A vanadium-doped lithium iron phosphate cathode material, its preparation method and application

Abstract

This invention provides a vanadium-doped lithium iron phosphate cathode material, its preparation method, and its application, belonging to the field of lithium-ion battery material technology. The vanadium-doped lithium iron phosphate cathode material prepared by the method of this invention has excellent electrochemical performance. When applied to the subsequent preparation of lithium-ion batteries, the resulting lithium-ion batteries have the characteristics of high discharge specific capacity and high initial charge-discharge efficiency. Furthermore, the preparation method of the vanadium-doped lithium iron phosphate cathode material provided by this invention has mild conditions and simple operation, which is beneficial to actual production.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery material technology, and particularly relates to a vanadium-doped lithium iron phosphate cathode material, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as a new generation of energy storage devices, have become the preferred choice for mobile power supplies due to their advantages such as high operating voltage, low self-discharge, and environmental friendliness. Lithium iron phosphate (LiFePO4), as one of its most promising cathode materials, boasts numerous advantages including low price, high capacity, long lifespan, and good thermal stability. However, its poor conductivity and low lithium-ion diffusion coefficient result in an actual capacity lower than the theoretical capacity, limiting its large-scale application in the power battery field. Currently, to address these issues and improve the electrochemical performance of cathode materials, common modification methods include reducing particle size, crystal phase doping, surface modification, and controlling particle morphology. Although these methods have effectively improved the electrochemical performance of LiFePO4, they still have the following drawbacks: nano-sizing reduces the material's packing density, affecting the volumetric energy density of the electrode material; poor control of process conditions makes it easy for doping modification to introduce impurity atoms, affecting the stability of the electrode material's crystal structure and electrochemical performance; surface modification, such as amorphous carbon coating, is difficult to uniformly coat and may generate byproducts, and it cannot fundamentally change the material's structure, which is detrimental to improving conductivity and hinders further improvements in battery performance. Furthermore, among the methods for preparing lithium iron phosphate materials, the high-temperature solid-state method has a long synthesis cycle, is prone to particle agglomeration, and has low uniformity, which is not conducive to lithium-ion diffusion. The sol-gel method and microwave synthesis method have problems such as high energy consumption and complex processes, which makes industrial production difficult. Therefore, developing a simple, reliable, and easily industrialized method to prepare lithium iron phosphate cathode materials that are conducive to lithium-ion diffusion and interfacial electrolyte penetration is of great significance for improving the overall electrochemical performance of lithium batteries. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a vanadium-doped lithium iron phosphate cathode material with a simple preparation method and excellent electrochemical performance, as well as its preparation method and application.

[0004] To achieve the above objectives, in a first aspect, the present invention provides a method for preparing a vanadium-doped lithium iron phosphate cathode material, the method comprising the following steps:

[0005] (1) Take MIL(V)-MOF and heat treat it in air atmosphere. After heat treatment, cool and grind it to obtain porous vanadium pentoxide microspheres.

[0006] (2) The porous vanadium pentoxide microspheres were dispersed in deionized water, and then the polymer monomer was added and heated to react. After the heating reaction was completed, the mixture was centrifuged, washed and dried to obtain the vanadium pentoxide / conductive polymer composite material.

[0007] (3) Vanadium pentoxide / conductive polymer composite material was dispersed in anhydrous ethanol, followed by the addition of ferrous salt and lithium salt. After stirring, phosphate was added, and the mixture was stirred again before hydrothermal reaction was carried out. After the hydrothermal reaction was completed, the mixture was centrifuged, washed, and dried to obtain lithium iron phosphate precursor.

[0008] (4) Calcining lithium iron phosphate precursor to obtain vanadium-doped lithium iron phosphate cathode material;

[0009] The polymer monomer includes at least one of pyrrole and aniline.

[0010] In the preparation of the vanadium-doped lithium iron phosphate cathode material provided by this invention, porous vanadium pentoxide (V2O5) microspheres are first prepared by annealing a high-porosity and high-specific-surface-area MIL(V)-based MOF material in air as a sacrificial template, wherein the V2O5 is arranged in layers. Then, the porous vanadium pentoxide microspheres are dispersed in deionized water and at least one of aniline or pyrrole is added as a polymerizing monomer and heated to react. The strong oxidizing property of pentavalent vanadium initiates the polymerization of the polymerizing monomers and their intercalation into the interlayer of vanadium pentoxide crystals, ultimately causing the layered structure of the porous vanadium pentoxide microspheres to peel off to form nanospheres. Vanadium pentoxide (V2O5) nanosheets are dispersed in a polymer until the V2O5 nanolayers are transformed into a large number of nanosheets. These nanosheets cross-link to form a porous network structure, resulting in a porous network structure of vanadium pentoxide / conductive polymer composite material. Subsequently, the vanadium pentoxide / conductive polymer composite material is dispersed in anhydrous ethanol, and ferrous salt, lithium salt, and phosphate are added sequentially and stirred for a period of time. Under hydrothermal conditions, a lithium iron phosphate precursor grows in situ on the surface of the vanadium pentoxide / conductive polymer composite material. After the reaction is complete, the resulting lithium iron phosphate precursor is calcined to obtain the vanadium-doped lithium iron phosphate cathode material. The vanadium-doped lithium iron phosphate cathode material prepared by the method of this invention possesses a porous network framework structure and vanadium doping, which synergistically promotes the rapid diffusion of lithium ions and electrolytes, improves the conductivity of the lithium iron phosphate cathode material, and enhances the overall electrochemical performance of the lithium-ion battery. Specifically, when the prepared vanadium-doped lithium iron phosphate cathode material is applied to the preparation of lithium-ion batteries, the resulting lithium-ion batteries exhibit high discharge specific capacity and high initial charge-discharge efficiency.

[0011] In one embodiment, in step (1), the heat treatment process is as follows: heating to 500-600℃ at a heating rate of 4-6℃ / min and holding at that temperature for 3-5 hours.

[0012] The present invention has found that the MIL(V)-MOF used in this invention has abundant porosity and high specific surface area. By annealing under the above-mentioned heat treatment parameters, porous vanadium pentoxide microspheres with layered arrangement can be prepared. The resulting layered structure is conducive to the subsequent insertion of monomers and helps to achieve the excellent comprehensive performance of the product.

[0013] In one embodiment, the grinding is performed until the particle size range is 1μm-3μm.

[0014] In one embodiment, the mass-to-volume ratio of the MIL(V)-MOF to the polymeric monomer is 1 g:(1-1.5) mL.

[0015] This invention has discovered that after preparing porous V₂O₅ microspheres from MIL(V)-MOF, the introduced polymeric monomers can penetrate into the interior of the porous V₂O₅ microspheres and insert themselves in situ into the interlayer of the porous V₂O₅ microspheres. Subsequently, during the subsequent heating reaction, in V₂O₅... 5+ Under the strong oxidizing properties of the polymer, the monomers continuously polymerize, eventually causing the layered structure of porous V2O5 microspheres to peel off and disperse in the polymer until the porous V2O5 microspheres are transformed into nanosheets. At the same time, due to the polymeric monomers inserting into the interlayer of porous V2O5 microspheres to form polymers, the nanosheets cross-link to form a porous network structure. When the mass-volume ratio of MIL(V)-MOF to polymeric monomers is further selected as 1 g:(1-1.5) mL, the polymer formed by the polymerization of polymeric monomers can effectively cross-link the nanosheets, thereby forming a stable porous network structure. The formation of the porous network structure can increase the specific surface area of ​​the product, which increases the contact area between the vanadium-doped lithium iron phosphate cathode material and the electrolyte in the subsequently prepared lithium-ion battery, and increases the number of electrochemical reaction active sites. This effectively shortens the lithium-ion transport path and promotes the lithiation / delithiation process, which is beneficial to improving the rate performance of the subsequently prepared lithium-ion battery.

[0016] In one embodiment, in step (2), the heating reaction process is as follows: after stirring for 25-35 minutes, the temperature is raised to 40-50°C and stirring is continued for 11-13 hours.

[0017] This invention has found that when the heating reaction operation and parameters are selected within the above-mentioned range during the heating process, the continuous polymerization of monomers can be effectively promoted, thereby transforming the layered porous V2O5 microspheres into cross-linked nanosheets, and then forming a porous network structure, thus improving the electrochemical performance of the prepared vanadium-doped lithium iron phosphate cathode material.

[0018] In one embodiment, in step (2), the washing is performed by washing with deionized water and anhydrous ethanol in sequence.

[0019] In one embodiment, in step (2), the drying temperature is 65-75°C and the drying time is 12-24h.

[0020] In one embodiment, in step (3), the temperature of the hydrothermal reaction is 150-180°C and the time of the hydrothermal reaction is 5-8h.

[0021] The present invention has found that when the temperature and time of the hydrothermal reaction in step (3) are within the range given in the present invention, on the one hand, the amino groups on the conductive polymer in the vanadium pentoxide / conductive polymer composite material have a better complexing effect on ferrous ions and lithium ions, which can effectively complex ferrous ions and lithium ions in the reaction system and promote the in-situ growth of ferrous ions and lithium ions on the porous network cross-linked nanosheets; on the other hand, the conductive polymer can better inhibit the oxidation of ferrous ions as a carbon reducing agent; thereby improving the overall performance of the product.

[0022] In one embodiment, the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is (0.03-0.07):1:1:1.

[0023] The present invention has found that in the preparation process of step (3), the vanadium pentoxide / conductive polymer composite material can serve as a supporting framework for the synthesis of lithium iron phosphate precursor, preventing the added substances from agglomerating and affecting the preparation efficiency. In other words, it can effectively improve the uniformity of particle size distribution of the synthesized lithium iron phosphate and effectively promote the transfer of ions and electrons. In particular, when the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is further selected as (0.03-0.07):1:1:1, it can not only provide a suitable supporting framework, but also better complex ferrous ions and lithium ions, thereby improving the overall performance of the product.

[0024] In one embodiment, in step (3), the washing is performed by washing with deionized water and anhydrous ethanol in sequence.

[0025] In one embodiment, in step (3), the drying temperature is 65-75°C and the drying time is 12-24h.

[0026] In one embodiment, in step (4), the calcination temperature is 650-750°C and the calcination time is 5-8 hours.

[0027] This invention has found that the introduction of the calcination process allows the introduction of vanadium dopant into the lithium iron phosphate precursor. Vanadium doping increases the lattice defects of lithium iron phosphate, reduces the energy barrier for lithium-ion diffusion in the lattice, and decreases the insertion / extraction resistance of lithium ions, thereby effectively improving the ion conductivity and electrical conductivity of the lithium iron phosphate cathode material. Specifically, in the hydrothermal reaction of step (3), the formed lithium iron phosphate precursor is grown in situ on V2O5 nanosheets coated or embedded with a conductive polymer. Subsequently, after calcination in step (4), vanadium doping is introduced into the lithium iron phosphate, while the conductive polymer is transformed into a conductive nitrogen-doped carbon layer embedded or coated on the surface of the lithium iron phosphate material, thus forming a vanadium-doped lithium iron phosphate cathode material. When the calcination temperature and time are further selected within the range given in this invention, the conversion can be better achieved, resulting in a product with superior overall performance.

[0028] In one embodiment, in step (4), the calcination atmosphere is an inert gas atmosphere.

[0029] For example, the inert gas includes rare gases such as argon.

[0030] In one embodiment, the MIL(V)-MOF includes at least one of MIL-101(V), MIL-53(V), and MIL-47(V).

[0031] This invention has found that when MIL(V)-MOF is selected as the above-mentioned material, porous vanadium pentoxide microspheres can be effectively arranged in a layered manner after heat treatment.

[0032] In one embodiment, the ferrous salt includes at least one of ferrous sulfate, ferrous oxalate, and ferrous chloride.

[0033] In one embodiment, the lithium salt includes at least one of lithium carbonate and lithium acetate.

[0034] In one embodiment, the phosphate includes at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate.

[0035] In a second aspect, the present invention provides a vanadium-doped lithium iron phosphate cathode material, wherein the vanadium-doped lithium iron phosphate cathode material is prepared by the preparation method described in the present invention.

[0036] The vanadium-doped lithium iron phosphate cathode material provided by this invention has excellent electrochemical performance, specifically, it has high discharge specific capacity and high first charge-discharge efficiency.

[0037] In a third aspect, the present invention provides the application of the vanadium-doped lithium iron phosphate cathode material in the preparation of lithium-ion batteries.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] In the preparation process of the vanadium-doped lithium iron phosphate cathode material provided by this invention, by using a vanadium pentoxide / conductive polymer composite material as the supporting framework for the synthesis of lithium iron phosphate precursor, the particle size distribution uniformity of the prepared lithium iron phosphate can be effectively improved, and the transfer of ions and electrons can be effectively promoted. Subsequently, through calcination, the vanadium in the vanadium pentoxide / conductive polymer composite material is doped into the lithium iron phosphate precursor. The introduction of vanadium doping can increase the lattice defects of lithium iron phosphate, reduce the energy barrier for lithium ion diffusion in the lattice, and reduce the insertion / extraction resistance of lithium ions, thereby effectively improving the ion conductivity and electron conductivity of lithium iron phosphate. The conductive polymer in the vanadium pentoxide / conductive polymer composite material is transformed into a nitrogen-doped carbon layer embedded or coated on the surface of lithium iron phosphate, further improving the conductivity and ion conductivity of lithium iron phosphate. Therefore, the vanadium-doped lithium iron phosphate cathode material prepared by the method of the present invention has excellent electrochemical performance. When applied to the subsequent preparation of lithium-ion batteries, the resulting lithium-ion batteries have the characteristics of high discharge specific capacity and high initial charge-discharge efficiency. Furthermore, the preparation method of the vanadium-doped lithium iron phosphate cathode material provided by the present invention has mild conditions and simple operation, which is beneficial to actual production. Attached Figure Description

[0040] Figure 1 The image shows an XRD comparison between the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 and the LiFePO4 standard card.

[0041] Figure 2 XPS comparison images of the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 and the pure lithium iron phosphate cathode material;

[0042] Figure 3 SEM image of the vanadium pentoxide / conductive polymer composite material prepared in Example 1;

[0043] Figure 4 The image shows a SEM image of the vanadium-doped lithium iron phosphate cathode material prepared in Example 1.

[0044] Figure 5 The image shows the charge-discharge curves of a lithium-ion battery prepared using the lithium iron phosphate cathode material prepared in Example 1. Detailed Implementation

[0045] To better illustrate the purpose, technical solution, and advantages of the present invention, the present invention will be further described below in conjunction with specific embodiments.

[0046] Unless otherwise specified, the reagents, methods and equipment used in this invention are all conventional reagents, methods and equipment in the field.

[0047] Example 1

[0048] This invention provides a vanadium-doped lithium iron phosphate cathode material, the preparation method of which includes the following steps:

[0049] (1) Disperse 3 mmol vanadium chloride and 3 mmol terephthalic acid in 50 mL anhydrous methanol, stir evenly, pour the mixture into 100 mL stainless steel autoclave, heat at 180 °C for 24 h, wash three times with methanol and ethanol in sequence after the reaction, and dry under vacuum at 60 °C for 12 h after washing to obtain 0.532 g MIL-101(V) powder.

[0050] (2) Place the MIL-101(V) powder prepared in step (1) in a muffle furnace, heat it to 500℃ at a rate of 5℃ / min in an air atmosphere and keep it at that temperature for 3h. After cooling to room temperature, collect the powder and grind it to a particle size range of 1μm-3μm to obtain porous V2O5 microspheres.

[0051] (3) The porous V2O5 microspheres prepared in step (2) were dispersed in 50 mL of deionized water. Then, aniline monomer was added at a mass-volume ratio of 1 g to 1 mL of MIL-101(V) powder and aniline monomer. After stirring for 0.5 h, the temperature was raised to 40 °C and stirring was continued for 12 h. After the reaction was completed, the product was collected by centrifugation and washed with deionized water and anhydrous ethanol in sequence. After washing, the product was placed in a vacuum oven at 70 °C and dried overnight to obtain V2O5@PANI composite material.

[0052] (4) The V2O5@PANI composite material prepared in step (3) was dispersed in 60 mL of anhydrous ethanol, and then 0.1 mol of ferrous sulfate and 0.1 mol of lithium acetate were added and stirred for 0.5 h. Then 0.1 mol of ammonium dihydrogen phosphate was added and stirred (the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt and phosphorus in phosphate was 0.03:1:1:1). After the mixture was evenly dispersed, it was poured into a 100 mL polytetrafluoroethylene stainless steel autoclave and placed in an oven at 150 °C for 6 h. After the reaction was completed, it was cooled to room temperature and then centrifuged to collect the product. The product was washed three times with deionized water and anhydrous ethanol. After washing, it was placed in a vacuum oven at 70 °C and dried overnight to obtain lithium iron phosphate precursor.

[0053] (5) The lithium iron phosphate precursor prepared in step (4) is placed in a tube furnace under an argon atmosphere and calcined at 650°C for 6 hours to obtain vanadium-doped lithium iron phosphate material.

[0054] Example 2

[0055] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), the amount of aniline monomer added is changed so that the mass-volume ratio of MIL-101(V) powder to aniline monomer is 1g:1.5mL.

[0056] Example 3

[0057] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), the amount of aniline monomer added is changed so that the mass-volume ratio of MIL-101(V) powder to aniline monomer is 1g:0.8mL.

[0058] Example 4

[0059] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), the amount of aniline monomer added is changed so that the mass-volume ratio of MIL-101(V) powder to aniline monomer is 1g:1.8mL.

[0060] Example 5

[0061] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), after stirring for 0.5 h, the temperature is raised to 50 °C and stirring is continued for 12 h.

[0062] Example 6

[0063] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), after stirring for 0.5 h, the temperature is raised to 30 °C and stirring is continued for 12 h.

[0064] Example 7

[0065] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (3), after stirring for 0.5 h, the temperature is raised to 60 °C and stirring is continued for 12 h.

[0066] Example 8

[0067] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (4), the material is placed in an oven and kept at 170°C for 8 hours.

[0068] Example 9

[0069] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (4), the material is placed in an oven and kept at 140°C for 7 hours.

[0070] Example 10

[0071] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that in step (4), the material is placed in an oven and kept at 190°C for 5 hours.

[0072] Example 11

[0073] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is changed (0.06 mol) by altering the molar number of ferrous sulfate, lithium acetate, and ammonium dihydrogen phosphate added, so that the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is 0.05:1:1:1.

[0074] Example 12

[0075] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is changed (0.15 mol) by altering the molar ratio of ferrous sulfate, lithium acetate, and ammonium dihydrogen phosphate added, so that the molar ratio is 0.02:1:1:1.

[0076] Example 13

[0077] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that the molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is changed (0.04 mol) by altering the molar ratio of ferrous sulfate, lithium acetate, and ammonium dihydrogen phosphate added, so that the molar ratio is 0.07:1:1:1.

[0078] Example 14

[0079] This invention provides a vanadium-doped lithium iron phosphate cathode material. The only difference between the preparation method of the vanadium-doped lithium iron phosphate cathode material and that of Example 1 is that pyrrole is used instead of aniline.

[0080] Comparative Example 1

[0081] This invention provides a vanadium-doped lithium iron phosphate cathode material, the preparation method of which includes the following steps:

[0082] 0.1 mol ferrous sulfate, 0.1 mol lithium acetate, and 0.1 mol ammonium dihydrogen phosphate were dispersed in 60 mL of anhydrous ethanol and stirred evenly. The mixture was then poured into a 100 mL polytetrafluoroethylene stainless steel autoclave and placed in an oven at 150 °C for 6 h. After cooling to room temperature, the product was collected by centrifugation and washed three times with deionized water and anhydrous ethanol. After washing, the product was dried in a vacuum oven at 70 °C to obtain a lithium iron phosphate precursor. Subsequently, the prepared lithium iron phosphate precursor was mixed with vanadium pentoxide and ground for 20 min. After grinding, the product was calcined in a tube furnace under an argon atmosphere at 650 °C for 6 h to obtain vanadium-doped lithium iron phosphate cathode material.

[0083] Comparative Example 2

[0084] This invention provides a lithium iron phosphate cathode material in a comparative example, the preparation method of which includes the following steps:

[0085] 0.1 mol ferrous sulfate, 0.1 mol lithium acetate, and 0.1 mol ammonium dihydrogen phosphate were dispersed in 60 mL of anhydrous ethanol and stirred until homogeneous. The mixture was then poured into a 100 mL polytetrafluoroethylene stainless steel autoclave and placed in an oven at 150 °C for 6 h. After cooling to room temperature, the product was collected by centrifugation and washed three times with deionized water and anhydrous ethanol. After washing, the product was dried in a vacuum oven at 70 °C to obtain the lithium iron phosphate precursor. Subsequently, the prepared lithium iron phosphate precursor was calcined in a tube furnace under an argon atmosphere at 650 °C for 6 h to obtain the lithium iron phosphate cathode material.

[0086] Comparative Example 3

[0087] This invention provides a lithium iron phosphate cathode material in a comparative example, the preparation method of which includes the following steps:

[0088] 0.1 mol ferrous sulfate, 0.1 mol lithium acetate, and 0.1 mol ammonium dihydrogen phosphate were dispersed in 60 mL of anhydrous ethanol. Then, 0.005 mol silica nanoparticles (D50 of silica nanoparticles was 50 nm) were added and stirred until homogeneous. The mixture was then poured into a 100 mL polytetrafluoroethylene stainless steel autoclave and placed in an oven at 150 °C for 6 h. After cooling to room temperature, the product was collected by centrifugation and washed three times with deionized water and anhydrous ethanol. After washing, the product was dried in a vacuum oven at 70 °C to obtain the lithium iron phosphate precursor. Subsequently, the prepared lithium iron phosphate precursor was mixed with vanadium pentoxide and ground for 20 min. After grinding, the product was calcined in a tube furnace under an argon atmosphere at 650 °C for 6 h to obtain the lithium iron phosphate cathode material.

[0089] Example of effect

[0090] The effectiveness examples of this invention verify the performance of the lithium iron phosphate cathode materials prepared in Examples 1-14 and Comparative Examples 1-3. Specifically, the lithium iron phosphate cathode materials prepared in Examples 1-14 and Comparative Examples 1-3, the binder PVDF, and the conductive agent were mixed in a mass ratio of 95:2.5:2.5 to obtain a cathode slurry, which was then coated onto the surface of aluminum foil to obtain a cathode sheet. Lithium metal was used as the anode, and the electrolyte ratio was 1.15M LiPF6EC:DMC (1:1 vo 1%). The coin cells were assembled into a coin cell. The obtained coin cells were tested at 25°C with a test voltage of 2.5-4.5V. The discharge specific capacity at 0.1C and 0.5C and the first charge-discharge efficiency were tested. The results are shown in Table 1.

[0091] Table 1

[0092]

[0093]

[0094] As can be seen from Table 1, when the technical solution provided by this invention is adopted, the lithium iron phosphate cathode material prepared and applied to the preparation of batteries results in batteries with excellent electrochemical performance. Specifically, the battery has a 0.1C discharge specific capacity of over 153.7 mAh / g, a 0.5C discharge specific capacity of over 147.5 mAh / g, and an initial charge-discharge efficiency of over 96.02%. Compared with the lithium iron phosphate cathode materials without porous network structure or without vanadium doping in Comparative Examples 1-3, the electrochemical performance is significantly improved.

[0095] In addition, the XRD pattern of the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 is compared with the LiFePO4 standard card as shown in the figure below. Figure 1 As shown, from Figure 1 As can be seen from the data, the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 has the same characteristic peaks as LiFePO4, with no other impurity peaks. The diffraction peaks are very intense and have sharp peak shapes. Figure 1 It can be seen that vanadium ions are doped into the interior of the iron phosphate crystal structure and form a continuous solid solution without affecting its phase structure.

[0096] The XPS spectrum of the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 is as follows: Figure 2 As shown, from Figure 2 It can be seen that the XPS spectrum of the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 shows peaks corresponding to Fe, Li, P and V elements, further indicating that vanadium element has been successfully doped into the structure of lithium iron phosphate.

[0097] The SEM image of the vanadium pentoxide / conductive polymer composite material prepared in Example 1 is shown below. Figure 3 As shown; the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 is from Figure 4 It can be seen that, from Figure 3-4 As can be seen from the above, both the vanadium pentoxide / conductive polymer composite material and the vanadium-doped lithium iron phosphate cathode material prepared in Example 1 exhibit a porous network structure.

[0098] The charge-discharge curves of the lithium-ion battery prepared using the lithium iron phosphate cathode material prepared in Example 1 are shown in the figure below. Figure 5 As shown, from Figure 5 It can be seen that lithium-ion batteries have a stable voltage plateau around 3.4V, which corresponds to the lithium ion insertion / extraction voltage in lithium iron phosphate crystals.

Claims

1. A method for preparing vanadium-doped lithium iron phosphate cathode material, characterized in that, The preparation method includes the following steps: (1) Take MIL(V)-MOF and heat treat it in air atmosphere. After heat treatment, cool and grind it to obtain porous vanadium pentoxide microspheres. (2) The porous vanadium pentoxide microspheres were dispersed in deionized water, and then the polymer monomer was added and heated to react. After the heating reaction was completed, the mixture was centrifuged, washed and dried to obtain the vanadium pentoxide / conductive polymer composite material. (3) Vanadium pentoxide / conductive polymer composite material was dispersed in anhydrous ethanol, followed by the addition of ferrous salt and lithium salt. After stirring, phosphate was added, and the mixture was stirred again before hydrothermal reaction was carried out. After the hydrothermal reaction was completed, the mixture was centrifuged, washed, and dried to obtain lithium iron phosphate precursor. (4) Calcining lithium iron phosphate precursor to obtain vanadium-doped lithium iron phosphate cathode material; The polymer monomer includes at least one of pyrrole and aniline.

2. The preparation method according to claim 1, characterized in that, In step (1), the heat treatment process is as follows: the temperature is raised to 500-600℃ at a heating rate of 4-6℃ / min and held for 3-5 hours.

3. The preparation method according to claim 1, characterized in that, The mass-to-volume ratio of MIL(V)-MOF to the polymer monomer is 1 g:(1-1.5) mL.

4. The preparation method according to claim 1, characterized in that, In step (2), the heating reaction process is as follows: after stirring for 25-35 minutes, the temperature is raised to 40-50℃ and stirring is continued for 11-13 hours.

5. The preparation method according to claim 1, characterized in that, In step (3), the temperature of the hydrothermal reaction is 150-180℃ and the time of the hydrothermal reaction is 5-8h.

6. The preparation method according to claim 1, characterized in that, The molar ratio of vanadium in MIL(V)-MOF, iron in ferrous salt, lithium in lithium salt, and phosphorus in phosphate is (0.03-0.07):1:1:

1.

7. The preparation method according to claim 1, characterized in that, In step (4), the calcination temperature is 650-750℃ and the calcination time is 5-8h.

8. The preparation method according to claim 1, characterized in that, Satisfy at least one of the following: (a) The MIL(V)-MOF includes at least one of MIL-101(V), MIL-53(V), and MIL-47(V); (b) The ferrous salts include at least one of ferrous sulfate, ferrous oxalate, and ferrous chloride; (c) The lithium salt includes at least one of lithium carbonate and lithium acetate; (d) The phosphate includes at least one of ammonium dihydrogen phosphate and ammonium hydrogen phosphate.

9. A vanadium-doped lithium iron phosphate cathode material, characterized in that, The vanadium-doped lithium iron phosphate cathode material is prepared by the preparation method described in any one of claims 1-8.

10. The application of the vanadium-doped lithium iron phosphate cathode material as described in claim 9 in the preparation of lithium-ion batteries.

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