molecular weight of the dialysis bag used for dialysis can be 500 D - 1500 D, such as 500 D, 800 D, 1000 D, 1200 D or 1500 D, etc. Through dialysis, a dialysate (i.e., an aqueous solution of carbon quantum dots) is obtained.

[0068] For reference, the temperature for drying the dialysate can be -20 °C to -60 °C, such as -20 °C, -40 °C or -60 °C, etc., and the drying time can be 10 h - 14 h, such as 10 h, 12 h or 14 h, etc. Exemplarily, the drying method can be freeze-drying.

[0069] In the present disclosure, the mass ratio of the carbon quantum dots to the divalent iron salt can be 5:100 to 20:100, such as 5:100, 8:100, 10:100, 12:100, 15:100, 18:100 or 20:100, etc., or any other arbitrary value within the range of 5:100 to 20:100.

[0070] Among them, the divalent iron salt can exemplarily but not restrictively include at least one of ferrous sulfate, ferrous nitrate and ferrous chloride.

[0071] It should be noted that the mixing order of the divalent iron salt and the carbon quantum dots is not limited. In some embodiments, the first mixed solution can be formed by adding carbon quantum dots to the divalent iron salt solution.

[0072] The molar ratio of the iron element in the divalent iron salt to the phosphorus element in the phosphate can be 1.1:1 to 1.5:1, such as 1.1:1, 1.2:1, 1.3:1, 1.4:1 or 1.5:1, etc., and can also be any other value within the range of 1.1:1 to 1.5:1.

[0073] Among them, the phosphate can exemplarily but not restrictively include at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate and ammonium phosphate.

[0074] In the present disclosure, the carbon-containing dispersant can include acrylic acid. By adding acrylic acid as a dispersant during the synthesis of iron phosphate, it can not only reduce the surface energy of the nano-crystals, change the thickness of the double electric layer and increase the steric hindrance effect between the grains, thereby playing a role in suppressing the agglomeration between the nano-crystals. At the same time, Fe 2+ Fe formed after being oxidized by the oxidant 3+ can also act as a catalyst to promote the oxidative polymerization reaction of acrylic acid on the surface of the iron phosphate dihydrate particles, generate polyacrylic acid and coat it on the surface of the iron phosphate dihydrate, further restricting the growth of the iron phosphate dihydrate particles.

[0075] For reference, the dosage of the carbon-containing dispersant can be 3wt%-10wt% of the divalent iron salt, such as 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt% or 10wt%, etc., and can also be any other value within the range of 3wt%-10wt%.

[0076] It should be noted that the mixing order of the phosphate and the carbon-containing dispersant is not limited. In some embodiments, the second mixed solution can be formed by adding the carbon-containing dispersant to the phosphate solution.

[0077] The above-mentioned oxidant can be, for example, hydrogen peroxide, and in addition, it can also be oxygen, etc.

[0078] For reference, in terms of molar amount, the addition amount of the oxidant can be 1 to 1.2 times the iron element in the divalent iron salt, such as 1 time, 1.1 times or 1.2 times, etc.

[0079] It should be noted that the mixing order of the first mixed solution, the second mixed solution and the oxidant is not limited. In some embodiments, the second mixed solution can be added to the first mixed solution and the oxidant is added.

[0080] In the present disclosure, the reaction temperature of the first mixture, the second mixture and the oxidant can be 100°C - 130°C, such as 100°C, 105°C, 110°C, 115°C, 120°C, 125°C or 130°C, etc., or any other value within the range of 100°C - 130°C. The reaction time can be 2h - 6h, such as 2h, 3h, 4h, 5h or 6h, etc., or any other value within the range of 2h - 6h.

[0081] Through the above reaction, the oxidant oxidizes the Fe in the ferrous salt 2+ to Fe 3+ and reacts with phosphate to in-situ generate iron phosphate dihydrate on the surface of the carbon quantum dots. The use of a dispersant can avoid agglomeration.

[0082] If the above reaction temperature is lower than 100°C, the reaction rate is too low, resulting in the iron element content in the generated iron phosphate dihydrate deviating from the theoretical value; if the above reaction temperature is higher than 130°C, it will cause the hydrolysis of ferric ions to generate iron hydroxide, reducing the purity of the generated iron phosphate dihydrate. If the above reaction time is shorter than 2h, the reaction is insufficient, and some residual raw materials do not participate in the reaction, resulting in insufficient purity of iron phosphate dihydrate; if the above reaction time is longer than 6h, it will cause abnormal growth of the iron phosphate dihydrate crystals and easily cause side reactions, resulting in a decrease in the purity of iron phosphate dihydrate.

[0083] In the present disclosure, the annealing calcination temperature can be 400°C - 600°C, such as 400°C, 450°C, 500°C, 550°C or 600°C, etc., or any other value within the range of 400°C - 600°C. The annealing calcination time can be 3h - 8h, such as 3h, 4h, 5h, 6h, 7h or 8h, etc., or any other value within the range of 3h - 8h. The annealing calcination can be carried out under vacuum conditions.

[0084] Taking polypropylene as the carbon-containing dispersant as an example, through annealing calcination, polyacrylic acid is carbonized to form a carbon coating layer.

[0085] If the above annealing calcination temperature is lower than 400°C, it is not conducive to the complete carbonization reaction, resulting in a decrease in the purity of iron phosphate dihydrate; if the above annealing calcination temperature is higher than 600°C, it will cause more crystal defects in the formed iron phosphate dihydrate and a decrease in the stability. If the above annealing calcination time is shorter than 3h, it will lead to a decrease in the crystallinity of iron phosphate dihydrate and an uneven carbon layer formation; if the above annealing calcination time is longer than 8h, it is easy to cause the generation of impurity phases.

[0086] Continuing from the above, the present disclosure improves the nucleation rate of iron phosphate dihydrate by adding carbon quantum dots at the nanoscale, and combines with the self-polymerization reaction of acrylic acid to limit grain growth. With the cooperation of the two, the nucleation rate of grains in the synthesis process of iron phosphate dihydrate can be made greater than the growth rate, thereby effectively controlling the particle size, facilitating the generation of nanoscale iron phosphate dihydrate, and forming more uniform crystals. The organic polymer on the surface of the iron phosphate dihydrate particles undergoes annealing carbonization to uniformly carbon coat the iron phosphate dihydrate particles, which is beneficial to the preparation of nanoscale iron phosphate materials.

[0087] In addition, the present disclosure also provides a lithium iron phosphate cathode material, the preparation raw materials of which include the above-mentioned iron phosphate material.

[0088] In some embodiments, the lithium iron phosphate cathode material provided by the present disclosure is at the nanoscale, which can shorten the electron and ion transport distances, has the ability of high-current charge and discharge, and has better electrochemical performance. For example, the lithium iron phosphate cathode material may have a large discharge specific capacity, excellent rate performance, and good cycle stability.

[0089] Furthermore, the present disclosure also provides a battery containing the above-mentioned lithium iron phosphate cathode material, which is beneficial to having better electrochemical performance.

[0090] The following further describes in detail the features and properties of the present disclosure in combination with embodiments.

[0091] Example 1

[0092] This example provides an iron phosphate material, which is carbon-coated nanoscale iron phosphate, denoted as LiFePO4 / C composite material. This iron phosphate material includes a carbon quantum dot core located inside and an iron phosphate shell formed in situ on the surface of the carbon quantum dot core, and the surface of the iron phosphate shell has a carbon coating layer.

[0093] The preparation method of this iron phosphate material includes:

[0094] S1: Dissolve citric acid and ethylenediamine in deionized water according to a mass ratio of 1:0.2 to obtain a modified solution with a citric acid mass fraction of 99.5%. Heat the modified solution to 230°C and react for 6 hours to obtain a carbon-containing solution.

[0095] S2: Mix the carbon-containing solution in S1 with a sodium hydroxide solution with a concentration of 20 g / L according to a volume ratio of 1:1.5. Centrifuge the mixed solution at a speed of 8000 r / min for 10 minutes and collect the supernatant. Transfer the supernatant to a dialysis bag with a cut-off molecular weight of 1000 D and dialyze for 12 hours to obtain a dialysate (aqueous solution of carbon quantum dots). Collect the dialysate and transfer it to a freeze dryer, and freeze-dry it at -40°C for 12 hours to obtain nanoscale carbon quantum dots.

[0096] S3: Dissolve ferrous sulfate and ammonium dihydrogen phosphate in deionized water respectively according to the Fe:P molar ratio of 1.2:1 to form solutions. Add 10% of the mass of ferrous sulfate of nano-carbon quantum dots to the ferrous sulfate solution, and mix to obtain a first mixed solution. Add 6% of the mass of ferrous sulfate of acrylic acid to the ammonium dihydrogen phosphate solution, and mix to obtain a second mixed solution.

[0097] S4: Add the second mixed solution to the first mixed solution and add hydrogen peroxide with a molar amount 1.1 times that of iron. React at 120 °C for 4 h, filter and wash to obtain iron phosphate dihydrate.

[0098] S5: Anneal and calcine iron phosphate dihydrate in a vacuum tube furnace at 500 °C for 5 h to obtain carbon-coated nano-iron phosphate.

[0099] The SEM image of the carbon-coated nano-iron phosphate is as Figure 1 shown. It can be seen from Figure 1 that the prepared iron phosphate material has regular morphology, uniform particles and small particle size.

[0100] The D 50 of the carbon-coated nano-iron phosphate is 120 nm, the D 50 of the carbon quantum dot core is 15 nm, and the thickness of the carbon coating layer is 2 nm; the mass of the carbon coating layer is 2 wt% of the carbon-coated nano-iron phosphate.

[0101] Example 2

[0102] This example provides an iron phosphate material, which is carbon-coated nano-iron phosphate, denoted as LiFePO4 / C composite material. The iron phosphate material includes a carbon quantum dot core located inside and an iron phosphate shell formed in-situ on the surface of the carbon quantum dot core, and the surface of the iron phosphate shell has a carbon coating layer.

[0103] The preparation method of the iron phosphate material includes:

[0104] S1: Dissolve citric acid and ethylenediamine in deionized water according to the mass ratio of 1:0.2 to obtain a modified solution with a citric acid mass fraction of 99.5%. Heat the modified solution to 180 °C and react for 8 h.

[0105] S2: Mix the carbon-containing solution of S1 and a sodium hydroxide solution with a concentration of 10 g / L according to a volume ratio of 1:1. Centrifuge the mixed solution at a speed of 8000 r / min for 10 min and collect the supernatant. Transfer the supernatant to a dialysis bag with a cut-off molecular weight of 1000 D and dialyze for 12 h to obtain a dialysate (aqueous solution of carbon quantum dots). Collect the dialysate and transfer it to a freeze dryer, and freeze-dry at -60 °C for 12 h to obtain nano-carbon quantum dots.

[0106] S3: Dissolve ferrous sulfate and ammonium dihydrogen phosphate in deionized water respectively according to the Fe:P molar ratio of 1.1:1 to prepare solutions. Add 10% of the mass of ferrous sulfate of nano-carbon quantum dots to the ferrous sulfate solution, and mix to obtain a first mixture. Add 3% of the mass of ferrous sulfate of acrylic acid to the ammonium dihydrogen phosphate solution, and mix to obtain a second mixture.

[0107] S4: Add the second mixture to the first mixture and add hydrogen peroxide with a molar amount 1 times that of iron, react at a temperature of 100 °C for 6 h, filter and wash to obtain iron phosphate dihydrate.

[0108] S5: Anneal and calcine iron phosphate dihydrate in a vacuum tube furnace at 600 °C for 3 h to obtain carbon-coated nano-iron phosphate.

[0109] Example 3

[0110] This example provides an iron phosphate material, which is carbon-coated nano-iron phosphate, denoted as LiFePO4 / C composite material. This iron phosphate material includes a carbon quantum dot core located inside and an iron phosphate shell formed in-situ on the surface of the carbon quantum dot core, and the surface of the iron phosphate shell has a carbon coating layer.

[0111] The preparation method of this iron phosphate material includes:

[0112] S1: Dissolve citric acid and ethylenediamine in deionized water according to the mass ratio of 1:0.5 to obtain a modified solution with a citric acid mass fraction of 99.5%, heat the modified solution to 280 °C, and react for 4 h.

[0113] S2: Mix the carbon-containing solution of S1 and a sodium hydroxide solution with a concentration of 20 g / L according to a volume ratio of 1:2, centrifuge the mixed solution at a speed of 8000 r / min for 10 min, and collect the supernatant. Transfer the supernatant to a dialysis bag with a cut-off molecular weight of 1000 D, dialyze for 12 h to obtain a dialysate (aqueous solution of carbon quantum dots). Collect the dialysate and transfer it to a freeze dryer, freeze-dry at -20 °C for 12 h to obtain nano-carbon quantum dots.

[0114] S3: Dissolve ferrous sulfate and ammonium dihydrogen phosphate in deionized water respectively according to the Fe:P molar ratio of 1.5:1 to prepare solutions. Add 20% of the mass of ferrous sulfate of nano-carbon quantum dots to the ferrous sulfate solution, and mix to obtain a first mixture. Add 10% of the mass of ferrous sulfate of acrylic acid to the ammonium dihydrogen phosphate solution, and mix to obtain a second mixture.

[0115] S4: Add the second mixture to the first mixture and add hydrogen peroxide with a molar amount 1.2 times that of iron, react at a temperature of 130 °C for 2 h, filter and wash to obtain iron phosphate dihydrate.

[0116] S5: Anneal and calcine iron phosphate dihydrate in a vacuum tube furnace at 400 °C for 8 h to obtain carbon-coated nano iron phosphate.

[0117] Comparative Example 1

[0118] The difference between this comparative example and Example 1 is that: carbon quantum dots were not added to the first mixed solution, and correspondingly, S1 and S2 were not carried out.

[0119] Comparative Example 2

[0120] The difference between this comparative example and Example 1 is that: acrylic acid was not added to the second mixed solution.

[0121] Comparative Example 3

[0122] The difference between this comparative example and Example 1 is that: carbon quantum dots were not added to the first mixed solution and acrylic acid was not added to the second mixed solution.

[0123] Comparative Example 4

[0124] The difference between this comparative example and Example 1 is that: lignin was used to replace citric acid.

[0125] Comparative Example 5

[0126] The difference between this comparative example and Example 1 is that: during the preparation of carbon quantum dots, ethylenediamine was not used to modify citric acid.

[0127] Comparative Example 6

[0128] The difference between this comparative example and Example 1 is that: trimethyloctadecylammonium bromide was used to replace acrylic acid in equal amount.

[0129] Comparative Example 7

[0130] The difference between this comparative example and Example 1 is that: glucose was used to replace acrylic acid in equal amount.

[0131] Performance Detection

[0132] The iron phosphate materials prepared in Examples 1-3 and Comparative Examples 1-7 were respectively prepared into positive electrode sheets in the following manner and assembled into button cells.

[0133] Preparation method: Weigh the iron phosphate material and lithium carbonate according to a molar ratio of P:Li of 1:1. After mixing the iron phosphate and sucrose with a mass fraction of 10% of the iron phosphate evenly, calcine it at 720 °C for 6 h under a nitrogen protection atmosphere, and then naturally cool it to room temperature to finally obtain the lithium iron phosphate cathode material. Mix the lithium iron phosphate cathode material, a conductive agent (Super P), and a binder polyvinylidene fluoride (PVDF) according to a mass ratio of 90:5:5, and add them to a solvent N-methylpyrrolidone (NMP), and stir to form a uniform positive electrode active material slurry. Coat the slurry evenly on the positive electrode current collector aluminum foil and dry it to obtain the positive electrode sheet.

[0134] Assembly method: Mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) according to a mass ratio of 1:1:1 to prepare an electrolyte with a LiPF6 concentration of 1.0 mol / L. Use a polyethylene film (PE) as the separator and a lithium metal sheet as the negative electrode to assemble a 2025 coin cell.

[0135] Perform electrochemical performance tests on the above coin cells, and conduct charge and discharge tests at room temperature from 2 V to 3.75 V. The results are shown in Table 1.

[0136] Table 1 Test results

[0137]

[0138]

[0139] As can be seen from Table 1, the iron phosphate material provided in this embodiment has a smaller particle size, and the corresponding lithium iron phosphate cathode material has good electrochemical performance.

[0140] Among them, it can be seen from Comparative Example 1 and Comparative Example 1 that adding carbon quantum dots can play a role in refining particles. It can be seen from comparing Example 1 and Comparative Example 2 that adding acrylic acid is beneficial to controlling the particle size of the material. The reason may be that it can reduce the agglomeration of particles. At the same time, it provides a carbon coating source. It can be seen from comparing Example 1 and Comparative Example 3 that through the cooperation of carbon quantum dots and acrylic acid, the size of iron phosphate can be better controlled, which is beneficial to the formation of uniformly sized nano iron phosphate, and thus is beneficial to significantly improving the electrochemical performance of the material.

[0141] In summary, the iron phosphate material provided by the present disclosure has regular morphology, uniform particles, and a small particle size, which is at the nano level. This iron phosphate material can be used to prepare a nano-level lithium iron phosphate cathode material, making it have good electrochemical performance.

[0142] Industrial applicability

[0143] The iron phosphate material provided by the present disclosure has regular morphology, uniform particles and small particle size, and can be used to prepare a lithium iron phosphate cathode material at the nanoscale. The lithium iron phosphate cathode material at the nanoscale can shorten the electron and ion transport distances, has the ability of high-current charge and discharge, and has better electrochemical performance. The preparation method of the iron phosphate material is simple, easy to operate, easy to control, the product has good stability, and is suitable for industrial production.

Claims

1. A lithium iron phosphate material, characterized in that, The iron phosphate material includes a carbon quantum dot core located inside and an iron phosphate shell formed on the surface of the carbon quantum dot core, and the surface of the iron phosphate shell has a carbon coating layer formed by a carbon-containing dispersant; The D of the iron phosphate material 50 does not exceed 200 nm; the carbon-containing dispersant includes acrylic acid.

2. The iron phosphate material according to claim 1, characterized in that, The iron phosphate material further has at least one of the following characteristics: Feature 1: The D of the carbon quantum dot core is 50 10 nm to 20 nm; Characteristic two: The iron phosphate shell is formed in situ on the surface of the carbon quantum dot core; Characteristic three: The thickness of the carbon coating layer is 1-3 nm; Characteristic four: The mass of the carbon coating layer does not exceed 5 wt% of the iron phosphate material.

3. The iron phosphate material according to claim 2, wherein The D of the iron phosphate material 50 is 20 nm to 200 nm; and / or, the mass of the carbon coating layer is 1 wt% - 5 wt% of the iron phosphate material.

4. A method for preparing the iron phosphate material according to any one of claims 1-3, characterized in that, It includes the following steps: mixing and reacting a first mixed solution, a second mixed solution, and an oxidant to obtain iron phosphate dihydrate; annealing and calcining the iron phosphate dihydrate to obtain the iron phosphate material; Among them, the first mixed solution includes a divalent iron salt and carbon quantum dots; the second mixed solution includes a phosphate and a carbon-containing dispersant; the carbon-containing dispersant includes acrylic acid; The preparation of the carbon quantum dots includes: modifying citric acid with an organic amine in a solution environment to obtain a modified solution; heating the modified solution to cause the cleavage of citric acid to obtain a carbon-containing solution; mixing the carbon-containing solution with an aqueous sodium hydroxide solution, performing solid-liquid separation, dialyzing the separated liquid phase, and drying the dialysis obtained dialysate.

5. The preparation method according to claim 4, characterized in that, The mass ratio of the citric acid to the organic amine is 1:0.1 - 1:0.

5.

6. The preparation method according to claim 4, characterized in that, The mass fraction of the citric acid in the modified solution is not less than 99.5%.

7. The preparation method according to any one of claims 4-6, characterized in that, The organic amine includes at least one of ethanolamine, ethylenediamine, and polyethylene diamine.

8. The preparation method according to claim 4, characterized in that, The heating temperature is 180°C - 280°C, and / or the heating time is 4 h - 8 h.

9. The preparation method according to claim 4, wherein The concentration of the aqueous sodium hydroxide solution is 10 g / L - 20 g / L, and the volume ratio of the carbon-containing solution to the aqueous sodium hydroxide solution is 1:1 - 1:

2.

10. The preparation method according to claim 4, characterized in that, The cut-off molecular weight of the dialysis bag used for dialysis is 500D - 1500D.

11. According to the preparation method described in claim 4, characterized in that, The drying temperature is -20°C to -60°C, and the drying time is 10 h - 14 h.

12. According to the preparation method described in claim 4, characterized in that, The mass ratio of the carbon quantum dots to the divalent iron salt is 5:100 to 20:

100.

13. The preparation method according to claim 12, characterized in that, The divalent iron salt includes at least one of ferrous sulfate, ferrous nitrate, and ferrous chloride.

14. The preparation method according to claim 4, characterized in that, The molar ratio of the iron element in the divalent iron salt to the phosphorus element in the phosphate is 1.1:1 to 1.5:

1.

15. The preparation method according to claim 14, wherein, The phosphate includes at least one of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate.

16. The preparation method according to claim 4, wherein The dosage of the carbon-containing dispersant is 3 wt% - 10 wt% of the divalent iron salt.

17. The preparation method according to claim 4, characterized in that, The oxidant includes hydrogen peroxide.

18. The preparation method according to claim 17, wherein In terms of molar amount, the added amount of the oxidant is 1 times - 1.2 times the iron element in the divalent iron salt.

19. The preparation method according to claim 4, characterized in that, The reaction temperature is 100°C - 130°C; and / or the reaction time is 2 h - 6 h.

20. The preparation method according to claim 4, wherein, The annealing and calcining include at least one of the following characteristics: Characteristic one: The temperature of the annealing and calcining is 400°C - 600°C; Characteristic two: The time of the annealing and calcining is 3 h - 8 h; Characteristic three: The annealing and calcining are carried out under vacuum conditions.

21. A lithium iron phosphate cathode material, characterized in that, The preparation raw materials of the lithium iron phosphate cathode material include the iron phosphate material according to any one of claims 1 - 3.

22. A battery, characterized in that, Containing the lithium iron phosphate cathode material according to claim 21.

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