Composite iron phosphate, and preparation method and application thereof

Abstract

This invention provides a composite iron phosphate, its preparation method, and its application. The preparation method includes the following steps: (1) mixing ferrous phosphate suspension with dopamine hydrochloride and reacting to obtain a mixed solution; (2) adjusting the pH of the mixed solution, adding a ferric iron source and a phosphorus source, and reacting by heating to obtain an intermediate; (3) mixing the intermediate with a phosphoric acid solution, reacting by heating and oxidation to obtain a mixture, and calcining the mixture to obtain the composite iron phosphate. This invention prepares a composite material of iron phosphate with large-particle-size hollow and small-particle-size solid particles. After the composite material is made into a lithium iron phosphate cathode material, it will inherit its morphology and structure. The structure can shorten the Li... + This improves the diffusion rate by optimizing the diffusion path. Simultaneously, the coexistence of large and small particle sizes increases the tap density of lithium iron phosphate, thereby reducing capacity loss caused by hollow lithium iron phosphate.

Description

Technical Field

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

[0002] Lithium iron phosphate (LiFePO4) is a type of lithium... + The main advantages of LiFePO4 as a battery cathode material include its wide availability of raw materials, environmental friendliness, high theoretical specific capacity (170 mAh / g), and stable discharge platform. Compared with layered materials, its most prominent advantage is that the phosphate ions in LiFePO4 are very stable anionic groups that do not decompose and release oxygen during charging and discharging. Furthermore, the material itself exhibits relatively small volume strain, thus attracting widespread attention.

[0003] However, the crystal structure of LiFePO4 results in its low electronic conductivity, which to some extent limits the potential of Li... + The diffusion motion causes it to diffuse along the one-dimensional channel, resulting in an extremely low ion diffusion rate (10⁻⁶) for LiFePO₄ materials. -17 ~10 -14 cm 2 / s).

[0004] CN115849321A discloses a method for preparing FePO4 hollow microspheres for lithium-ion battery cathode materials. The preparation method includes the preparation of Fe(OH)3 embedded carbon sphere precursor, the pretreatment of Fe(OH)3 / C sphere precursor, and the formation and removal of FePO4 hollow microspheres and carbon spheres.

[0005] CN117509593A discloses a lithium iron phosphate material and its preparation method. The method uses a microwave-assisted solvothermal method to construct nano-lithium iron phosphate material, which has a hollow bowl-shaped loose structure.

[0006] The hollow lithium iron phosphate material prepared by the above method has a low volumetric capacity and poor performance, which is not conducive to practical applications. Summary of the Invention

[0007] The purpose of this invention is to provide a composite iron phosphate, its preparation method, and its application. This invention prepares a composite material of large-particle-size hollow iron phosphate and small-particle-size solid iron phosphate. When this composite material is used to make a lithium iron phosphate cathode material, it will inherit its morphology and structure. This structure can shorten the lifespan of the lithium iron phosphate cathode. + This improves the diffusion rate by optimizing the diffusion path. Simultaneously, the coexistence of large and small particle sizes increases the tap density of lithium iron phosphate, thereby reducing capacity loss caused by hollow lithium iron phosphate.

[0008] To achieve this objective, the present invention employs the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing composite iron phosphate, the method comprising the following steps:

[0010] (1) The ferrous phosphate suspension was mixed with dopamine hydrochloride and reacted to obtain a polydopamine-coated ferrous phosphate suspension.

[0011] (2) After adjusting the pH of the polydopamine-coated ferrous phosphate suspension, a ferric source and a phosphorus source were added, and an intermediate was obtained by heating in one step.

[0012] (3) The intermediate is mixed with phosphoric acid solution, and after a two-step heating reaction, an oxidant is added to carry out an oxidation reaction to obtain a mixture. The mixture is then calcined to obtain the composite iron phosphate.

[0013] This invention utilizes dopamine hydrochloride to coat ferrous phosphate, forming a polydopamine layer. Then, leveraging the adsorption properties of polydopamine, a ferric salt solution and a phosphorus source are added. After heating and reaction, an ferric phosphate layer is formed outside the polydopamine layer. The resulting material is then placed in a phosphoric acid solution to dissolve the ferric hydroxide and ferrous phosphate formed from the ferric iron. The dissolved solution contains hollow ferric phosphate, ferrous ions, and phosphate ions. Hydrogen peroxide is added to the solution to oxidize the ferrous iron to ferric ions, which then combine with the phosphate ions to form new ferric phosphate particles. Ultimately, hollow, large-particle-size ferric phosphate and secondary-formed, small-particle-size ferric phosphate are formed.

[0014] Preferably, step (1) is performed under a protective atmosphere.

[0015] Preferably, the protective atmosphere includes any one or a combination of at least two of nitrogen, helium, or argon, with typical but non-limiting combinations including combinations of nitrogen and helium, combinations of nitrogen and argon, etc.

[0016] Preferably, the ferrous phosphate suspension in step (1) is prepared by the following method:

[0017] After adjusting the pH of the phosphorus source solution, it was mixed with a ferrous iron source and stirred to obtain the ferrous phosphate suspension.

[0018] Preferably, the pH is 3.8 to 4.0, for example: 3.8, 3.85, 3.9, 3.95 or 4.0, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] Preferably, the pH adjuster includes any one or a combination of at least two of ammonia, sodium hydroxide, sodium bicarbonate, or sodium acetate. Typical but non-limiting combinations include a combination of ammonia and sodium hydroxide, a combination of ammonia and sodium bicarbonate, or a combination of sodium acetate and sodium hydroxide.

[0020] Preferably, the phosphorus source includes any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, Na2HPO4, or NaH2PO4. Typical but non-limiting combinations include H3PO4, a combination of (NH4)2HPO4 and NH4H2PO4, a combination of H3PO4 and (NH4)2HPO4, a combination of (NH4)3PO4 and NaH2PO4, etc.

[0021] Preferably, the divalent iron source includes any one or a combination of at least two of ferrous sulfate, ferrous nitrate, ferrous oxalate, or ferrous chloride.

[0022] Preferably, the molar ratio of phosphorus to iron in the phosphorus source solution is 1:(1.4 to 1.5), for example: 1:1.4, 1:1.42, 1:1.45, 1:1.48 or 1:1.5, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] Preferably, the stirring reaction speed is 600-800 rpm, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] Preferably, the pH of the stirring reaction is <6.0.

[0025] Preferably, the mass ratio of divalent iron source in the dopamine hydrochloride and ferrous phosphate suspension in step (1) is (0.5-1):(5-12), for example: 1:5, 0.6:8, 1:10, 0.8:10 or 0.5:12, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Preferably, the reaction in step (1) includes an ultrasonic reaction.

[0027] Preferably, the reaction time in step (1) is 20 to 30 minutes, for example: 20 minutes, 22 minutes, 25 minutes, 28 minutes or 30 minutes, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0028] Preferably, in step (2), the second pH is 2 to 5, for example: 2, 2.2, 3, 3.5 or 5, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0029] Preferably, the molar ratio of ferric ions in the ferric iron source to ferrous ions in the ferrous phosphate suspension in step (2) is 1:(0.3 to 0.6), for example: 1:0.3, 1:0.35, 1:0.4, 1:0.5 or 1:0.6, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0030] Preferably, the trivalent iron source in step (2) includes any one or a combination of at least two of ferric nitrate, ferric sulfate, or ferric chloride.

[0031] Preferably, the molar ratio of iron in the trivalent iron source and phosphorus in the phosphorus source in step (2) is (0.8 to 1.2):1, for example: 0.8:1, 0.9:1, 1:1, 1.1:1 or 1.2:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0032] Preferably, the temperature of the one-step heating reaction in step (2) is 50 to 70°C, for example: 50°C, 55°C, 60°C, 65°C or 70°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0033] Preferably, the stirring speed of the one-step heating reaction in step (2) is 600 to 1000 rpm, for example: 600 rpm, 700 rpm, 800 rpm, 900 rpm or 1000 rpm, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0034] Preferably, the step (2) involves filtration, washing, and drying after the heating reaction.

[0035] Preferably, the pH of the phosphoric acid solution in step (3) is 1.5 to 2.2, for example: 1.5, 1.8, 2, 2.1 or 2.2, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0036] Preferably, the temperature of the two-step heating reaction in step (3) is 80 to 100°C, for example: 80°C, 85°C, 90°C, 95°C or 100°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0037] Preferably, the stirring speed of the two-step heating reaction in step (3) is 600 to 800 rpm, for example: 600 rpm, 650 rpm, 700 rpm, 750 rpm or 800 rpm, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0038] Preferably, the two-step heating reaction time in step (3) is 12 to 36 hours, for example: 12 hours, 18 hours, 24 hours, 30 hours or 36 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0039] Preferably, the oxidant in step (3) includes hydrogen peroxide.

[0040] Preferably, the mass ratio of the oxidant and the divalent iron source in the reaction system in step (3) is (2.5 to 5):1, for example: 2.5:1, 3:1, 3.5:1, 4:1 or 5:1, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0041] Preferably, the oxidation reaction time in step (3) is 4 to 6 hours, for example: 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0042] Preferably, the calcination temperature in step (3) is 500 to 700°C, for example: 500°C, 550°C, 600°C, 650°C or 700°C, etc., not limited to the listed values, and other unlisted values ​​within this range are also applicable.

[0043] Preferably, the calcination time in step (3) is 8 to 12 hours, for example: 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, etc., and is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0044] In a second aspect, the present invention provides a composite iron phosphate, which is prepared by the method described in the first aspect.

[0045] Preferably, the hollow iron phosphate has a particle size of 5-6 μm, such as 5 μm, 5.2 μm, 5.5 μm, 5.8 μm or 6 μm.

[0046] Preferably, the particle size of the solid iron phosphate is 1 to 2 μm, for example: 1 μm, 1.2 μm, 1.5 μm, 1.8 μm or 2 μm, etc.

[0047] Thirdly, the present invention provides a lithium iron phosphate cathode material, which is prepared by sintering a composite iron phosphate and a lithium source as described in the second aspect.

[0048] Fourthly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material as described in the third aspect.

[0049] Compared with the prior art, the present invention has the following beneficial effects:

[0050] (1) This invention prepares a composite phosphoric acid containing large-particle-size hollow ferric phosphate and small-particle-size solid ferric phosphate, and utilizes the difference in particle size to improve the tap density; the large-particle-size ferric phosphate has a hollow structure, which improves the rate performance; at the same time, the tap density is improved by mixing large and small particles, and the mixture of large hollow and small solid ferric phosphate alleviates the "overburning" and "underburning" phenomena in the calcination process; the dopamine coating layer used in the process is uniformly located inside the large particles, and does not form carbon after calcination, thus improving the conductivity of the material.

[0051] (2) The tap density of the lithium iron phosphate cathode material prepared by the method of the present invention can reach 1.20 g / cm³. 3 The resulting batteries can achieve a capacity of over 158 mAh / g at 0.2C, over 150 mAh / g at 1C, over 142 mAh / g at 2C, over 133 mAh / g at 5C, and over 124 mAh / g at 10C. After 100 cycles, the capacity retention rate can reach over 97.1%. Attached Figure Description

[0052] Figure 1 This is a SEM image of the composite iron phosphate prepared in Example 1 of the present invention. Detailed Implementation

[0053] 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 the present invention and should not be construed as limiting the invention.

[0054] Example 1

[0055] This embodiment provides a composite ferric phosphate, and the preparation method of the composite ferric phosphate is as follows:

[0056] (1) Disodium hydrogen phosphate was added to a deionized water solution and stirred until dissolved. Then, phosphoric acid and sodium acetate were added to adjust the pH to 3.8. Ferrous sulfate was then added to make the molar ratio of P:Fe in the system 1:1.5. The mixture was stirred until dissolved and stirred at a constant speed of 600 rpm. The entire reaction was carried out under a nitrogen atmosphere, and the pH was kept below 6 throughout the process. Fe3(PO4)2·8H2O was obtained after the reaction was complete. The above reaction solution was ultrasonically treated to disperse it completely. Then, dopamine hydrochloride was added. The mass ratio of dopamine hydrochloride to ferrous sulfate was 1:12. The mixture was ultrasonically reacted for 25 min to obtain a polydopamine-coated ferrous phosphate suspension.

[0057] (2) Adjust the pH of the polydopamine-coated ferrous phosphate suspension to 4-5 with phosphoric acid and maintain it, then add ferric chloride solution, Fe 3+ :Fe 2+The molar ratio of ammonium phosphate to ferric chloride is 1:0.5. After thorough mixing, ammonium hydrogen phosphate is added and stirred until homogeneous. The molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. The reaction temperature is then increased to 60℃ and the reaction is carried out at 700 rpm. A shell of ferric phosphate mixed with ferric hydroxide continues to grow on ferrous phosphate. The product is filtered, washed, and dried to obtain an intermediate.

[0058] (3) The intermediate was placed in a phosphoric acid solution with a pH of 1.8 and stirred at 700 rpm for 24 h at 90 °C to dissolve ferric hydroxide and ferrous hydroxide. Hydrogen peroxide was added at the same speed, with a mass ratio of hydrogen peroxide to ferrous sulfate of 3.5:1, to oxidize ferrous ions to ferric ions. After reacting for 5 h, small-sized ferric phosphate particles were formed in the solution. The obtained small-sized ferric phosphate and hollow large-sized ferric phosphate were then filtered, washed, and dried, and then calcined at 700 °C for 10 h in an inert atmosphere to obtain the composite ferric phosphate. The hollow ferric phosphate particles in the composite ferric phosphate had a particle size D50 of 4.5 μm, and the solid ferric phosphate particles had a particle size D50 of 1.5 μm.

[0059] The SEM image of the composite ferric phosphate is shown below. Figure 1 As shown, by Figure 1 It can be seen that the composite ferric phosphate of the present invention contains both large-particle-size hollow ferric phosphate and small-particle-size ferric phosphate.

[0060] Example 2

[0061] This embodiment provides a composite ferric phosphate, and the preparation method of the composite ferric phosphate is as follows:

[0062] (1) Add ammonium phosphate to a deionized water solution and stir until dissolved. Then add phosphoric acid and sodium bicarbonate to adjust the pH to 3.9. Then add ferrous sulfate to make the molar ratio of P:Fe in the system 1:1.45. Stir until dissolved and uniform. Stir at a constant speed of 700 rpm. The entire reaction process is carried out under a nitrogen atmosphere. The pH is maintained below 6.0 throughout the process. Fe3(PO4)2·8H2O is obtained after complete reaction. The above reaction solution is ultrasonically treated to make it fully dispersed. Then add dopamine hydrochloride. The mass ratio of dopamine hydrochloride to ferrous sulfate is 1:5. Sonicate for 20 min to obtain a polydopamine-coated ferrous phosphate suspension.

[0063] (2) Adjust the pH of the polydopamine-coated ferrous phosphate suspension to 3-4 with phosphoric acid and maintain it, then add ferric chloride solution, Fe 3+ :Fe 2+The molar ratio of ammonium phosphate to ferric chloride is 1:0.3. After thorough mixing, ammonium hydrogen phosphate is added and stirred until homogeneous. The molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. The reaction temperature is then increased to 50°C and the reaction is carried out at 1000 rpm. A shell of ferric phosphate mixed with ferric hydroxide continues to grow on ferrous phosphate. The product is filtered, washed, and dried to obtain an intermediate.

[0064] (3) The intermediate was placed in a phosphoric acid solution with a pH of 2.2 and stirred at 700 rpm for 36 h at 80 °C to dissolve ferric hydroxide and ferrous hydroxide. Hydrogen peroxide was added at the same speed, with a mass ratio of hydrogen peroxide to ferrous sulfate of 2.5:1, to oxidize ferrous ions to ferric ions. After reacting for 6 h, small-sized ferric phosphate particles were formed in the solution. The obtained small-sized ferric phosphate and hollow large-sized ferric phosphate were then filtered, washed, and dried, and then calcined at 500 °C for 12 h in an inert atmosphere to obtain the composite ferric phosphate. The hollow ferric phosphate particles in the composite ferric phosphate had a particle size D50 of 5.5 μm, and the solid ferric phosphate particles had a particle size D50 of 1.8 μm.

[0065] Example 3

[0066] This embodiment provides a composite ferric phosphate, and the preparation method of the composite ferric phosphate is as follows:

[0067] (1) Add (NH4)2HPO4 to a deionized water solution and stir until dissolved. Then add phosphoric acid and sodium bicarbonate to adjust the pH to 4.0. Then add ferrous chloride to make the molar ratio of P:Fe in the system 1:1.4. Stir until dissolved and uniform. Stir at a constant speed of 600 rpm. The entire reaction process is carried out under a nitrogen atmosphere. The pH is maintained below 6.0 throughout the process. Fe3(PO4)2·8H2O is obtained after complete reaction. The above reaction solution is ultrasonically treated to make it fully dispersed. Then add dopamine hydrochloride. The mass ratio of dopamine hydrochloride to ferrous chloride is 0.5:12. Sonicate for 20 min to obtain a polydopamine-coated ferrous phosphate suspension.

[0068] (2) Adjust the pH of the polydopamine-coated ferrous phosphate suspension to 2-3 with phosphoric acid and maintain it, then add ferric chloride solution, Fe 3+ :Fe 2+ The molar ratio of ammonium phosphate to ferric chloride is 1:0.6. After thorough mixing, ammonium hydrogen phosphate is added and stirred until homogeneous. The molar ratio of ammonium hydrogen phosphate to ferric chloride is 1:1. The reaction temperature is then increased to 70℃ and the reaction is carried out at 600 rpm. A shell of ferric phosphate mixed with ferric hydroxide continues to grow on ferrous phosphate. After filtration, washing and drying, the intermediate is obtained.

[0069] (3) The intermediate was placed in a phosphoric acid solution with a pH of 1.5 and stirred at 100°C and 800 rpm for 12 hours to dissolve ferric hydroxide and ferrous hydroxide. Hydrogen peroxide was added at the same stirring speed, with a mass ratio of hydrogen peroxide to ferrous chloride of 5:1, to oxidize ferrous ions to ferric ions. After reacting for 4 hours, small-sized ferric phosphate particles were formed in the solution. The obtained small-sized ferric phosphate and hollow large-sized ferric phosphate were then filtered, washed, and dried, and then calcined at 700°C for 8 hours in an inert atmosphere to obtain the composite ferric phosphate. The hollow ferric phosphate particles in the composite ferric phosphate had a particle size D50 of 4.1 μm, and the solid ferric phosphate particles had a particle size D50 of 1.6 μm.

[0070] Example 4

[0071] The only difference between this embodiment and embodiment 1 is that the mass ratio of ferrous iron source and dopamine hydrochloride in step (1) is 3:1, while the other conditions and parameters are exactly the same as in embodiment 1.

[0072] Example 5

[0073] The only difference between this embodiment and Embodiment 1 is that the mass ratio of ferrous iron source and dopamine hydrochloride in step (1) is 15:0.5, while the other conditions and parameters are exactly the same as in Embodiment 1.

[0074] Example 6

[0075] The only difference between this embodiment and embodiment 1 is that step (2) Fe 3+ :Fe 2+ The molar ratio was 1:0.1, and other conditions and parameters were exactly the same as in Example 1.

[0076] Example 7

[0077] The only difference between this embodiment and embodiment 1 is that step (2) Fe 3+ :Fe 2+ The molar ratio was 1:0.8, and other conditions and parameters were exactly the same as in Example 1.

[0078] Comparative Example 1

[0079] In this comparative example, ordinary ferric phosphate was prepared by precipitation method, as follows:

[0080] Add 1 mol / L ferric chloride solution to 1 mol / L ammonium hydrogen phosphate solution to make the iron-to-phosphorus ratio 1:1. Stir well and react for 4-6 hours at 60℃ and pH 1.8-2.2 to allow ferric phosphate to precipitate completely. Filter and dry the precipitated ferric phosphate material. After freezing, heat and dry the material, then calcine it at 600℃ for 6 hours to obtain anhydrous ferric phosphate.

[0081] Comparative Example 2

[0082] The only difference between this comparative example and Example 1 is that dopamine hydrochloride is not added; all other conditions and parameters are exactly the same as in Example 1.

[0083] Comparative Example 3

[0084] The only difference between this comparative example and Example 1 is that in step (1), the divalent iron source is replaced with a trivalent iron source, while the other conditions and parameters are exactly the same as in Example 1.

[0085] Performance testing:

[0086] The iron phosphate and lithium hydroxide prepared in the examples and comparative examples were mixed at a molar ratio of 1:1, and then 6% of glucose by total mass was added. Then, an appropriate amount of ethanol was added as a dispersant. The mixture was ground and stirred in a grinder for 4 hours, and then dried in an 80°C drying oven. The dried mixture was placed in a tube furnace and kept at 800°C for 12 hours under an inert atmosphere to obtain lithium iron phosphate cathode material.

[0087] The tap density of the obtained lithium iron phosphate cathode material was tested.

[0088] Weigh the mass m1 of the drying graduated cylinder. Add a certain amount of solid sample (to about 5 ml) into the graduated cylinder, plug the cylinder opening, and shake the graduated cylinder vertically until the sample volume no longer decreases. Record the sample volume V (ml). Weigh the mass m2 (g) of the graduated cylinder plus the sample. Use the formula ρ=(m2-m1) / V to obtain ρ (g / cm³). 3 The tap density of the sample to be tested is obtained.

[0089] The electrical performance of the obtained lithium iron phosphate cathode material was tested:

[0090] A uniform slurry was prepared by mixing the positive electrode material, acetylene black, and PVDF in a mass ratio of 75:15:10. This slurry was then uniformly coated onto an aluminum foil substrate to serve as the positive electrode of the simulated battery. The negative electrode of the simulated battery used a lithium sheet, and the separator was a polypropylene porous membrane. The electrolyte was 1 mol of LiPF6 dissolved in 1 L of a mixed solvent of EC and DMC (volume ratio 1:1). The positive electrode, negative electrode, electrolyte, and separator were assembled into a battery in an argon-protected glove box. The battery was first charged at a constant current to 4.2V, then discharged at a higher rate to 2.0V. The capacity discharged at that rate was the discharge capacity. After discharge, the battery was discharged again at a constant current to 2.0V. The next rate test was then conducted, and the results are shown in Table 1.

[0091] Table 1

[0092]

[0093]

[0094] As shown in Table 1, and based on Examples 1-3, the tap density of the lithium iron phosphate cathode material made from the composite iron phosphate of the present invention can reach 1.25 g / cm³. 3 The resulting battery exhibits a discharge capacity of over 160 mAh / g at 0.2C, over 153 mAh / g at 1C, over 146 mAh / g at 2C, over 138 mAh / g at 5C, and over 127 mAh / g at 10C. It also retains over 97.5% of its capacity after 100 cycles.

[0095] A comparison of Examples 1 and 4-5 shows that the amount of dopamine hydrochloride added during the preparation of the composite iron phosphate of the present invention affects its performance. Controlling the mass ratio of ferrous iron source to dopamine hydrochloride at (5-12):(0.5-1) yields composite iron phosphate with better performance. Excessive dopamine hydrochloride addition leads to an increase in carbon content in the cathode material, resulting in a decrease in active material. Insufficient dopamine hydrochloride addition prevents iron phosphate from growing under the coating of ferrous phosphate, ultimately affecting the formation of hollow iron phosphate. Simultaneously, the reduced dopamine content affects the carbon network structure after calcination, decreasing electronic conductivity.

[0096] A comparison of Examples 1 and 6-7 shows that the molar ratio of ferric ions in the ferric iron source to ferrous ions in the ferrous iron source affects the performance of the composite ferric phosphate in the preparation process of the present invention. Controlling the molar ratio of ferric ions in the ferric iron source to ferrous ions in the ferrous iron source at 1:0.3-0.6 yields composite ferric phosphate with better performance. If the amount of ferric salt added is too large, the ferric phosphate layer coating ferrous phosphate will be too thick, leading to an increase in the particle size of large ferric phosphate particles and a decrease in the proportion of small ferric phosphate particles, thus affecting the tap density. If the amount of ferric salt added is too small, the proportion of ferrous iron will increase, ultimately leading to an increase in the particle size of small ferric phosphate particles.

[0097] As can be seen from the comparison between Example 1 and Comparative Example 1, the lithium iron phosphate material prepared by the composite iron phosphate of the present invention has better rate performance, tap density and cycle performance than the iron phosphate prepared by the precipitation method commonly used in the prior art.

[0098] As can be seen from the comparison between Example 1 and Comparative Example 2, without the addition of dopamine hydrochloride, ferric phosphate cannot grow well in the coating of ferrous phosphate, which ultimately affects the formation of hollow ferric phosphate. At the same time, without dopamine, a carbon network structure cannot be formed after calcination, resulting in a decrease in electronic conductivity and a decrease in rate performance.

[0099] As can be seen from the comparison between Example 1 and Comparative Example 3, when ferric phosphate is prepared directly using ferric iron, the internal ferric phosphate cannot be dissolved in the subsequent acid solution. Therefore, hollow ferric phosphate cannot be obtained, which affects the rate performance, tap density and cycling performance, and reduces them.

[0100] 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 composite ferric phosphate, characterized in that, The preparation method includes the following steps: (1) The ferrous phosphate suspension was mixed with dopamine hydrochloride and reacted to obtain a polydopamine-coated ferrous phosphate suspension; In step (1), the mass ratio of the divalent iron source in the dopamine hydrochloride and ferrous phosphate suspension is (0.5~1): (5~12); (2) After adjusting the pH of the polydopamine-coated ferrous phosphate suspension, add ferric iron source and phosphorus source, and obtain the intermediate by heating reaction in one step; The pH value in step (2) is 2-5; In step (2), the molar ratio of ferric ions in the ferric iron source to ferrous ions in the ferrous phosphate suspension is 1:(0.3~0.6); In step (2), the molar ratio of iron in the trivalent iron source to phosphorus in the phosphorus source is (0.8~1.2):1; (3) The intermediate is mixed with phosphoric acid solution, and after a two-step heating reaction, an oxidant is added to carry out an oxidation reaction to obtain a mixture. The mixture is then calcined to obtain the composite iron phosphate. The pH of the phosphoric acid solution in step (3) is 1.5~2.2; The temperature of the two-step heating reaction in step (3) is 80~100℃; In step (3), the mass ratio of the oxidant to the divalent iron source in the reaction system is (2.5~5):

1.

2. The preparation method according to claim 1, characterized in that, Step (1) is carried out under a protective atmosphere.

3. The preparation method according to claim 2, characterized in that, The protective atmosphere includes any one or a combination of at least two of nitrogen, helium, or argon.

4. The preparation method according to claim 1, characterized in that, The ferrous phosphate suspension in step (1) is prepared by the following method: After adjusting the pH of the phosphorus source solution, it was mixed with a ferrous iron source and stirred to obtain the ferrous phosphate suspension.

5. The preparation method according to claim 4, characterized in that, The pH is 3.8 to 4.

0.

6. The preparation method according to claim 4, characterized in that, The pH adjuster includes any one or a combination of at least two of ammonia, sodium hydroxide, sodium bicarbonate, or sodium acetate.

7. The preparation method according to claim 4, characterized in that, The phosphorus source includes any one or a combination of at least two of H3PO4, (NH4)2HPO4, NH4H2PO4, (NH4)3PO4, Na2HPO4, or NaH2PO4.

8. The preparation method according to claim 4, characterized in that, The divalent iron source includes any one or a combination of at least two of ferrous sulfate, ferrous nitrate, ferrous oxalate, or ferrous chloride.

9. The preparation method according to claim 4, characterized in that, The molar ratio of phosphorus to iron in the phosphorus source solution is 1:(1.4~1.5).

10. The preparation method according to claim 4, characterized in that, The stirring reaction is carried out at a speed of 600-800 rpm.

11. The preparation method according to claim 4, characterized in that, The pH of the stirred reaction is <6.

0.

12. The preparation method according to claim 1, characterized in that, The reaction described in step (1) includes an ultrasonic reaction.

13. The preparation method according to claim 1, characterized in that, The reaction time in step (1) is 20-30 min.

14. The preparation method according to claim 1, characterized in that, The trivalent iron source in step (2) includes any one or a combination of at least two of ferric nitrate, ferric sulfate, or ferric chloride.

15. The preparation method according to claim 1, characterized in that, The temperature of the one-step heating reaction in step (2) is 50~70℃.

16. The preparation method according to claim 1, characterized in that, The stirring speed for the one-step heating reaction in step (2) is 600~1000 rpm.

17. The preparation method according to claim 1, characterized in that, After the one-step heating reaction described in step (2), the mixture is filtered, washed and dried.

18. The preparation method according to claim 1, characterized in that, The stirring speed for the two-step heating reaction in step (3) is 600~800 rpm.

19. The preparation method according to claim 1, characterized in that, The two-step heating reaction in step (3) takes 12 to 36 hours.

20. The preparation method according to claim 1, characterized in that, The oxidant in step (3) includes hydrogen peroxide.

21. The preparation method according to claim 1, characterized in that, The oxidation reaction in step (3) takes 4 to 6 hours.

22. The preparation method according to claim 1, characterized in that, The calcination temperature in step (3) is 500~700℃.

23. The preparation method according to claim 1, characterized in that, The calcination time in step (3) is 8~12h.

24. A composite iron phosphate, characterized in that, The composite ferric phosphate is prepared by the method described in any one of claims 1-23.

25. The composite ferric phosphate as described in claim 24, characterized in that, The composite ferric phosphate includes hollow ferric phosphate and solid ferric phosphate.

26. The composite iron phosphate as described in claim 25, characterized in that, The hollow iron phosphate particles have a particle size of 5~6μm.

27. The composite iron phosphate as described in claim 25, characterized in that, The particle size of the solid iron phosphate is 1~2μm.

28. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by sintering the composite iron phosphate and lithium source as described in any one of claims 24-27.

29. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the lithium iron phosphate cathode material as described in claim 28.

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