Iron phosphate composite material, preparation method and application thereof

Abstract

This invention belongs to the field of lithium-ion battery material technology, and specifically relates to an iron phosphate composite material, its preparation method, and its application. The iron phosphate composite material comprises titanium-doped iron phosphate and nitrogen-carbon quantum dots. The preparation method includes the following steps: (1) uniformly mixing trivalent iron salt, two-dimensional titanium carbide multilayer nanosheets, and nitrogen-carbon quantum dots, and obtaining a flower-shaped framework precursor material through a hydrothermal reaction; (2) mixing the flower-shaped framework precursor material with a phosphorus source, adjusting the pH, and obtaining a flower-shaped hydrated iron phosphate material through a co-precipitation reaction; (3) sintering the hydrated iron phosphate material at high temperature and acid treatment to obtain the iron phosphate composite material. The construction of the three-dimensional flower-shaped structure, the doping of nitrogen-carbon quantum dots, and a small amount of titanium in this invention synergistically promote the rapid diffusion of lithium ions and electrolytes, improving the specific capacity of the lithium battery cathode material and its overall electrochemical performance.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to an iron phosphate composite material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as a new type of high-energy battery, have been widely used in the electric vehicle field. The main types include ternary lithium batteries and lithium iron phosphate batteries. Compared to ternary lithium batteries, lithium iron phosphate batteries with an olivine crystal structure have high safety, long cycle life, low cost, and environmental friendliness, making them the most promising lithium-ion cathode material. However, due to its structural characteristics, lithium ions can only diffuse through a one-dimensional channel, resulting in a low lithium-ion diffusion rate and poor conductivity in lithium iron phosphate (LiFePO4) materials, thus limiting the further commercial application of this cathode material. Iron phosphate (FePO4) is an important precursor for the preparation of lithium iron phosphate, and changes in its microstructure and chemical composition directly affect the performance of LiFePO4 materials. While carbon coating can improve the conductivity of iron phosphate cathode materials to some extent, it cannot fundamentally solve the problem of poor conductivity and also leads to a decrease in the battery's specific capacity. Conventional preparation methods, such as co-precipitation and high-temperature solid-state methods, often produce iron phosphate particles with severe agglomeration, large particle size, and small specific surface area. This limits the diffusion and transport of lithium ions and affects the electrical performance of lithium batteries.

[0003] Therefore, how to improve the conductivity of lithium iron phosphate while reducing its impact on specific capacity has become an urgent technical problem to be solved; and developing a simple and reliable modification method to prepare physicochemically stable iron phosphate is of great significance for obtaining high-performance lithium iron phosphate cathode materials. Summary of the Invention

[0004] In view of the problems existing in the prior art, the purpose of this invention is to provide an iron phosphate composite material, its preparation method and application, which aims to improve the specific capacity of lithium iron phosphate cathode material and enhance the overall electrochemical performance of lithium battery.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides an iron phosphate composite material, the iron phosphate composite material comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots;

[0007] The iron phosphate composite material has a three-dimensional flower-shaped morphology.

[0008] In an optional embodiment, the specific surface area of ​​the iron phosphate composite material is 7 m². 2 / g-10m 2 / g;

[0009] In an optional embodiment, the titanium content in the iron phosphate composite material is 5.0%-8.5%.

[0010] In an optional embodiment, the average particle size of the nitrogen-carbon quantum dots is 5.5 nm to 9.0 nm.

[0011] In the iron phosphate composite material prepared by this invention, the construction of a three-dimensional flower-shaped structure, the doping of nitrogen and carbon quantum dots and a small amount of titanium elements are conducive to synergistically promoting the rapid diffusion of lithium ions and electrolytes, improving the specific capacity of lithium iron phosphate cathode material and enhancing the overall electrochemical performance of lithium battery.

[0012] Secondly, the present invention provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0013] (1) A suspension was obtained by mixing ferric salt, two-dimensional titanium carbide multilayer nanosheets and nitrogen-carbon quantum dots in water, and a hydrothermal reaction was carried out to obtain a flower-shaped framework precursor material.

[0014] (2) The three-dimensional flower-shaped framework precursor material was dispersed in water and mixed with a phosphorus source to carry out a co-precipitation reaction to obtain flower-shaped hydrated iron phosphate material.

[0015] (3) The hydrated iron phosphate material is sintered to obtain titanium-doped anhydrous iron phosphate material;

[0016] (4) The titanium-doped anhydrous iron phosphate material was immersed in an acid solution to obtain an iron phosphate composite material.

[0017] This invention first uses titanium aluminum carbide (Ti3AlC2) as a precursor to prepare two-dimensional titanium carbide (Ti3C2T) by chemical etching. x Multilayer nanosheets are prepared by dissolving ferric salt in water and mixing it with the nanosheets. Under electrostatic forces, ferric ions intercalate the negatively charged titanium carbide nanosheets, thereby expanding the interlayer spacing and achieving the exfoliation of nanosheet layers to form few-layer titanium carbide nanosheets. At the same time, ferric ions adhere to the surface of the few-layer titanium carbide nanosheets. Nitrogen-carbon quantum dots (NCQDs) are added to the above mixture and stirred evenly. Then, a three-dimensional flower-shaped framework precursor material is generated by self-assembly under high temperature and high pressure using a hydrothermal method. Subsequently, the above framework precursor material is mixed with a phosphate solution, the pH of the solution is adjusted, and iron phosphate particles are grown in situ on the surface of the flower-shaped titanium carbide sheets by co-precipitation to obtain iron phosphate material. After the reaction, the flower-shaped hydrated iron phosphate material is first placed in a muffle furnace for high-temperature calcination, and then the crude product after calcination is placed in an acid solution to dissolve excess titanium elements, resulting in titanium-doped iron phosphate / nitrogen-carbon quantum dot material, i.e., iron phosphate composite material.

[0018] Titanium carbide (Ti3C2T) prepared using titanium aluminum carbide (Ti3AlC2) as a precursor x The multilayer nanosheets serve as a titanium doping source on one hand, and on the other hand, they self-assemble with iron ions in the presence of nitrogen-carbon quantum dots (NCQD) to form a three-dimensional flower-shaped structure, which also serves as a template for the generation of iron phosphate particles. After high-temperature sintering, both iron phosphate and lithium iron phosphate can inherit the three-dimensional flower-shaped morphology of the precursor material. The large specific surface area of ​​this structure is conducive to the migration and transport of lithium ions.

[0019] In an optional embodiment, the mass ratio of the trivalent iron salt to the two-dimensional titanium carbide multilayer nanosheets in step (1) is 1:(1-1.5), for example, it can be 1:1, 1:1.1, 1:1.2, 1:1.3, 1:1.4 or 1:1.5, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0020] In an optional embodiment, the nitrogen-carbon quantum dots (NCQD) mentioned in step (1) are a nitrogen-carbon quantum dot dispersion with a mass concentration of 2 mg / mL. The solid-liquid ratio of the trivalent iron salt to the nitrogen-carbon quantum dot dispersion is 1 g: (5-12) mL, for example, it can be 1 g: 5 mL, 1 g: 6 mL, 1 g: 7 mL, 1 g: 8 mL, 1 g: 9 mL, 1 g: 10 mL, 1 g: 11 mL or 1 g: 12 mL, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0021] In an optional embodiment, the temperature of the hydrothermal reaction in step (1) is 150-180°C, for example, 150°C, 155°C, 160°C, 165°C, 170°C, 175°C or 180°C, and the reaction time is 5-8h, for example, 5h, 6h, 7h or 8h, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0022] This invention introduces nitrogen-carbon quantum dots (NCQDs) into the preparation of lithium iron phosphate composite materials. These not only improve the conductivity of the cathode material, but also serve as an effective framework to support the precursor material. This helps to strengthen the structure of the flower-shaped template agent and alleviate the structural degradation and volume change of the cathode material during charging and discharging. In addition, nitrogen-carbon quantum dots can also serve as part of the carbon source for the subsequent high-temperature calcination preparation of lithium iron phosphate materials.

[0023] In an optional embodiment, after mixing in step (2), the pH value of the mixture is adjusted to 1.5-2.2 by adding ammonia or sodium hydroxide solution. For example, it can be 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1 or 2.2, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0024] In an optional embodiment, the temperature of the heating coprecipitation reaction in step (2) is 80-90°C, for example, 80°C, 82°C, 85°C, 87°C or 90°C, and the coprecipitation reaction time is 6-8h, for example, 6h, 6.5h, 7h, 7.5h or 8h, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0025] In an optional embodiment, after the coprecipitation reaction in step (2) is completed, a further aging treatment is carried out for 3-5 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours or 5 hours, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0026] In an optional embodiment, the sintering temperature in step (3) is 550-750℃, for example, 550℃, 600℃, 650℃, 700℃ or 750℃, and the sintering time is 4-6h, for example, 4h, 4.5h, 5h, 5.5h or 6h, but not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0027] In an optional embodiment, the atmosphere of the sintering process in step (3) includes an oxygen-nitrogen atmosphere, wherein the volume percentage of nitrogen in the oxygen-nitrogen atmosphere is 95% and the volume percentage of oxygen is 4% to 5%, for example, it can be 4%, 4.2%, 4.4%, 4.6%, 4.8% or 5%, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0028] In an optional embodiment, the acid solution in step (4) includes hydrofluoric acid, and the molar concentration of the hydrofluoric acid solution is 0.1-0.2 mol / L, for example, it can be 0.1 mol / L, 0.12 mol / L, 0.15 mol / L, 0.18 mol / L or 0.2 mol / L, and the soaking time is 2-3 h, for example, it can be 2 h, 2.2 h, 2.5 h, 2.7 h or 3 h, but it is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0029] In an optional embodiment, the trivalent ferric salt includes any one or a combination of at least two of ferric chloride, ferric sulfate, or ferric nitrate.

[0030] In an optional embodiment, the phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate.

[0031] In an optional embodiment, the molar ratio of iron in the trivalent iron salt to phosphorus in the phosphorus source is 1:1.

[0032] The iron phosphate particles prepared by this invention grow in situ on the surface of two-dimensional titanium carbide nanosheets that make up the flower-shaped precursor material. The titanium dioxide formed by calcining titanium carbide is removed by immersion in hydrofluoric acid, while some of the residual titanium elements are doped into the lattice of iron phosphate, causing internal lattice defects in the material and inhibiting lattice distortion. This not only reduces the lithium ion insertion / extraction resistance, but the fine grains formed can also improve the electronic / ionic conductivity of the material.

[0033] Thirdly, the present invention provides a lithium iron phosphate cathode material, which is prepared by sintering the iron phosphate composite material described in the second aspect with a lithium source and a carbon source.

[0034] Fourthly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material as described in the third aspect.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] (1) The iron phosphate composite material prepared by the present invention has a three-dimensional flower-shaped structure. Compared with ordinary spherical particles, this structure has a highly open framework, which is conducive to the full exposure of active sites and the full contact between the cathode material and the battery electrolyte. At the same time, the material of this structure has a large specific surface area, which effectively shortens the lithium ion transport path and promotes its lithiation / delithiation process, thereby improving the rate performance of lithium iron phosphate batteries.

[0037] (2) The composite iron phosphate material prepared by the present invention has good performance indicators. The lithium iron phosphate cathode material prepared by it also exhibits excellent electrochemical performance after being assembled into a lithium battery. Its charge and discharge capacity at 0.1C and 0.5C can reach up to 156.6mAh / g and 149.7mAh / g, respectively; and its first charge and discharge efficiency can reach up to 97.45%.

[0038] After reading and understanding the accompanying diagrams and detailed descriptions, the other aspects can be understood. Attached Figure Description

[0039] The accompanying drawings are used to provide a further understanding of the technical solutions in this paper and form part of the specification. They are used together with the embodiments of this application to explain the technical solutions in this paper and do not constitute a limitation on the technical solutions in this paper.

[0040] Figure 1 The XRD pattern of the iron phosphate composite material prepared in Example 1 of this invention is shown below.

[0041] Figure 2 The XPS spectrum of the iron phosphate composite material prepared in Example 1 of this invention;

[0042] Figure 3This is a SEM image of the iron phosphate composite material prepared in Example 1 of this invention;

[0043] Figure 4 This is a SEM image of the lithium iron phosphate material prepared in Example 1 of this invention;

[0044] Figure 5 This is a TEM image of the iron phosphate composite material prepared in Example 1 of this invention. Detailed Implementation

[0045] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of this disclosure and should not be construed as limiting the scope of the invention.

[0046] It should be noted that, unless otherwise specified in the examples, the conditions should be performed under standard conditions or conditions recommended by the manufacturer. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0047] The two-dimensional titanium carbide (Ti3C2T) used in the embodiments and comparative examples of this invention x Both the multilayer nanosheets and the nitrogen-carbon quantum dot (NCQD) dispersions were prepared using known existing techniques, and the preparation methods included the following steps:

[0048] (1) Dissolve 1.26 g NaF in hydrochloric acid solution (100 mL, 0.3 mol / L), stir well, and then add 3.92 g titanium aluminum carbide powder (Ti3AlC2). Continue stirring at 40 °C for 24 h. Wash the product with distilled water until the pH of the supernatant is close to neutral. Then wash with anhydrous ethanol and vacuum dry to obtain multilayer blocky titanium carbide (Ti3C2T). x (T) x (Refers to surface groups -F, -OH, -O). The above multilayered bulk Ti3C2T... x The precipitate was dispersed in dimethyl sulfoxide (DMSO) solution and stirred at room temperature for 12 h. The precipitate was then collected by centrifugation. Subsequently, the precipitate was dispersed in 100 ml of deionized water, sonicated for 3 h, and centrifuged for 1 h to obtain a black supernatant. This supernatant was collected by filtration and vacuum dried to obtain two-dimensional titanium carbide (Ti3C2T). x Multilayer nanosheet powder.

[0049] (2) Dissolve 0.50g citric acid and 0.25g acrylamide in 30mL deionized water, mix evenly by ultrasonication, pour the solution into a 50mL polytetrafluoroethylene-lined reactor, and heat it in an oven at 180℃ for 5h. After the reaction is completed, filter the reaction solution through a 0.22μm filter membrane, then dialyze it in deionized water for 48h using a dialysis bag (molecular weight cutoff 1000), freeze-dry it for 24h to obtain a light yellow carbon dot powder, which is NCQD. Finally, make up to 2mg / mL of nitrogen-carbon quantum dot dispersion and store it in a refrigerator at 4℃ for later use.

[0050] The specific surface area of ​​the iron phosphate composite material prepared by the method of the present invention was determined by a specific surface area analyzer; the titanium content was determined by an inductively coupled plasma mass spectrometer; and the particle size of the nitrogen and carbon quantum dots was determined by a transmission electron microscope.

[0051] Example 1

[0052] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 9.3 m². 2 / g; the titanium content in the iron phosphate composite material is 7.65%; the average particle size of the nitrogen-carbon quantum dots is 7.0nm.

[0053] This embodiment provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0054] (1) Dissolve 0.562 g (0.001 mol) of ferric sulfate nonahydrate in 70 mL of deionized water, and then take 0.731 g of Ti3C2T x Multilayer nanosheets were added to the above solution and ultrasonically mixed until homogeneous. Then, 4.50 mL of NCQD (2 mg / mL) dispersion was added to the mixture, and stirring was continued for 0.5 h. The mixture was then placed in a 100 mL polytetrafluoroethylene-lined reactor and subjected to a hydrothermal reaction at 170 °C for 6 h. The product was then collected by centrifugation and washed multiple times with deionized water and anhydrous ethanol to obtain a three-dimensional flower-shaped framework precursor material. Among these, trivalent iron salts and Ti3C2T... x The mass ratio is 1:1.3; the solid-liquid ratio of ferric salt to NCQD dispersion is 1g:8mL.

[0055] (2) The three-dimensional flower-shaped framework precursor material was dispersed in 50 mL of deionized water, and then 0.132 g (0.001 mol) of diammonium hydrogen phosphate was added. After stirring evenly, ammonia water was continuously added dropwise to adjust the pH of the mixture to 1.8. Then, a co-precipitation reaction was carried out: the reaction temperature of the system was raised to 85 °C and kept under water bath conditions for 6 h. After the reaction was completed, the mixture was aged for 4 h, centrifuged to collect and wash the product, and vacuum dried to obtain the hydrated iron phosphate material with a three-dimensional flower-shaped structure.

[0056] (3) The hydrated iron phosphate material was placed in a muffle furnace and heated to 650°C at a rate of 10°C / min and sintered in an oxygen-nitrogen atmosphere for 4 hours to obtain titanium-doped anhydrous iron phosphate material.

[0057] (4) The titanium-doped anhydrous iron phosphate material was immersed in hydrofluoric acid solution (0.15 mol / L) for 2 hours to dissolve the excess titanium element. The product was then collected by centrifugation, washed, and dried under vacuum to obtain the iron phosphate composite material.

[0058] The XRD pattern of the iron phosphate composite material prepared in this embodiment is as follows: Figure 1 As shown, the XPS spectrum is as follows Figure 2 As shown. From Figure 1 As can be seen from the data, the titanium-doped iron phosphate composite material of Example 1 has the same characteristic peaks as the FePO4 standard card, with no other impurity diffraction peaks, indicating that the residual titanium element has entered the interior of the iron phosphate crystal structure and formed a continuous solid solution without affecting its phase structure; from Figure 2 As can be seen from the XPS spectrum of the titanium-doped iron phosphate composite material in Example 1, peaks corresponding to Fe, Li, P, and Ti elements appear, further indicating that titanium elements have been successfully doped into the structure of iron phosphate.

[0059] SEM images of the iron phosphate composite material and lithium iron phosphate material prepared in this embodiment are shown below. Figure 3 and 4 As can be seen, both exhibit a three-dimensional flower-shaped structure.

[0060] The TEM image of the iron phosphate composite material prepared in this embodiment is shown below. Figure 5 As shown, nitrogen-carbon quantum dots are uniformly distributed on the iron phosphate composite material, with an average particle size of 7.0 nm.

[0061] Example 2

[0062] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 8.5 m². 2 / g; the titanium content in the iron phosphate composite material is 5.35%; the average particle size of the nitrogen-carbon quantum dots is 6.6nm.

[0063] This embodiment provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0064] (1) Dissolve 0.270 g (0.001 mol) of ferric chloride hexahydrate in 70 mL of deionized water, and then take 0.297 g of Ti3C2T xMultilayer nanosheets were added to the above solution and stirred until homogeneous. Then, 2.16 mL of NCQD (2 mg / mL) dispersion was added to the mixture, and stirring continued for 0.5 h. The mixture was then placed in a 100 mL polytetrafluoroethylene-lined reactor and subjected to a hydrothermal reaction at 175 °C for 7 h. The product was then collected by centrifugation and washed multiple times with deionized water and anhydrous ethanol to obtain a three-dimensional flower-shaped framework precursor material. Among these, trivalent iron salts and Ti3C2T... x The mass ratio is 1:1.1; the solid-liquid ratio of ferric salt to NCQD dispersion is 1g:8mL.

[0065] (2) The three-dimensional flower-shaped framework precursor material was dispersed in 50 mL of deionized water, and then 0.115 g (0.001 mol) of ammonium dihydrogen phosphate was added. After stirring evenly, ammonia water was continuously added dropwise to adjust the pH of the mixture to 1.5. Then, a co-precipitation reaction was carried out: the reaction temperature of the system was raised to 85 °C and kept under water bath conditions for 7 h. After the reaction was completed, the mixture was aged for 3 h, centrifuged to collect and wash the product, and vacuum dried to obtain the hydrated iron phosphate material with a three-dimensional flower-shaped structure.

[0066] (3) The hydrated iron phosphate material was placed in a muffle furnace and heated to 550°C at a rate of 10°C / min and sintered in an oxygen-nitrogen atmosphere for 6 hours to obtain titanium-doped anhydrous iron phosphate material.

[0067] (4) The titanium-doped anhydrous iron phosphate material was immersed in hydrofluoric acid solution (0.1 mol / L) for 2 hours to dissolve the excess titanium element. The product was then collected by centrifugation, washed, and dried under vacuum to obtain the iron phosphate composite material.

[0068] Example 3

[0069] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 9.0 m². 2 / g; the titanium content in the iron phosphate composite material is 8.28%; the average particle size of the nitrogen-carbon quantum dots is 8.3nm.

[0070] This embodiment provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0071] (1) Dissolve 0.404 g (0.001 mol) of ferric nitrate nonahydrate in 70 mL of deionized water, and then take 0.566 g of Ti3C2T xMultilayer nanosheets were added to the above solution and ultrasonically mixed thoroughly. Then, 4.04 mL of NCQD (2 mg / mL) dispersion was added to the mixture, and stirring was continued for 0.5 h. The mixture was then placed in a 100 mL polytetrafluoroethylene-lined reactor and subjected to a hydrothermal reaction at 165 °C for 8 h. The product was then collected by centrifugation and washed multiple times with deionized water and anhydrous ethanol to obtain a three-dimensional flower-shaped framework precursor material. Among these, trivalent iron salts and Ti3C2T... x The mass ratio is 1:1.4; the solid-liquid ratio of ferric salt to NCQD dispersion is 1g:10mL.

[0072] (2) The three-dimensional flower-shaped framework precursor material was dispersed in 50 mL of deionized water, and then 0.132 g (0.001 mol) of diammonium hydrogen phosphate was added. After stirring evenly, ammonia water was continuously added dropwise to adjust the pH of the mixture to 2.2. Then, a co-precipitation reaction was carried out: the reaction temperature of the system was raised to 85 °C and kept under water bath conditions for 8 h. After the reaction was completed, it was aged for 5 h. The product was collected by centrifugation, washed, and vacuum dried to obtain the hydrated iron phosphate material with a three-dimensional flower-shaped structure.

[0073] (3) The hydrated iron phosphate material was placed in a muffle furnace and heated to 750°C at a rate of 10°C / min and sintered in an oxygen-nitrogen atmosphere for 5 hours to obtain titanium-doped anhydrous iron phosphate material.

[0074] (4) The titanium-doped anhydrous iron phosphate material was immersed in hydrofluoric acid solution (0.2 mol / L) for 2 hours to dissolve the excess titanium element. The product was then collected by centrifugation, washed, and dried under vacuum to obtain the iron phosphate composite material.

[0075] Example 4

[0076] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 8.4 m². 2 / g; the titanium content in the iron phosphate composite material is 5.0%; the average particle size of the nitrogen-carbon quantum dots is 7.3nm.

[0077] This embodiment provides a method for preparing an iron phosphate composite material. The difference between this method and Example 1 lies only in the change of Ti3C2T. x The nanosheets weighed 0.562 g, and all other conditions and procedures were exactly the same as in Example 1. (Fe3C2T) x The mass ratio of the two is 1:1.

[0078] Example 5

[0079] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 8.7 m². 2 / g; the titanium content in the iron phosphate composite material is 8.50%; the average particle size of the nitrogen-carbon quantum dots is 6.8nm.

[0080] This embodiment provides a method for preparing an iron phosphate composite material. The difference between this method and Example 1 lies only in the change of Ti3C2T. x The nanosheets weighed 0.843 g, and all other conditions and procedures were exactly the same as in Example 1. (Fe3+ and Ti3C2T) x (The mass ratio of the two is 1:1.5)

[0081] Example 6

[0082] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 7.8 m². 2 / g; the titanium content in the iron phosphate composite material is 7.61%; the average particle size of the nitrogen-carbon quantum dots is 5.8nm.

[0083] This embodiment provides a method for preparing an iron phosphate composite material. The only difference between this method and Example 1 is that the volume of the NCQD dispersion is changed to 2.81 mL; all other conditions and steps are exactly the same as in Example 1. (The solid-liquid ratio of ferric salt to NCQD dispersion is 1 g: 5 mL.)

[0084] Example 7

[0085] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 7.6 m². 2 / g; the titanium content in the iron phosphate composite material is 7.68%; the average particle size of the nitrogen-carbon quantum dots is 9.0nm.

[0086] This embodiment provides a method for preparing an iron phosphate composite material. The only difference between this method and Example 1 is that the volume of the NCQD dispersion is changed to 6.75 mL; all other conditions and steps are exactly the same as in Example 1. (The solid-liquid ratio of ferric salt to NCQD dispersion is 1 g: 12 mL.)

[0087] Example 8

[0088] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 8.0 m². 2 / g; the titanium content in the iron phosphate composite material is 7.64%; the average particle size of the nitrogen-carbon quantum dots is 7.5nm.

[0089] This embodiment provides a method for preparing an iron phosphate composite material. The only difference between this method and Example 1 is that the hydrothermal reaction temperature is changed to 150°C and the reaction time is changed to 6 hours. All other conditions and steps are exactly the same as in Example 1.

[0090] Example 9

[0091] This embodiment provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 8.3 m². 2 / g; the titanium content in the iron phosphate composite material is 7.70%; the average particle size of the nitrogen-carbon quantum dots is 7.3nm.

[0092] This embodiment provides a method for preparing an iron phosphate composite material. The only difference between this method and Example 1 is that the hydrothermal reaction temperature is changed to 180°C and the reaction time is changed to 6 hours. All other conditions and steps are exactly the same as in Example 1.

[0093] Comparative Example 1

[0094] Compared with Example 1, iron phosphate material modified with nitrogen and carbon quantum dots without titanium element doping was prepared directly using the co-precipitation method.

[0095] This comparative example provides an iron phosphate composite material, comprising iron phosphate and nitrogen-carbon quantum dots. The iron phosphate composite material has an irregular granular morphology and a specific surface area of ​​2.8 m². 2 / g, the average particle size of nitrogen-carbon quantum dots is 7.3nm.

[0096] This comparative example provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0097] 0.562 g (0.001 mol) of ferric sulfate nonahydrate and 0.132 g (0.001 mol) of diammonium hydrogen phosphate were dissolved in 50 mL of deionized water and stirred evenly. Then, 4.50 mL of nitrogen-carbon quantum dot (NCQD) dispersion was added to the mixture. Ammonia was continuously added dropwise to adjust the pH of the mixture to 1.5. The temperature of the reaction system was then raised to 85 °C and maintained in a water bath for 6 h. After the reaction was completed, the mixture was allowed to stand and age for 4 h. The product was collected and washed with deionized water and anhydrous ethanol in sequence. After vacuum drying, crude hydrated ferric phosphate was obtained. The above material was placed in a muffle furnace and sintered at 650 °C at a rate of 10 °C / min in an oxygen-nitrogen atmosphere for 4 h to obtain the ferric phosphate composite material.

[0098] Comparative Example 2

[0099] This comparative example provides an iron phosphate composite material, including titanium-doped iron phosphate. The iron phosphate composite material has an irregular granular morphology and a specific surface area of ​​3.2 m². 2 / g, the titanium content in the iron phosphate composite material is 7.60%.

[0100] This comparative example provides a method for preparing an iron phosphate composite material. Compared with Example 1, the method does not add nitrogen carbon quantum dot (NCQD) dispersion to the mixture, while the other conditions and steps are exactly the same as in Example 1.

[0101] Comparative Example 3

[0102] Compared to Example 1, Fe was not used. 3+ For Ti3C2T x Intercalation of multilayer nanosheets: First, prepare NCQD-Ti3C2T x Flower-shaped structural materials are then used as templates to prepare iron phosphate materials.

[0103] This comparative example provides an iron phosphate composite material, comprising titanium-doped iron phosphate and nitrogen-carbon quantum dots. The specific surface area of ​​the iron phosphate composite material is 4.0 m². 2 / g; the titanium content in the iron phosphate composite material is 7.67%; the average particle size of the nitrogen-carbon quantum dots is 7.4nm.

[0104] This comparative example provides a method for preparing an iron phosphate composite material, the method comprising the following steps:

[0105] Take 0.675g Ti3C2T xMultilayer nanosheets and 4.50 mL of nitrogen-carbon quantum dot (NCQD) dispersion were added to 70 mL of deionized water and stirred until homogeneous. The mixture was then placed in a 100 mL polytetrafluoroethylene-lined reactor and heated at 170 °C for 6 h. The product was then collected by centrifugation and washed to obtain a three-dimensional flower-shaped framework precursor material. The washed material was dispersed in 50 mL of deionized water, and 0.562 g (0.001 mol) of ferric sulfate nonahydrate and 0.132 g (0.001 mol) of diammonium hydrogen phosphate were added sequentially. After stirring until homogeneous, ammonia was continuously added dropwise to adjust the pH of the mixture. With an H value of 1.5, the temperature of the reaction system was then raised to 85°C and maintained in a water bath for 6 hours. After the reaction, the mixture was allowed to stand and age for 4 hours. The product was collected by centrifugation, washed, and then dried under vacuum to obtain a hydrated iron phosphate material with a three-dimensional flower-shaped structure. The material was then placed in a muffle furnace and sintered at 650°C at a rate of 10°C / min in an oxygen-nitrogen atmosphere for 4 hours to obtain a titanium-doped anhydrous iron phosphate material. Subsequently, the material was immersed in a hydrofluoric acid solution (0.1 mol / L) for 2 hours to dissolve excess titanium elements. The product was then collected by centrifugation, washed, and dried under vacuum to obtain an iron phosphate composite material.

[0106] The iron phosphate composite materials obtained in Examples 1-9 and Comparative Examples 1-3 were used to prepare lithium iron phosphate cathode materials. These lithium iron phosphate cathode materials were then formulated into coin cells, and their electrochemical performance was evaluated. The specific steps included:

[0107] (1) Preparation of lithium iron phosphate cathode material

[0108] Lithium carbonate, iron phosphate composite material, and glucose were dispersed in anhydrous ethanol according to a stoichiometric ratio of lithium source, iron source, and carbon source of 1:1:0.05. The mixture was ball-milled for 3 hours at 3000 rpm until homogeneous, followed by spray drying to obtain precursor powder. Subsequently, the precursor powder was heated to 400℃ for 3 hours under a high-purity argon atmosphere at a heating rate of 8℃ / min, and then calcined at 700℃ for 8 hours to obtain lithium iron phosphate cathode material.

[0109] (2) Preparation of button cells

[0110] Lithium iron phosphate cathode material, conductive agent acetylene black, and adhesive polyvinylidene fluoride were uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to form a slurry. The slurry was then coated onto aluminum foil and dried in a vacuum drying oven. The cathode was then pressed into a positive electrode sheet using a tablet press, while the negative electrode sheet was a lithium metal sheet. The electrolyte was a 1 mol / L lithium hexafluorophosphate-ethylene carbonate:dimethyl carbonate (LiPF6-EC:DMC, volume ratio 1:1). A polypropylene porous membrane was used as the separator. The battery assembly was carried out in an argon glove box.

[0111] (3) Electrochemical performance testing

[0112] The assembled coin cells were subjected to electrochemical performance tests within a charge / discharge voltage range of 2.5-4.5V. First, they were charged to 4.5V using a constant current, then discharged to 2.5V using a higher rate current. The capacity discharged was the discharge capacity at that rate. After discharge, they were discharged again to 2.5V using a constant current current. The next rate test was then conducted. The test results are shown in Table 1.

[0113] Table 1

[0114]

[0115] As can be seen from Table 1, as obtained from Examples 1-9, the lithium iron phosphate cathode material prepared by the method of the present invention can be configured into a coin cell with a discharge specific capacity of over 156.6 mAh / g at 0.1C, a discharge specific capacity of over 149.7 mAh / g at 0.5C, and an initial charge-discharge efficiency of over 97.45%.

[0116] A comparison of Examples 1 and 4-5 shows that, in the preparation process of the iron phosphate composite material of the present invention, the trivalent iron salt reacts with Ti3C2T x The mass ratio of titanium to phosphate affects the performance of the cathode material. If the mass ratio is too low, the amount of titanium doping will be low, which is not conducive to improving the conductivity and rate performance of the cathode material. If the mass ratio is too high, the amount of titanium doping will increase, which will make the structure of the cathode material unstable, reduce the electron and ion transport efficiency, and thus affect the electrical performance of the lithium battery.

[0117] A comparison of Examples 1 and 6-7 reveals that the solid-liquid ratio of ferric salt to NCQD dispersion affects the performance of the iron phosphate composite material described in this invention. If the solid-liquid ratio is too low, the amount of carbon nitrogen-carbon quantum dots (NCQDs) added is insufficient, making it difficult to support the flower-shaped framework of the precursor material, thus affecting the subsequent formation of the iron phosphate material and hindering the improvement of the cathode material's conductivity. Conversely, if the solid-liquid ratio is too high, excessive NCQD addition not only clogs the pore structure of the precursor material but also occupies active sites on the material surface, affecting the Fe... 3+ With PO4 3+ The combination of [the elements] and the formation of iron phosphate materials.

[0118] A comparison of Examples 1 and 8-9 shows that during the preparation of the iron phosphate composite material of the present invention, the temperature of the hydrothermal reaction during the synthesis of the precursor material affects its performance. The construction of the three-dimensional flower-shaped structure of the precursor material requires the high temperature and high pressure generated during the hydrothermal process as a driving force. A lower reaction temperature will affect the construction of its three-dimensional flower-shaped structure, thereby affecting the electrical performance of the subsequently prepared lithium battery cathode material. On the other hand, a higher reaction temperature will cause particle agglomeration to a certain extent, resulting in larger particle size, which in turn affects the migration and transport of lithium ions in the cathode material.

[0119] Comparing Example 1 and Comparative Examples 1-2, it can be seen that the iron phosphate composite material prepared by the present invention has a three-dimensional flower-shaped structure and contains titanium and nitrogen-carbon quantum dots, while the iron phosphate materials of Comparative Examples 1-2 are all solid and irregular structures. The iron phosphate of Comparative Example 1 is directly synthesized by co-precipitation without the addition of titanium, and the iron phosphate of Comparative Example 2 does not contain nitrogen-carbon quantum dots during synthesis, so there is no effective framework to support the flower-shaped structure of the precursor material. This results in both having lower specific surface area, poorer conductivity, and slower lithium-ion diffusion rate, so the electrical performance of the lithium battery cathode material prepared from them is also poor.

[0120] A comparison of Example 1 and Comparative Example 3 shows that the lithium cathode material prepared by the iron phosphate composite material of the present invention has superior electrical properties, while Comparative Example 3, due to the lack of prior use of Fe... 3+ For Ti3C2T x Nanosheets are intercalated to increase the interlayer spacing, hence Ti3C2T x Nanosheets may stack during the self-assembly of precursor materials to form flower-like structures, and Fe 3+ It also cannot be evenly distributed on its surface, which to some extent affects the structure of the product and thus the electrical performance of the lithium battery.

[0121] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing an iron phosphate composite material, characterized in that, Includes the following steps: (1) A suspension was obtained by mixing trivalent iron salt, two-dimensional titanium carbide multilayer nanosheets and nitrogen carbon quantum dots in water, and a hydrothermal reaction was carried out to obtain a flower-shaped framework precursor material. (2) The flower-shaped framework precursor material is dispersed in water and mixed with a phosphorus source to obtain a mixed solution, and a co-precipitation reaction is carried out to obtain a flower-shaped hydrated iron phosphate material; (3) The hydrated iron phosphate material is sintered to obtain titanium-doped anhydrous iron phosphate material; (4) The titanium-doped anhydrous iron phosphate material was immersed in an acid solution to obtain an iron phosphate composite material.

2. The preparation method according to claim 1, characterized in that, In step (1), ferric salt and two-dimensional titanium carbide multilayer nanosheets are first dispersed in water to form a dispersion, then nitrogen-carbon quantum dots are dispersed in water to obtain a nitrogen-carbon quantum dot dispersion, and finally the two dispersions are mixed to obtain a suspension.

3. The preparation method according to claim 2, characterized in that, The mass ratio of the trivalent iron salt to the two-dimensional titanium carbide multilayer nanosheets is 1:(1-1.5).

4. The preparation method according to claim 2, characterized in that, The mass concentration of the nitrogen-carbon quantum dot dispersion is 2 mg / mL, and the solid-liquid ratio of the trivalent iron salt to the nitrogen-carbon quantum dot dispersion is 1 g: (5-12) mL.

5. The preparation method according to claim 1, characterized in that, The hydrothermal reaction temperature is 150-180℃, and the hydrothermal reaction time is 5-8 hours.

6. The preparation method according to claim 1, characterized in that, In step (2), one or more of the following features one to three are satisfied: Feature 1: After mixing, the pH of the mixture is adjusted to 1.5-2.2 by adding ammonia or sodium hydroxide solution; Feature 2: The temperature of the coprecipitation reaction is 80-90℃, and the time of the coprecipitation reaction is 6-8h; Feature 3: After the co-precipitation reaction is completed, a further aging treatment is carried out for 3-5 hours.

7. The preparation method according to claim 1, characterized in that, The sintering temperature in step (3) is 550-750℃, and the sintering time is 4-6h; The atmosphere for the sintering process includes oxygen-containing nitrogen gas, in which nitrogen accounts for 95% by volume and oxygen accounts for 4% to 5%.

8. The preparation method according to claim 1, characterized in that, The acid solution mentioned in step (4) includes hydrofluoric acid, with a molar concentration of 0.1-0.2 mol / L and a soaking time of 2-3 h.

9. The preparation method according to claim 1, characterized in that, The trivalent ferric salt includes any one or a combination of at least two of ferric chloride, ferric sulfate, or ferric nitrate. The phosphorus source includes any one or a combination of at least two of phosphoric acid, ammonium dihydrogen phosphate, or diammonium hydrogen phosphate. The molar ratio of iron in the ferric salt to phosphorus in the phosphorus source is 1:

1.

10. The preparation method according to claim 1, characterized in that, The iron phosphate composite material includes titanium-doped iron phosphate and nitrogen-carbon quantum dots; The iron phosphate composite material has a three-dimensional flower-shaped morphology.

11. The preparation method according to claim 1, characterized in that, The iron phosphate composite material satisfies one or more of the following characteristics (1 to 3): Feature 1: The specific surface area of ​​the iron phosphate composite material is 7 m². 2 / g-10m 2 / g; Feature 2: The titanium content in the iron phosphate composite material is 5.0%-8.5%; Feature 3: The average particle size of the nitrogen-carbon quantum dots is 5.5nm-9.0nm.

12. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by mixing and sintering the lithium iron phosphate composite material provided by the preparation method according to any one of claims 1 to 11 with a lithium source and a carbon source.

13. A lithium-ion battery, characterized in that, The lithium-ion battery includes the lithium iron phosphate cathode material as described in claim 12.

Images