A composite iron phosphate, its preparation method and application

Abstract

This invention provides a composite iron phosphate, its preparation method, and its application. The preparation method includes the following steps: (1) mixing an iron salt solution and hydrochloric acid, heating and stirring, adding phosphate, adjusting the pH to obtain a mixed solution, mixing the mixed solution with hydrogen peroxide, and then performing a hydrothermal reaction to obtain a mixture of iron phosphate and iron hydroxide; (2) placing the mixture of iron phosphate and iron hydroxide and sodium hypophosphite in different regions of the same device, and heating the device to obtain the composite iron phosphate. This invention prepares a composite iron phosphate uniformly doped with iron phosphate through a simple hydrothermal method. The lithium iron phosphate cathode material made from the composite iron phosphate has high ionic conductivity, and thus exhibits high cycle stability and rate capacity.

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 cathode materials have rapidly captured the current market due to their advantages such as good safety performance, environmental friendliness, high capacity, and cost-effective raw materials.

[0003] The drawback of LiFePO4 is its low electronic and ionic conductivity, which are 10⁻⁶ and 10⁻⁶, respectively. -9 S∙cm -1 and 10 -10 ~10 -15 cm 2 ∙s -1 This directly results in poor specific capacity and rate charge / discharge performance, limiting its application.

[0004] Iron phosphide has good electrical conductivity, and at higher rates, it improves the electrochemical kinetics of the material, exhibiting excellent rate performance.

[0005] CN103094565A discloses a lithium iron phosphate / iron phosphide / carbon composite nanofiber, comprising the following components in mass percentage: 50-99% lithium iron phosphate, 0.5-30% iron phosphide, and 1-20% carbon. The average diameter of the lithium iron phosphate / iron phosphide / carbon composite nanofiber is between 50 and 500 nm, and the fiber contains spherical particles of iron phosphide.

[0006] CN112864362A discloses a method for converting iron phosphate into iron phosphate to obtain iron phosphate-doped lithium iron phosphate material by high temperature. It utilizes the reaction of concentrated phosphoric acid with lithium iron phosphate to generate a layer of iron phosphate on its surface, which is then reduced by hydrogen at high temperature to form an iron phosphate coating layer; and then carbon coating layer is obtained by vapor deposition on the surface of the iron phosphate coating layer.

[0007] The lithium iron phosphate produced by the above method has coarse grain growth, which leads to a decrease in the ionic conductivity of the material, and thus affects the cycle stability and rate capacity of the electrode material. Summary of the Invention

[0008] The purpose of this invention is to provide a composite iron phosphate, its preparation method, and its application. This invention uses a simple hydrothermal method to prepare a composite iron phosphate uniformly doped with iron phosphide. The lithium iron phosphate cathode material made from the composite iron phosphate has high ionic conductivity, and thus has high cycle stability and rate capacity.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing composite iron phosphate, the method comprising the following steps:

[0011] (1) Mix iron salt solution and hydrochloric acid, heat and stir, add phosphate, adjust pH to obtain mixed solution, mix the mixed solution with hydrogen peroxide and carry out hydrothermal reaction to obtain a mixture of iron phosphate and iron hydroxyl oxide;

[0012] (2) The mixture of ferric phosphate and ferric hydroxide and sodium hypophosphite are placed in different areas of the same device and the device is heated to obtain the composite ferric phosphate.

[0013] In the preparation process of the composite iron phosphate described in this invention, part of the iron source reacts hydrothermally with hydrogen peroxide to form iron hydroxyl oxide. The iron hydroxyl oxide is generated simultaneously with iron phosphate, so the iron hydroxyl oxide is uniformly distributed in the iron phosphate. Then, it is placed in the device together with sodium hypophosphite. The sodium hypophosphite is pyrolyzed to generate PH3, which fully phosphates the iron hydroxyl oxide in situ to FeP, thus obtaining FeP-doped iron phosphate. The FeP is uniformly distributed in the iron phosphate, which is beneficial to fully exert its conductivity and provide the rate performance of the material.

[0014] Preferably, the iron salt solution in step (1) includes any one or a combination of at least two of the following: an aqueous solution of ferric chloride, an aqueous solution of ferric nitrate, or an aqueous solution of ferric sulfate.

[0015] Preferably, the concentration of the iron salt solution is 0.5~2 mol / L, for example: 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L, etc.

[0016] Preferably, the heating and stirring temperature is 80~90℃, for example: 80℃, 82℃, 85℃, 88℃ or 90℃, etc.

[0017] Preferably, the heating and stirring time is 10 to 20 minutes, for example: 10 minutes, 12 minutes, 15 minutes, 18 minutes or 20 minutes.

[0018] Preferably, the phosphate in step (1) includes any one or a combination of at least two of hydrogen phosphate, ammonium phosphate, or ammonium dihydrogen phosphate.

[0019] Preferably, the molar ratio of iron in the iron salt solution to phosphorus in the phosphate is 1:(0.97~0.99), for example: 1:0.97, 1:0.975, 1:0.98, 1:0.985 or 1:0.99, etc.

[0020] Preferably, the addition of phosphate is followed by stirring.

[0021] Preferably, the stirring time is 10 to 20 minutes, for example: 10 minutes, 12 minutes, 15 minutes, 18 minutes or 20 minutes.

[0022] Preferably, the pH adjuster in step (1) includes hydrochloric acid or sodium hydroxide.

[0023] Preferably, the pH is 2 to 2.5, for example: 2, 2.1, 2.2, 2.4 or 2.5, etc.

[0024] Preferably, the mass concentration of the hydrogen peroxide is 20% to 40%, for example: 20%, 25%, 30%, 35% or 40%, etc.

[0025] Preferably, the mass ratio of hydrogen peroxide to the mixed solution is (40~50):100, for example: 40:100, 42:100, 45:100, 48:100 or 50:100, etc.

[0026] Preferably, the temperature of the hydrothermal reaction in step (1) is 120~180℃, for example: 120℃, 130℃, 150℃, 160℃ or 180℃, etc.

[0027] Preferably, the hydrothermal reaction time is 3 to 10 hours, for example: 3 hours, 5 hours, 7 hours, 8 hours, or 10 hours.

[0028] Preferably, the mass ratio of the total mass of the mixture of ferric phosphate and ferric hydroxide in step (2) to the mass of sodium hypophosphite is (18~20):(0.5~2.5), for example: 18:0.5, 18:1, 19:1.5, 20:1 or 20:2.5, etc.

[0029] Preferably, the mixture of ferric phosphate and ferric hydroxide does not come into contact with sodium hypophosphite in the device.

[0030] Preferably, the temperature of the heat treatment in step (2) is 500~600℃, for example: 500℃, 520℃, 550℃, 580℃ or 600℃, etc.

[0031] Preferably, the heating treatment time is 4 to 8 hours, for example: 4 hours, 5 hours, 6 hours, 7 hours or 8 hours.

[0032] Preferably, the atmosphere for the heat treatment includes an inert atmosphere.

[0033] In a second aspect, the present invention provides a composite iron phosphate, which is prepared by the method described in the first aspect.

[0034] Preferably, the composite ferric phosphate comprises anhydrous ferric phosphate and ferric phosphide.

[0035] 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.

[0036] Preferably, the lithium iron phosphate cathode material is an iron phosphate-doped lithium iron phosphate cathode material.

[0037] Fourthly, the present invention provides a lithium-ion battery comprising the lithium iron phosphate cathode material as described in the third aspect.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) The present invention prepares a composite iron phosphate uniformly doped with iron phosphate by a simple hydrothermal method. The iron phosphate is uniformly distributed in the lithium iron phosphate cathode material made of the composite iron phosphate, which is beneficial to give full play to its conductivity and provide the rate performance of the material.

[0040] (2) The battery made from the composite iron phosphate material described in this invention can achieve a discharge specific capacity of more than 165.2 mAh / g at 0.1C, more than 162.7 mAh / g at 0.2C, more than 158 mAh / g at 1C, more than 151.6 mAh / g at 2C, more than 142.9 mAh / g at 5C, and more than 133.6 mAh / g at 10C. Attached Figure Description

[0041] Figure 1 This is the XRD pattern of the composite iron phosphate described in Example 1.

[0042] Figure 2 This is a comparison curve of the rate performance of lithium iron phosphate cathode materials prepared by Application Example 1 and Comparative Application Example 1-2. Detailed Implementation

[0043] 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 in any way.

[0044] Example 1

[0045] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0046] (1) At 85℃, 1 mol / L ferric chloride aqueous solution was mixed with hydrochloric acid and stirred for 15 min. Then, ammonium phosphate was added at a molar ratio of 1:0.98 for iron to phosphorus and stirred for another 15 min. The pH was adjusted to 2.3 (hydrochloric acid or sodium hydroxide was added depending on the actual situation). 30% hydrogen peroxide (the mass of hydrogen peroxide was 40% of the mass of the mixed solution) was added and transferred to a polytetrafluoroethylene reactor. The mixture was hydrothermally reacted at 135℃ for 6 h. After washing and drying with deionized water, a mixture of ferric phosphate and ferric hydroxide was obtained.

[0047] (2) The mixture of ferric phosphate and ferric hydroxide and sodium hypophosphite were placed at both ends of an atmosphere furnace at a mass ratio of 20:1.5 to prevent them from mixing. The mixture was heated at 550°C for 5 hours under an argon atmosphere to obtain the composite ferric phosphate.

[0048] The XRD pattern of the composite iron phosphate is as follows: Figure 1 As shown, by Figure 1 It can be seen that the composite iron phosphate prepared by the method of the present invention contains iron phosphide.

[0049] Example 2

[0050] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0051] (1) At 80℃, 1 mol / L ferric chloride aqueous solution was mixed with hydrochloric acid and stirred for 20 min. Then, ammonium phosphate was added at a molar ratio of 1:0.97 for iron to phosphorus and stirred for another 20 min. The pH was adjusted to 2 (hydrochloric acid or sodium hydroxide was added depending on the actual situation). 20% hydrogen peroxide (the mass of hydrogen peroxide was 50% of the mass of the mixed solution) was added and transferred to a polytetrafluoroethylene reactor. The mixture was hydrothermally reacted at 120℃ for 10 h. After washing and drying with deionized water, a mixture of ferric phosphate and ferric hydroxide was obtained.

[0052] (2) The mixture of ferric phosphate and ferric hydroxide and sodium hypophosphite were placed at both ends of an atmosphere furnace in a mass ratio of 18:0.5 to prevent them from mixing. The mixture was heated at 500°C for 8 hours under an argon atmosphere to obtain the composite ferric phosphate.

[0053] Example 3

[0054] This embodiment provides a composite iron phosphate, which is prepared by the following method:

[0055] (1) At 90℃, 1 mol / L ferric chloride aqueous solution was mixed with hydrochloric acid and stirred for 10 min. Then, ammonium phosphate was added at a molar ratio of 1:0.99 for iron to phosphorus and stirred for another 10 min. The pH was adjusted to 2.5 (hydrochloric acid or sodium hydroxide was added depending on the actual situation). Hydrogen peroxide with a mass concentration of 40% (the mass of the hydrogen peroxide was 30% of the mass of the mixed solution) was added and transferred to a polytetrafluoroethylene reactor. The mixture was hydrothermally reacted at 180℃ for 3 h. After washing and drying with deionized water, a mixture of ferric phosphate and ferric hydroxide was obtained.

[0056] (2) The mixture of ferric phosphate and ferric hydroxide and sodium hypophosphite were placed at both ends of an atmosphere furnace in a mass ratio of 20:2.5 to prevent them from mixing. The mixture was heated at 600°C for 4 hours under an argon atmosphere to obtain the composite ferric phosphate.

[0057] Example 4

[0058] The only difference between this embodiment and Example 1 is that ammonium phosphate is added at an iron to phosphorus molar ratio of 1:0.96. All other conditions and parameters are exactly the same as in Example 1.

[0059] Example 5

[0060] The only difference between this embodiment and Example 1 is that ammonium phosphate is added at a molar ratio of iron to phosphorus of 1:0.995. All other conditions and parameters are exactly the same as in Example 1.

[0061] Comparative Example 1

[0062] This comparative example provides an iron phosphate, the preparation method of which is as follows:

[0063] FeCl3·6H2O was dissolved in deionized water, and then fully dissolved in hydrochloric acid solution at 85℃ for 15 minutes. Ammonium phosphate was then added and fully dissolved, and stirred for 15 minutes to achieve a 1:1 molar ratio of iron to phosphorus. The pH was adjusted to 2-2.5 using hydrochloric acid and sodium hydroxide. The mixture was placed in a polytetrafluoroethylene reactor and reacted at 135℃ for 6 hours. After washing and drying with deionized water, a mixture of ferric phosphate and ferric hydroxide was obtained. The obtained ferric phosphate was calcined and heated at 500℃ for 5 hours to obtain anhydrous ferric phosphate.

[0064] Application Example 1

[0065] This application example provides a lithium iron phosphate cathode material, and the preparation method of the lithium iron phosphate cathode material is as follows:

[0066] The composite iron phosphate obtained in Example 1 was mixed with lithium hydroxide at a lithium:iron molar ratio of 1:1 using ethanol as a dispersant to obtain a mixed material. The mixed material was stirred and ground in a grinder for 2 hours, and then dried in a drying oven at 70°C. The dried material was placed in a tube furnace and calcined at 750°C for 10 hours under a nitrogen atmosphere to obtain the lithium iron phosphate cathode material.

[0067] Application Example 2

[0068] The only difference between this application example and application example 1 is that the composite iron phosphate described in example 1 is replaced with the composite iron phosphate described in example 2, while the other conditions and parameters are exactly the same as in application example 1.

[0069] Application Example 3

[0070] The only difference between this application example and application example 1 is that the composite iron phosphate described in example 1 is replaced with the composite iron phosphate described in example 3, while the other conditions and parameters are exactly the same as in application example 1.

[0071] Application Example 4

[0072] The only difference between this application example and application example 1 is that the composite iron phosphate described in example 1 is replaced with the composite iron phosphate described in example 4, while the other conditions and parameters are exactly the same as in application example 1.

[0073] Application Example 5

[0074] The only difference between this application example and application example 1 is that the composite iron phosphate described in example 1 is replaced with the composite iron phosphate described in example 5, while the other conditions and parameters are exactly the same as in application example 1.

[0075] Comparative Application Example 1

[0076] This comparative application example provides a lithium iron phosphate cathode material, and the preparation method of the composite lithium iron phosphate cathode material is as follows:

[0077] Anhydrous iron phosphate prepared in Comparative Example 1 was mixed with lithium hydroxide at a lithium:iron molar ratio of 1:1. Then, 1% by mass of glucose was added as a carbon source, and ethanol was used as a dispersant for mixing to obtain a mixed material. The mixed material was stirred and ground in a grinder for 2 hours, and then dried in a drying oven at 70°C. The dried material was placed in a tube furnace and calcined at 750°C for 10 hours under a nitrogen atmosphere to obtain lithium iron phosphate electrode material.

[0078] Comparative Application Example 2

[0079] This comparative application example provides a composite lithium iron phosphate cathode material, and the preparation method of the composite lithium iron phosphate cathode material is as follows:

[0080] (1) Dissolve FeCl3·6H2O in deionized water, then dissolve it completely in hydrochloric acid solution at 85℃ for 15 min. Then add ammonium phosphate and dissolve it completely, stirring for 15 min to make the iron to phosphorus molar ratio 1:1. Adjust the pH to 2-2.5 using hydrochloric acid and sodium hydroxide. Place it in a polytetrafluoroethylene reaction vessel and react at 135℃ for 6 h. Wash and dry with deionized water to obtain ferric phosphate. Calcine the obtained ferric phosphate and heat it at 500℃ for 5 h to obtain anhydrous ferric phosphate.

[0081] Anhydrous ferric phosphate and lithium hydroxide were mixed at a lithium-to-iron molar ratio of 1:1 using ethanol as a dispersant to obtain a mixed material. The mixed material was stirred and ground in a grinder for 2 hours, and then dried in a drying oven at 70°C. The dried material was placed in a tube furnace and calcined at 750°C for 10 hours under a nitrogen atmosphere. Then, it was stirred in concentrated phosphoric acid with a solid-liquid mass ratio of 1:1 to obtain a mixture. This mixture was placed in a polytetrafluoroethylene reactor and reacted at 80°C for 100 minutes. After filtration and washing, it was dried at 70°C for 10 hours to obtain ferric phosphate-coated lithium iron phosphate.

[0082] (3) The lithium iron phosphate coated with iron phosphate was placed in a reactor and heated to 800°C in a nitrogen atmosphere. The atmosphere was then switched to hydrogen atmosphere and reacted for 1 hour. After that, it was cooled to room temperature with nitrogen. The lithium iron phosphate material coated with iron phosphate was obtained (i.e., the composite lithium iron phosphate cathode material).

[0083] Performance testing:

[0084] The lithium iron phosphate cathode materials prepared in the application examples and comparative application examples were mixed with PVDF and conductive carbon black at a mass ratio of 8:1:1, and N-methylpyrrolidone was added to form a slurry. This slurry was then uniformly coated onto aluminum foil, vacuum dried, rolled, and punched into circular electrode sheets, which were then assembled into coin cells in a glove box. Constant current charge-discharge cycle tests were performed on the coin cells, with charge-discharge voltages ranging from 2.5 to 4.2 V. The test results are shown in Table 1.

[0085] Table 1

[0086]

[0087] As can be seen from Table 1, based on Application Examples 1-3, the batteries made from the composite iron phosphate material described in this invention can achieve a discharge specific capacity of over 165.2 mAh / g at 0.1C, over 162.7 mAh / g at 0.2C, over 158 mAh / g at 1C, over 151.6 mAh / g at 2C, over 142.9 mAh / g at 5C, and over 133.6 mAh / g at 10C.

[0088] Comparing Application Example 1 and Application Examples 4-5, it can be seen that in the preparation process of the composite ferric phosphate described in this invention, the molar ratio of iron in the iron salt solution and phosphorus in the phosphate affects its performance. Controlling the molar ratio of iron in the iron salt solution and phosphorus in the phosphate to 1:(0.97~0.99) results in better performance of the composite ferric phosphate. If the amount of phosphate added is too large, the content of ferric hydroxide will be too small, resulting in too little ferric phosphide, weak conductivity, and reduced rate performance. If the amount of phosphate added is too small, the content of ferric hydroxide will be too large, resulting in relatively more ferric phosphide, strong conductivity, increased rate performance, but reduced capacity.

[0089] The rate performance comparison curves of lithium iron phosphate cathode materials prepared in Application Example 1 and Comparative Application Example 1-2 are shown in the figure below. Figure 2 As shown. A comparison of Application Example 1 and Comparative Application Example 1 reveals that, compared to lithium iron phosphate prepared by adding a carbon source after obtaining iron phosphate via a conventional hydrothermal method, the rate performance of the lithium iron phosphate cathode material prepared by the method of the present invention is significantly improved. A comparison of Application Example 1 and Comparative Application Example 2 reveals that the prior art converts the surface lithium iron phosphate into iron phosphide by adding concentrated phosphoric acid, and then reduces it to an iron phosphide shell layer by high-temperature hydrogen. This method is complex, and iron phosphide can only be generated on the surface, resulting in poor dispersion uniformity and inability to achieve complete coverage; therefore, its rate performance is relatively poor.

[0090] 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) Mix iron salt solution and hydrochloric acid, heat and stir, add phosphate, adjust pH to obtain mixed solution, mix the mixed solution with hydrogen peroxide and carry out hydrothermal reaction to obtain a mixture of iron phosphate and iron hydroxyl oxide; (2) The mixture of ferric phosphate and ferric hydroxide and sodium hypophosphite are placed in different areas of the same device, and the device is heated to obtain the composite ferric phosphate; The molar ratio of iron in the iron salt solution to phosphorus in the phosphate is 1:(0.97~0.99), the mass ratio of the total mass of the mixture of iron phosphate and iron hydroxide to sodium hypophosphite is (18~20):(0.5~2.5), and the composite iron phosphate includes anhydrous iron phosphate and iron phosphide.

2. The preparation method according to claim 1, characterized in that, The iron salt solution in step (1) includes any one or a combination of at least two of the following: an aqueous solution of ferric chloride, an aqueous solution of ferric nitrate, or an aqueous solution of ferric sulfate.

3. The preparation method according to claim 1, characterized in that, The concentration of the iron salt solution is 0.5~2 mol / L.

4. The preparation method according to claim 1, characterized in that, The heating and stirring temperature is 80~90℃.

5. The preparation method according to claim 1, characterized in that, The heating and stirring time is 10-20 minutes.

6. The preparation method according to claim 1, characterized in that, The phosphate in step (1) includes any one or a combination of at least two of hydrogen phosphate, ammonium phosphate, or ammonium dihydrogen phosphate.

7. The preparation method according to claim 1, characterized in that, The phosphate is added and then stirred.

8. The preparation method according to claim 7, characterized in that, The stirring process takes 10-20 minutes.

9. The preparation method according to claim 1, characterized in that, The pH adjuster mentioned in step (1) includes hydrochloric acid or sodium hydroxide.

10. The preparation method according to claim 1, characterized in that, The pH is 2 to 2.

5.

11. The preparation method according to claim 1, characterized in that, The mass concentration of the hydrogen peroxide is 20% to 40%.

12. The preparation method according to claim 1, characterized in that, The mass ratio of hydrogen peroxide to the mixed solution is (40~50):

100.

13. The preparation method according to claim 1, characterized in that, The temperature of the hydrothermal reaction in step (1) is 120~180℃.

14. The preparation method according to claim 1, characterized in that, The hydrothermal reaction takes 3 to 10 hours.

15. The preparation method according to claim 1, characterized in that, The mixture of ferric phosphate and ferric hydroxide does not come into contact with sodium hypophosphite in the device.

16. The preparation method according to claim 1, characterized in that, The temperature of the heat treatment in step (2) is 500~600℃.

17. The preparation method according to claim 1, characterized in that, The heat treatment time is 4 to 8 hours.

18. The preparation method according to claim 1, characterized in that, The atmosphere for the heat treatment includes an inert atmosphere.

19. A composite iron phosphate, characterized in that, The composite ferric phosphate is prepared by the method described in any one of claims 1-18.

20. 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 claim 19.

21. The lithium iron phosphate cathode material as described in claim 20, characterized in that, The lithium iron phosphate cathode material is an iron phosphate doped lithium iron phosphate cathode material.

22. A lithium-ion battery, characterized in that, The lithium-ion battery comprises the lithium iron phosphate cathode material as described in claim 20 or 21.

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