Composite lithium iron phosphate positive electrode material, preparation method therefor, and use thereof

Abstract

A composite lithium iron phosphate positive electrode material, a preparation method therefor, and the use thereof. The preparation method comprises the following steps: (1) mixing an iron source and a phosphorus source with a solvent to obtain a mixed solution, adjusting the pH value of the mixed solution to a first pH value and carrying out a first reaction, and then adjusting the pH value to a second pH value and carrying out a second reaction, so as to obtain composite iron phosphate containing ferric hydroxide; (2) sintering the composite iron phosphate, mixing same with a lithium source, and calcining same to obtain modified lithium iron phosphate; and (3) mixing the modified lithium iron phosphate with a silver salt solution, and carrying out a replacement reaction, so as to obtain the composite lithium iron phosphate positive electrode material. The method fully uses ferric hydroxide generated during the precipitation process of iron phosphate, so that no impurity removal step is required, and silver can be uniformly distributed in the ferric phosphate product, so as to form a uniformly dispersed conductive network, thus solving the problems of agglomeration and non-uniform dispersion.

Description

A composite lithium iron phosphate positive electrode material and its preparation method and application Technical Field

[0001] The present application relates to the technical field of lithium-ion batteries, for example, a composite lithium iron phosphate positive electrode material and its preparation method and application. Background Art

[0002] The performance of power battery cathode materials is crucial for the development of electric vehicles. Currently, commercially available secondary batteries for electric vehicles primarily include lead-acid, nickel-cadmium, nickel-metal hydride, and lithium-ion batteries. Compared to traditional lead-acid, nickel-cadmium, and nickel-metal hydride batteries, lithium-ion secondary batteries offer numerous advantages, including high operating voltage, excellent cycle performance, high energy density, superior safety, and environmental friendliness.

[0003] Currently, the lithium-ion battery cathode materials that are widely used or have practical potential include lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium iron phosphate, and lithium nickel cobalt manganese oxide. Compared with other types of lithium-ion battery cathode materials, lithium iron phosphate (LiFePO4) has superior safety and cycle performance, especially in electric vehicles, where this performance indicator is particularly important.

[0004] Olivine structure LiFePO4 is a polyanion framework structure, and its structural units are LiO6 octahedron, FeO6 octahedron and PO4 tetrahedron. The strong PO covalent bond in the phosphate group stabilizes the oxygen atoms in the fully charged state, preventing them from being oxidized to produce oxygen and release. The existence of this structure makes LiFePO4 a safe positive electrode material. Moreover, due to its stable polyanion framework structure, LiFePO4 has very good cycle performance. The disadvantage of LiFePO4 is that its electronic conductivity and ionic conductivity are low, which are 10 -9 S cm -1 and 10 -10 ~10 -15 cm 2 ·s -1 , which directly leads to its poor specific capacity and rate charge and discharge performance, limiting its application.

[0005] CN104979557A discloses a method for preparing a high-rate lithium iron phosphate cathode material and a lithium-ion battery cathode sheet made from the material. The method includes preparing the lithium iron phosphate cathode material, electrolessly plating aluminum on the surface of the lithium iron phosphate, and preparing a lithium iron phosphate-graphene composite material.

[0006] CN109935802A discloses a lithium iron phosphate positive electrode material, which comprises, from the inside to the outside, lithium iron phosphate particles, a graphene / organic carbon source synergistic coating layer, and a polyaniline deposition layer for adsorbing zinc oxide quantum dots.

[0007] The above solution improves the conductivity of lithium iron phosphate by coating the conductive layer. However, the bonding strength and stability of the conductive layer and the lithium iron phosphate material are poor, resulting in increased resistivity and reduced cycle performance.

[0008] Summary of the Invention

[0009] The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

[0010] The present application provides a composite lithium iron phosphate positive electrode material, a preparation method and an application thereof. The method described in the present application fully utilizes the iron hydroxide produced in the iron phosphate precipitation process, reduces the impurity removal step, and can evenly distribute metallic silver in the iron phosphate product, thereby realizing a uniformly dispersed conductive network and eliminating the problems of agglomeration and uneven dispersion.

[0011] In a first aspect, the present application provides a method for preparing a composite lithium iron phosphate positive electrode material, the preparation method comprising the following steps:

[0012] (1) mixing an iron source and a phosphorus source with a solvent to obtain a mixed solution, adjusting the pH of the mixed solution to a first pH for a one-step reaction, and then adjusting the pH to a second pH for a two-step reaction to obtain a composite iron phosphate containing iron hydroxide;

[0013] (2) sintering the composite iron phosphate and mixing it with a lithium source, and calcining it to obtain modified lithium iron phosphate;

[0014] (3) The modified lithium iron phosphate is mixed with a silver salt solution, and subjected to a replacement reaction to obtain the composite lithium iron phosphate positive electrode material.

[0015] The step (1) of the present application of mixing the iron source and the phosphorus source with the solvent may be to mix the iron source and the phosphorus source directly with the solvent, or to mix the iron source solution and the phosphorus source solution, or to mix the solution of one of the iron source and the phosphorus source with the other.

[0016] This application controls the pH of the coprecipitation reaction to produce iron hydroxide impurities, converts the iron hydroxide impurities into iron oxide through calcination, and then uses a carbon monoxide atmosphere to reduce the iron oxide to elemental iron. The iron element then reacts with silver nitrate to replace the silver element in situ, forming a conductive silver network. Metallic silver has excellent electrical conductivity, with the conductivity of silver reaching 6.3×10 7 S / m, its electrical conductivity is one order of magnitude higher than that of other metals. The electrical conductivity of lithium iron phosphate as a positive electrode material is only 10 -9S / cm, so using silver as its conductive network can significantly enhance its electronic conductivity, reduce resistance, and increase its conductivity, thereby increasing the charge and discharge rate of the battery and thus improving the rate performance.

[0017] In one embodiment, the iron source in step (1) comprises ferric chloride and / or ferric nitrate.

[0018] In one embodiment, the phosphorus source includes any one of ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate or phosphoric acid, or a combination of at least two thereof.

[0019] In one embodiment, the solvent comprises water.

[0020] In one embodiment, the mixing stirring speed is 300-800 rpm.

[0021] In one embodiment, the first pH in step (1) is 1.2 to 2.0, for example, 1.2, 1.4, 1.6, 1.8 or 2.0.

[0022] In one embodiment, the one-step reaction time is 0.5 to 2 h, for example, 1.2, 1.4, 1.6, 1.8 or 2.0 h.

[0023] In one embodiment, the second pH is 2.0 to 2.4, for example, 2.0, 2.1, 2.2, 2.3 or 2.4.

[0024] In one embodiment, the two-step reaction time is 0.5 to 2 h, for example, 0.5 h, 1 h, 1.5 h, 2 h, 3 h or 4 h.

[0025] In one embodiment, the pH regulator for adjusting the mixed solution is aqueous ammonia.

[0026] In one embodiment, the concentration of the ammonia water is 0.1 to 2 mol / L, for example, 0.1 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L or 2 mol / L.

[0027] In one embodiment, the sintering temperature in step (2) is 500-700°C, for example, 500°C, 550°C, 600°C, 650°C or 700°C.

[0028] In one embodiment, the atmosphere of the sintering process includes oxygen.

[0029] In one embodiment, the sintering treatment time is 4 to 10 hours, for example, 4 hours, 5 hours, 6 hours, 8 hours or 10 hours.

[0030] In one embodiment, the lithium source includes any one of lithium hydroxide, lithium carbonate, lithium nitrate, or lithium oxalate, or a combination of at least two thereof.

[0031] In one embodiment, the molar ratio of the lithium element in the lithium source to the iron element in the composite iron phosphate is (1-1.07):1, for example: 1:1, 1.02:1, 1.03:1, 1.05:1 or 1.07:1.

[0032] In one embodiment, the mixture is mixed with the lithium source and then sand-milled and spray-dried.

[0033] In one embodiment, the atmosphere of the calcination treatment in step (2) comprises hydrogen and / or carbon monoxide.

[0034] In one embodiment, the calcination treatment is performed at a pressure of 1 to 10 kPa, for example, 1 kPa, 2 kPa, 5 kPa, 8 kPa, or 10 kPa.

[0035] In one embodiment, the calcination temperature is 600-800°C, for example, 600°C, 650°C, 700°C, 750°C or 800°C.

[0036] In one embodiment, the calcination treatment time is 8 to 15 hours, for example, 8 hours, 10 hours, 12 hours, 14 hours or 15 hours.

[0037] In one embodiment, the concentration of silver ions in the silver salt solution in step (3) is 0.02 to 0.2 mol / L, for example, 0.02 mol / L, 0.05 mol / L, 0.1 mol / L, 0.15 mol / L or 0.2 mol / L.

[0038] In one embodiment, the replacement reaction in step (3) is carried out for 2 to 5 hours, for example, 2 hours, 2.5 hours, 3 hours, 4 hours or 5 hours.

[0039] In one embodiment, the replacement reaction is followed by filtration, washing and drying.

[0040] In a second aspect, the present application provides a composite lithium iron phosphate positive electrode material, which is prepared by the method described in the first aspect.

[0041] In one embodiment, the composite lithium iron phosphate positive electrode material is a lithium iron phosphate material having a uniform silver conductive network.

[0042] In a third aspect, the present application provides a positive electrode plate, which comprises the composite lithium iron phosphate positive electrode material as described in the second aspect.

[0043] In a fourth aspect, the present application provides a lithium-ion battery, wherein the lithium-ion battery comprises the positive electrode sheet as described in the third aspect.

[0044] Compared with the related art, this application has the following beneficial effects:

[0045] (1) This application utilizes iron hydroxide produced during the precipitation of iron phosphate, converts the iron hydroxide into elemental iron, and utilizes silver nitrate to achieve in-situ iron replacement of silver, thereby forming an effective 3D silver conductive network inside the lithium iron phosphate, greatly improving its conductivity.

[0046] (2) After the lithium iron phosphate material prepared in this application is made into a button-type battery, the 0.1C discharge specific capacity can reach more than 162mAh / g, the 1C discharge specific capacity can reach more than 156mAh / g, the 3C discharge specific capacity can reach more than 146mAh / g, the 5C discharge specific capacity can reach more than 139mAh / g, and the 10C discharge specific capacity can reach more than 130mAh / g; the resistivity can reach less than 8.7Ω·cm.

[0047] Still other aspects will become apparent upon reading and understanding the accompanying drawings and detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The accompanying drawings are used to provide a further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

[0049] FIG1 is a SEM image of the composite lithium iron phosphate positive electrode material prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0050] The technical solution of the present application is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only used to help understand the present application and should not be regarded as specific limitations of the present application.

[0051] Example 1

[0052] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0053] (1) Mixing a 1.02 mol / L ferric nitrate solution and a 1 mol / L ammonium hydrogen phosphate solution, controlling the first pH value to be between 1.5 and 2.0 with 1 mol / L ammonia water, stirring at a speed of 500 rpm, stirring the mixture for 0.5 h, then adjusting the second pH value to between 2.0 and 2.4, and reacting the mixture for 2 h to obtain a composite ferric phosphate containing ferric hydroxide;

[0054] (2) filtering, washing, and drying the composite iron phosphate, and then sintering it in an oxygen atmosphere at 600° C. for 6 hours to obtain iron phosphate, and converting iron hydroxide into iron oxide; in a carbon monoxide atmosphere, mixing the obtained iron phosphate with deionized water and lithium hydroxide in a ratio of lithium to iron of 1.02, controlling the pressure in the furnace to 5 kPa, setting the temperature to 750° C., and calcining for 10 hours to obtain modified lithium iron phosphate;

[0055] (3) The modified lithium iron phosphate was placed in a 0.1 mol / L silver nitrate solution, stirred and reacted for 4 hours, and then filtered and dried to obtain the composite lithium iron phosphate positive electrode material. The SEM image of the composite lithium iron phosphate positive electrode material is shown in Figure 1. As can be seen from Figure 1, the surface of the obtained lithium iron phosphate is relatively regular.

[0056] Example 2

[0057] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0058] (1) Ferric nitrate and 1 mol / L ammonium hydrogen phosphate solution were mixed, and the first pH value was controlled to be between 1.2 and 1.5 with 0.1 mol / L ammonia water. The mixture was stirred at a stirring speed of 500 rpm for 2 hours, and then the second pH value was adjusted to be between 2.2 and 2.4, and the mixture was reacted for 0.5 hours to obtain a composite iron phosphate containing iron hydroxide;

[0059] (2) filtering, washing, and drying the composite iron phosphate, sintering it in an oxygen atmosphere at 600° C. for 6 h to obtain iron phosphate, and then placing it in a carbon monoxide atmosphere, mixing the obtained iron phosphate with deionized water and lithium hydroxide in a ratio of lithium to iron of 1.02, controlling the pressure in the furnace to 10 kPa, setting the temperature to 700° C., and calcining for 4 h to obtain modified lithium iron phosphate;

[0060] (3) The modified lithium iron phosphate was placed in a 0.2 mol / L silver nitrate solution, stirred to react for 2 h, and then filtered and dried to obtain the composite lithium iron phosphate positive electrode material.

[0061] Example 3

[0062] This embodiment provides a composite lithium iron phosphate positive electrode material, and the preparation method of the composite lithium iron phosphate positive electrode material is as follows:

[0063] (1) Ferric nitrate and 1 mol / L ammonium hydrogen phosphate solution were mixed, and the first pH value was controlled to be between 1.8 and 2.0 with 0.1 mol / L ammonia water. The mixture was stirred at a stirring speed of 800 rpm for 2 hours. The second pH value was then adjusted to between 2.0 and 2.2, and the mixture was reacted for 1 hour to obtain a composite iron phosphate containing iron hydroxide;

[0064] (2) filtering, washing, and drying the composite iron phosphate, sintering it in an oxygen atmosphere at 600° C. for 6 h to obtain iron phosphate, and then placing it in a hydrogen atmosphere. The obtained iron phosphate is mixed with deionized water and lithium hydroxide in a ratio of 1.02 to iron. The pressure in the furnace is controlled to be 1 kPa, the temperature is set to 700° C., and calcined for 15 h to obtain modified lithium iron phosphate;

[0065] (3) The modified lithium iron phosphate was placed in a 0.02 mol / L silver nitrate solution, stirred to react for 5 h, and then filtered and dried to obtain the composite lithium iron phosphate positive electrode material.

[0066] Example 4

[0067] The only difference between this embodiment and embodiment 1 is that the second pH in step (1) is 2.5, and the other conditions and parameters are exactly the same as those in embodiment 1.

[0068] Comparative Example 1

[0069] The only difference between this comparative example and Example 1 is that the pH of the reaction in step (1) is 1.5-2.1 (without adjusting the pH, very little iron hydroxide is generated at a normal pH), and the other conditions and parameters are exactly the same as those in Example 1.

[0070] Comparative Example 2

[0071] The only difference between this comparative example and Example 1 is that the pH of the reaction in step (1) is 1 to 1.8, and the other conditions and parameters are exactly the same as those in Example 1 (the pH is not adjusted to avoid the formation of ferric hydroxide at low pH).

[0072] Comparative Example 3

[0073] The only difference between this comparative example and Example 1 is that the modified lithium iron phosphate described in step (3) is placed in a 0.02 mol / L silver nitrate solution and directly spray-dried and then calcined at 800°C for 6 h (i.e., directly coated with silver nitrate). The other conditions and parameters are exactly the same as those in Example 1.

[0074] Performance testing:

[0075] A mixture of lithium iron phosphate positive electrode material, acetylene black and polyvinylidene fluoride (weight ratio 97:1.5:1.5) prepared in the examples and comparative examples, a metal lithium sheet as the other electrode, and an electrolyte of 1 mol / L LiPF6 dissolved in a solvent of EC / DMC / EMC (volume ratio 1:1:1) were assembled into button test cells with constant current charge and discharge voltage range of 2.0-3.7V.

[0076] Charge to 3.7V with 0.1C constant current, wait 15 minutes, then discharge to 2V with 0.1C constant current, repeat the test 10 times.

[0077] The conductivity of the embodiment and the comparative example was tested, and the resistivity of the prepared lithium iron phosphate electrode was tested by a four-probe resistivity tester. The test results are shown in Table 1:

[0078] Table 1

[0079] As can be seen from Table 1, from Examples 1-3, the lithium iron phosphate material prepared in this application can be made into button-type batteries, and the 0.1C discharge capacity can reach above 162 mAh / g, the 1C discharge capacity can reach above 156 mAh / g, the 3C discharge capacity can reach above 146 mAh / g, the 5C discharge capacity can reach above 139 mAh / g, and the 10C discharge capacity can reach above 130 mAh / g; the resistivity can reach below 8.7 Ω·cm. By adjusting the reaction time at different pH values ​​during the reaction process, the formation of iron hydroxide can be controlled, ultimately affecting the conductivity of the material and the electrochemical performance of the battery material.

[0080] By comparison between Example 1 and Example 4, it can be seen that in the preparation process of the composite lithium iron phosphate positive electrode material described in the present application, the second pH will affect the content of iron hydroxide in the composite iron phosphate, and thus affect the content of the silver network in the subsequently prepared composite lithium iron phosphate positive electrode material. The second pH is controlled at 2.0-2.4, and the performance of the prepared composite lithium iron phosphate positive electrode material is better. If the second pH is too high, it will affect the performance of iron phosphate formation at high pH. If the second pH is too low (refer to Comparative Example 2), the required amount of iron hydroxide cannot be formed.

[0081] By comparison of Example 1 and Comparative Examples 1-2, it can be seen that the present application controls the pH of the coprecipitation reaction to produce iron hydroxide impurities, converts the iron hydroxide impurities into iron oxide by calcination, and then uses a carbon monoxide atmosphere to reduce the iron oxide to elemental iron, and then reacts the iron element with silver nitrate to replace the silver element in situ to form a conductive silver network. If the pH is not adjusted, regardless of whether the pH is high or low, an appropriate amount of iron hydroxide cannot be obtained, resulting in the inability to improve the conductivity of the material.

[0082] From the comparison between Example 1 and Comparative Example 3, it can be seen that direct coating with silver nitrate solution can only form silver ions adsorbed on the surface, forming silver on the surface of lithium iron phosphate, and the silver ions cannot be evenly coated on the surface of the material, and the adhesion amount cannot be controlled, so its conductivity is low.

[0083] The applicant declares that the above is only a specific implementation method of the present application, but the protection scope of the present application is not limited thereto. Technical personnel in the relevant technical field should understand that any changes or substitutions that can be easily thought of by technical personnel in the relevant technical field within the technical scope disclosed in this application fall within the protection scope and disclosure scope of this application.

Claims

1. A preparation method of a composite lithium iron phosphate cathode material, which comprises the following steps: (1) Mix an iron source and a phosphorus source with a solvent to obtain a mixed solution. After adjusting the pH of the mixed solution to a first pH for a one-step reaction and then adjusting it to a second pH for a two-step reaction, a composite iron phosphate containing iron hydroxide is obtained; (2) After sintering the composite iron phosphate, mix it with a lithium source and obtain modified lithium iron phosphate through a calcination treatment; (3) Mix the modified lithium iron phosphate with a silver salt solution, and through a displacement reaction, obtain the composite lithium iron phosphate cathode material.

2. The preparation method according to claim 1, wherein, The iron source in step (1) includes ferric chloride and / or ferric nitrate.

3. The preparation method according to claim 1 or 2, wherein The phosphorus source includes any one or a combination of at least two of ammonium phosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, or phosphoric acid.

4. The preparation method according to claim 1 or 2, wherein, The solvent includes water; Optionally, the stirring speed of the mixing is 300 - 800 rpm.

5. The preparation method according to any one of claims 1-4, wherein, The first pH in step (1) is 1.2 - 2.0; Optionally, the time of the one-step reaction is 0.5 - 2 h.

6. The preparation method according to any one of claims 1-5, wherein, The second pH is 2.0 - 2.4; Optionally, the time of the two-step reaction is 0.5 - 2 h; Optionally, the pH regulator for adjusting the mixed solution is ammonia water; Optionally, the concentration of the ammonia water is 0.1 - 2 mol / L.

7. The preparation method according to any one of claims 1-6, wherein, The temperature of the sintering treatment in step (2) is 500 - 700 °C; Optionally, the atmosphere of the sintering treatment includes oxygen; Optionally, the time of the sintering treatment is 4 - 10 h.

8. The preparation method according to any one of claims 1-7, wherein, The lithium source includes any one or a combination of at least two of lithium hydroxide, lithium carbonate, lithium nitrate, or lithium oxalate; Optionally, the molar ratio of lithium element in the lithium source to iron element in the composite iron phosphate is (1 - 1.07):1; Optionally, after mixing with the lithium source, sanding and spray drying treatments are performed.

9. The preparation method according to any one of claims 1-8, wherein, The atmosphere of the calcination treatment in step (2) includes hydrogen and / or carbon monoxide; Optionally, the pressure of the calcination treatment is 1 - 10 kPa.

10. The preparation method according to any one of claims 1-9, wherein, The temperature of the calcination treatment is 600 - 800 °C; Optionally, the time of the calcination treatment is 8 - 15 h.

11. The preparation method according to any one of claims 1-10, wherein, The concentration of silver ions in the silver salt solution in step (3) is 0.02 - 0.2 mol / L.

12. The preparation method according to any one of claims 1-11, wherein, The time of the displacement reaction in step (3) is 2 - 5 h; Optionally, after the displacement reaction, filtration, washing, and drying treatments are performed.

13. A composite lithium iron phosphate cathode material, wherein, The composite lithium iron phosphate cathode material is prepared by the method described in any one of claims 1 - 12; Optionally, the composite lithium iron phosphate cathode material is a lithium iron phosphate cathode material with a uniformly coated conductive silver network.

14. A positive electrode plate, wherein, The positive electrode plate contains the composite lithium iron phosphate cathode material described in claim 13.

15. A lithium-ion battery, wherein, The lithium ion battery contains the positive electrode plate described in claim 14.

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